The biomechanics of fast bowling in men's cricket: A review

Transcription

1 Journal of Sports Sciences, 1996, 14, The biomechanics of fast bowling in men's cricket: A review R.M. BARTLETT, 1 * N.P. STOCKILL, 2 B.C. ELLIOTT 3 and A.F. BURNETT 3 1 Department of Exercise and Sport Science, The Manchester Metropolitan University, Crewe + Alsager Faculty, Alsager ST7 2HL, UK, 2 The Lilleshall Sports Injury and Human Performance Centre, National Sports Centre, Lilleshall, Shropshire TF10 9AT, UK and 3 Department of Human Movement, University of Western Australia, Nedlands, Perth, WA 6907, Australia Accepted 9 December 1995 This review concentrates on synthesizing and analysing the biomechanical research which has been carried out on fast bowling in men's cricket. Specifically, it relates to those elements of the bowling technique which contribute towards a fast ball release, the aerodynamics and technique of swing bowling, and the association between fast bowling and lower back injury. With regard to bowling technique, no firm conclusions are drawn on the relationships between elements of the fast bowling technique and ball release speed. Recommendations for future research in this area include intra-player studies to establish the bowler-specific factors which contribute to fast ball release and features of body segment dynamics. There is general agreement that the phenomenon of differential boundary layer separation is the reason for normal and reverse cricket ball swing. Systematic research to establish the essential aspects of the bowling technique which contribute to successful swing bowling is recommended, along with studies of the behaviour of the ball in games to ascertain the effects of ball asymmetries on ball swing. There is sufficient evidence in the literature to establish a strong link between injury to the lower back and the use of the mixed technique. Recommendations are made for screening and intervention to reduce the use of the mixed technique, and for research into other aspects of injury. Fundamental research to develop biomechanical models of the lower back in fast bowling is strongly recommended. Keywords: Biomechanics, cricket fast bowling, injury, ball swing. Introduction Cricket has not been well served by biomechanical research. There are a number of coaching texts in which techniques for the various skills that make up the game are presented. Almost all of these are based on previous texts of a similar nature, observation of top players or anecdotal evidence. There is a total lack of published biomechanical research on women's cricket, and this review will therefore focus only on the men's game. Most of the scientific research to date into the biomechanics of men's cricket has been carried out on the technique of fast or fast-medium bowling. This may be because of the importance which this element of the game has acquired, particularly in the last decade or so, *Author to whom all correspondence should be addressed / E. & F.N. Spon when good pairings, trios or even quartets of fast bowlers have been a major feature in success at Test (International) level. It may also be because it is widely considered that fast bowling in cricket is one of the non-contact activities most susceptible to injury (e.g. Fitch, 1989), as evidenced by the almost epidemic proportions which injuries to the lower back have reached among fast bowlers (Elliott et al., 1992). Success in fast bowling is determined by a combination of many factors, one extremely important variable being the speed at which the ball is released. A fast ball release speed reduces the time available for the batsman to make correct decisions about the path of the ball, thus increasing the demands on the effector mechanisms responsible for executing the correct shot. An 'optimal' fast bowling technique could be defined as one that allows the bowler to bowl fast with a relatively low injury risk. Another factor that can be successfully

2 404 Bartlett et al. exploited by the fast bowler (and by bowlers of medium pace and upwards) is the phenomenon of swing. The exploits of the Pakistan fast bowlers produced a flurry of articles in the British 'popular' press during the England vs Pakistan Test series of 1992, related especially to so-called reverse swing. Many researchers (e.g. Elliott and Foster, 1989; Elliott et al., 1992; Stockill and Bartlett, 1992a) have identified and classified three main techniques prevalent in cricket fast bowling, although the boundaries between them are not fixed and they exist within a continuum of techniques. These three techniques are shown in Figs 1-3. The side-on technique (Fig. 1) has been advocated as the correct and most effective way to bowl. Until recently, it was the only technique recognized by the Marylebone Cricket Club (MCC) Coaching Book (MCC, 1976). It is typified by a relatively low run-up speed at the start of the delivery stride, a rear foot position which is parallel to the popping crease, and a shoulder alignment at rear foot strike that points down the wicket, such that the angle between the wickets and the line joining the shoulders is approximately 180 (see also Fig. 4a). The front-on technique (Fig. 2) is typified by a higher run-up speed, a rear foot position that points more towards the direction of ball travel after release, and a more open chested position at rear foot strike with the shoulders at an angle which considerably exceeds 180 (see also Fig. 4b). This technique is often used by the West Indian fast bowlers (Elliott et al., 1990). Figure 1 Computer reproduction of a side-on bowling action: side view (top) and front view (bottom). From left: back foot strike, mid-delivery stride, front foot strike, ball release. Figure 2 Computer reproduction of a front-on bowling action: side view (top) and front view (bottom). From left: back foot strike, mid-delivery stride, front foot strike, ball release.

3 The biomechanics of fast bowling in men's cricket 405 A mixed technique (Fig. 3) has also been identified (e.g. Foster et al, 1989; Elliott et al, 1992), which is used by a large number of fast bowlers. This technique, as the name suggests, is a mixture of the side-on and front-on techniques. It is characterized by bowlers adopting a front-on foot and shoulder orientation at back foot strike, which is followed by a realignment of the shoulders to a more side-on position during the delivery stride. This technique is believed to be more likely to lead to a high incidence of lower back problems (bony abnormalities such as pedicle sclerosis, spondylolysis and spondylolisthesis; disc degeneration and bulging). This occurs because of the spine adopting a twisted and hyperextended position (Foster et al, 1989) at a time (front foot strike) when ground reaction forces are high. Rasch (1989) identified trunk rotation, hyperextension and high axial compression (load bearing) as three potential causes of lower back pain. Bartlett (1992), for example, noted that all three of these components are present in the mixed technique. Reflecting the bias of the scientific literature on cricket, this review will concentrate on synthesizing and analysing the biomechanical research which has been carried out on the activity of men's fast bowling. Specifically, this will relate to those elements of the bowling technique which contribute towards a fast ball release, the aerodynamics and technique of swing bowling and the association between fast bowling and lower back injury. The fast bowling technique Because of the relative scarcity of published results and analyses of the forces involved in fast bowling, this section concentrates mainly on the kinematics of the fast bowling technique. Particular emphasis will be placed on the effect of various kinematic parameters upon the ball release speed. For the purpose of this review, the action is divided into the four distinct stages of the runup, the pre-delivery stride, the delivery stride and the follow-through. Figure 3 Computer reproduction of a mixed bowling action: side view (top) andfrontview (bottom). From left: back foot strike, mid-delivery stride, front foot strike, ball release. The run-up The run-up commences when the bowler walks or jogs over his marker, gradually increasing speed on his approach to the wicket, and ends as he leaps into the air at the start of the pre-delivery stride in preparation for the back foot to strike the ground, which marks the commencement of the delivery stride. The length of the run-up varies between bowlers and there is no universal agreement as to its optimal length. Davis and Blanksby (1976a), in a study of 19 club level fast bowlers, concluded that a run-up of 14 paces is sufficient to release the ball at 37 m s" 1. Davis and Blanksby (1976b), in a study of 17 fast bowlers, reported that the 6 fastest bowlers used a run-up that was 2.14 m longer than the 6 slowest bowlers (distances not reported), but offered no explanation as to how this affected the run-up speed. Elliot and Foster (1989) disagreed with the findings of Davis and Blanksby (1976b), suggesting a runup length between 15 and 30 m, with the emphasis on a balanced and rhythmical running technique. In the athletics event of javelin throwing, where similar approach speeds have been recorded to those in cricket fast bowling (e.g. Komi and Mero, 1985), run-ups (including the transition period before cross-over) as short as 12 paces have been cited (Flatten, 1980; Hay, 1985).

4 406 Bartlett et al. Speed of run-up In addition to an emphasis on rhythm, Elliott and Foster (1989) recommended that the build up of speed should be gradual with maximum speed being reached three or four strides from the end of the run-up. In an attempt to determine how their speed varied as bowlers approached the wicket, Mason et al. (1989) analysed 15 medium-fast junior bowlers and reported mean runup speeds of 6.1, 6.1, 5.7 and 5.6 m s" 1 at distances from the wickets of 12-16, 8-12, 4-8 and 0-4 metres respectively. Their results suggested that the bowlers reached maximum speed at 8-16 m from the crease and then slowed down slightly in preparation for the pre-delivery stride. The majority of studies, however, have reported the run-up speed at the end of the predelivery stride (back foot strike). A summary of reported run-up speeds is given in Table 1. The run-up speeds reported by Elliott and Foster (1984), Foster and Elliott (1985) and Elliott et al. (1986, 1992) are similar to the run-up speeds for top senior male javelin throwers of 5.3 ± 0.7 m s" 1 (n = 7), 5.2 ± 0.6 m s" 1 (n = 5) and 5.6 ± 1.0 m s" 1 (n = 11) reported by Ikegami et al. (1981), Komi and Mero (1985) and Mero et al. (1994) respectively. Stockill and Bartlett (1992a) reported mass centre speeds immediately before back foot strike of 6.8 ±1.7 m s~ l (n = 17 International seniors) but suggested that the unusually high values may have been due to the limited three-dimensional calibration volume used (2 x 2 x 1.4 m), and the potential inaccuracy of reconstruction of points which lie outside the defined volume (Wood and Marshall, 1986; Challis and Kerwin, 1992). The very fast run-up speed (9.3 m s" 1 ) reported by Penrose et al. (1976) for one elite bowler is unrealistically high, being similar to that reported by Hay et al. (1986) for an elite male long jumper. This suggests methodological problems in the study of Penrose et al. (1976). Elliott and Foster (1984) considered that the run-up speed should be sufficient to produce as high a linear velocity of the body as possible for ball release, but also must allow the correct delivery technique to be adopted. It is reasonable to assume, therefore, that the runup speed is dependent upon the individual's needs and that a bowler who, for example, wishes to adopt a sideon position, may need to approach at a slower rate than a front-on bowler. This was supported by Elliott and Foster (1984), who reported run-up speeds immediately before back foot strike for Australian representative fast bowlers of 3.9 io.lm s" 1 for two sideon bowlers compared to 4.5 ± 0.1 m s" 1 for three front-on bowlers. To summarize, the majority of the studies in the literature report run-up speeds (the horizontal component of the bowler's mass centre or hip velocity at the end of the pre-delivery stride) of between 3.9 and 5.5 m s" 1. Effect of run-up speed on ball release speed From a purely mechanical point of view, the bowler's run-up speed must contribute to the forward motion of the ball. Attempts have been made to calculate the percentage contribution of the run-up speed to ball release speed (Davis and Blanksby, 1976a; Elliott et al., 1986). By subtracting the bowler's mass centre speed at ball release from ball release speed and expressing it as a percentage of final ball speed, both groups of authors found similar results (19%, Davis and Blanksby, 1976a; 15%, Elliott et al., 1986). This simplistic method of determination is flawed because the studies assumed that the techniques adopted by each bowler were the same. As suggested earlier and demonstrated by Elliott and Foster (1984), this was not the case, as there are considerable differences in modes of delivery and run-up speeds. As a result, the percentage contribution of the run-up to ball release speed will vary between bowlers. Brees (1989) investigated the effect of experimentally manipulating run-up speed on the ball release speed, the kinematics of the delivery stride and accuracy. Using two-dimensional cinematography to analyse the technique, and a target designed by Stretch and Goslin (1987) to test for accuracy, seven college standard bowlers were instructed to bowl as fast and as accurately as possible at three different approach speeds (normal, slow and fast). The results revealed a positive correlation (P < 0.05) between run-up speed and ball release speed but a negative correlation (P < 0.05) between run-up speed and accuracy, suggesting that the bowlers 'normally' selected an approach speed that produced optimal ball speeds and optimal accuracy. The most obvious changes in kinematics owing to the increased run-up speed were a decreased contribution from trunk lateral flexion and trunk flexion and an increased amount of knee flexion between front foot strike and ball release. Although a relationship was found between run-up speed and ball release speed, the testing protocol required the bowlers to adopt approach speeds that were considerably different from those used in matches, and therefore the generality of these results to competition was not established. Burden (1990), in a two-dimensional cinematographic study of 10 college bowlers, revealed run-up speeds of 6.0 ± 0.6 m s" 1. Low order polynomials revealed no relationship (r 2 = 0.01) between the speed of the bowler at ball release and the release speed of the ball, thus questioning the suggestions of Davis and Blanksby (1976b) and Elliott et al. (1986). In summary, available results do not support the conclusion

5 The biomechanics of fast bowling in men's cricket 407 that the run-up speed makes a significant contribution to ball release speed under match conditions. Pre-delivery stride The pre-delivery stride, which separates the run-up from the delivery stride, begins, for a right-handed bowler, with a jump off the left foot and is completed as the bowler lands on the right or back foot (MCC, 1976). During this stride, with the shoulders pointing down the wicket, the right foot passes in front of the left with the right foot turning to land parallel to the bowling crease. Few data have been published regarding the optimum length of the pre-delivery stride, though it is likely to be longer than a normal stride, as it must allow time for the feet to cross in preparation for the right foot of the side-on bowler to land in a side-, on position. This is not the case for a front-on bowler, who is not required to attain this side-on foot position and hence does not need to make significant adjustments to his stride length. Davis and Blanksby (1976b) found that the pre-delivery stride was 0.42 m (22%) longer than the last run-up stride for the six fastest bowlers analysed, while for the six slowest bowlers the difference was only 0.05 m (5%). They reported that this increase in stride length was caused by the apparent necessity to decelerate in the final stride and was probably associated with the need to 'gather' for the final thrust. Davis and Blanksby (1976b) considered that if the bowler continued to accelerate, insufficient time would be available to transfer from a front-on position in the approach to a more side-on delivery position, resulting in an 'inefficient' delivery technique. These different requirements for side-on and front-on bowlers have implications for the height of the jump, or the bound (Andrew, 1984), in the pre-delivery stride, though no data have been reported in the literature. It may be inefficient and potentially injurious (as it would increase ground reaction forces experienced at back foot strike) to coach a front-on bowler to leap into the air when there is no technical reason for doing so. Delivery stride The MCC (1976) pointed out that at back foot strike, the start of the delivery stride, the weight should be on this foot with the body leaning away from the batsman. Figures 1-3 show the three key events in the delivery stride: back foot strike, front foot strike and ball release. For the purpose of this review, these key events will be outline chronologically. Figure 4 shows the reference system used to define the back foot, hip and shoulder alignment angles. When angles of the back foot, and the alignment angles of the hips and the shoulders, are quoted, they refer to the direction in which the following lines point: from the heel through the centre of the back foot (the midline of the foot); from left to right hip (hip alignment); and from left to right shoulder (shoulder alignment). The angles are calculated by measuring (in an anticlockwise direction) the angle between the 'zero line' and the midline of the foot, and the hip and the shoulder alignments. Figure 4 shows typical shoulder axis angles for side-on (180 ) and front-on (240 ) bowlers. Action classification Many coaching and scientific texts (e.g. MCC, 1976; Tyson, 1976; Elliott and Foster, 1984) suggest that the (a) ' 90' return crease off side direction of ball travel popping crease on side direction of ball travel Figure 4 Plan view showing how hip and shoulder alignments are defined. Typical shoulder alignment of (a) a side-on bowler (180 ) and (b) a front-on bowler (240 ).

6 408 Bartlett et al. position of the back foot at back foot strike is a good indicator of how far the body has turned towards a side-on position. In order to attain a perfect side-on position, the back foot should land parallel to the popping crease (270 ; see Fig. 4) with the hip and the shoulder alignments at 180, as in Fig 4a. In practice, very few bowlers attain this parallel foot position (Davis and Blanksby, 1976b: 4/12 bowlers; Elliott and Foster, 1984: 1/5 bowlers; Elliott et al, 1992: 3/20 bowlers; Stockill and Bartlett, 1992a: 3/17 bowlers). The mean back foot angles in Table 1 support these data, showing that the majority of bowlers tend to have a back foot angle greater than 270, suggesting a more open action than has been traditionally advocated. There are, however, examples of bowlers who attain ultra side-on back foot positions with a back foot angle less than 270. Davis and Blanksby (1976b) reported two bowlers with ultra side-on back foot positions. Elliott and Foster (1984) reported another example of this position, demonstrating an angle of 225. Stockill and Bartlett (1992a) highlighted one bowler with a back foot angle of 210 and Stockill (1994) reported 4 of 24 bowlers with a back foot angle of less than 270. Few data have been reported on the alignment angle of the hips, possibly because of the difficulty of digitizing both sides of the body using two-dimensional cinematography. Two groups of investigators (Stockill and Bartlett, 1992a,b, 1993, 1994; Stockill, 1994; and Burnett et al., 1995) have reported hip alignments from three-dimensional cinematography. The results of the studies of Stockill and Bartlett (1992a) and Burnett et al. (1995) showed very similar hip alignments at back foot strike, the minimum angle, front foot strike and ball release (Table 1). The angle of the shoulders in relation to the wicket provides additional information as to the type of action adopted by the bowler, providing the analyst with a method of classifying bowlers into the side-on, front-on and mixed categories. Foster et al. (1989) were the first to propose such a classification system. This system was limited in that it only viewed shoulder alignment but nevertheless still proved useful, particularly as it reported levels of trunk rotation that increased the potential injury risks to bowlers (see below). Foster et al. (1989) defined a front-on bowler as having a shoulder alignment of greater than 200 at back foot or front foot strikes, and a side-on bowler as having a shoulder alignment of less than 190 between back foot and front foot strikes. Their extensive study involved kinanthropometric, physiological, kinematic, kinetic and computerized tomography data, to investigate the main causes of the high incidence of back injuries in fast bowlers. The kinematic analysis revealed that only 9/82 (11%) bowlers were side-on (shoulder angle < 190 ), the remainder being front-on (n = 56; 68%) or mixed (n = 17; 21%). The mixed action, for the purpose of their analysis, referred to those bowlers who counter-rotated their trunk from a front-on to a more side-on orientation by greater than 40 between back foot and front foot strikes. No significant relationship was found between the mode of delivery (side-on, front-on or mixed) and the ball release speed (as also reported by Stockill and Bartlett, 1993). Elliott etal. (1992) reviewed this classification system and concluded that foot position must also be considered. They suggested that a side-on bowler be referred to as having a shoulder alignment of 190 or less and a back foot angle of 280 or less, while a front-on bowler had a shoulder alignment greater than 280. A mixed action was characterized as having a shoulder alignment of greater than 190, any foot placement and a counter-rotation of the shoulders of greater than 10. This suggestion by Elliott et al. (1992) had a marked effect on how the bowlers were subsequently classified. For example, Stockill and Bartlett (1992b) reported that of 17 senior bowlers, 3 (18%) used a pure side-on action, 11 (64%) used a front-on action and 3 (18%) used a mixed action using the classification system proposed by Foster et al. (1989). However, when the criteria of Elliott et al. (1992) were used, only one of the bowlers analysed by Stockill and Bartlett (1992b) was side-on, one was front-on and the remaining 15 (88%) were mixed. Although this demonstrates some subjectivity in classifying the different types of bowling action, the criteria of Elliott et al. (1992) do take into account more important factors than the earlier classification of Foster et al. (1989). Back foot strike At the start of the delivery stride, the bowler's weight is on the previously planted back foot with the body leaning away from the batsman. This leaning back of the trunk is similar to that observed in some styles of javelin throwing and may serve the purpose of increasing the acceleration path of the implement, as suggested by Bartlett and Best (1988). The degree of trunk lean away from the batsman is a function of the type of action used by each bowler, with the angle being more pronounced in side-on bowlers, as the lean is due to lateral flexion of the spine. In front-on bowlers, the lean results from the more restrictive hyperextension of the spine (Penrose et al., 1976; Elliott et al., 1986). Mason et al. (1989), using dual planar cinematography with 15 junior bowlers (age not stated), reported 10 of trunk lean with respect to the vertical and 10 right lateral flexion at back foot strike, but did not attempt to relate these angles to the adoption of side-on or front-on back foot, hip or shoulder positions. Indeed, Mason et al. (1989) reported that 14 of 15

7 Ball release speed (m s- 1 ) H = 36.4, L= Table 1 Summary of relevant published literature on important kinematic features of the fast bowling technique (mean ± S.D. unless stated) Authors Subjects Davis and 6 low- and 6 Blanksby high-ability (1976a) seniors Penrose et al. 6 international (1976) seniors Elliott and 4 international Foster (1984) seniors Elliott and 1 international Foster (1985) senior Elliott et al. (1986) Foster et al. (1989) 15 elite seniors Shoulder alignment angle ( ) Release Run-up Back foot Change Del. stride Del. stride height speed" angle BFS FFS to length (m or alignment^ (% (m s- 1 ) ( ) BFS6 FFS' BR* BR % height) (m) height) 7.53± ± ± ± ±11 198±9 299± ± representative 4.95± ± ± juniors (age 16.8) Mason et al. 15 medium fast 5.6 (1989) juniors Burden and 10 college 5.95±0.56 Bartlett (1989) players Elliott et al. (1992) Stockill and Bartlett (1992a) 20 representative 5.1 ±0.9 juniors (age 17.9) 17 elite seniors Elliott et al. 24 juniors (age (1993) 13.7) Group Group Burnett et al. (1995) 9 elite seniors % % % % Vert. vel. N-B elbow (m s- 1 )' " At back foot strike; * back foot strike; ' front foot strike; d ball release; * vertical velocity of non-bowling elbow; I + to off-side, - to on-side.

8 410 Bartlett et al. bowlers analysed were side-on, but unfortunately failed to describe the classification of side-on and front-on bowling used. It may be hypothesized that an increase in the acceleration path is less a requirement for the front-on bowler than the side-on bowler, as the former receives a greater contribution to ball release speed from the run-up. Consequently, the front-on bowler may rely less on the contribution from trunk flexion. The back foot strike involves smaller impact forces than those associated with front foot strike, as can be seen from Table 2, where the results of the few reported force-platform studies which have measured back foot forces are summarized. Front foot strike Ground contact forces. As the delivery stride proceeds, the front (left) foot strikes the ground. This event is commonly referred to as front foot contact, or more appropriately, bearing in mind the large resultant ground reaction forces experienced, front foot impact or front foot strike. The values for peak vertical impact force from most force-platform studies (Table 2) are in the region of times body weight with anteriorposterior braking forces of around two times body weight. The exception to this is the much higher mean peak value of nine times body weight occurring 0.01 s after impact reported by Mason et al. (1989). A lack of detailed methodological reporting prevents any evaluation of the differences between these studies. Mason et al. (1989) reported a sampling frequency of 500 Hz, which should be adequate for accurately capturing impact. The natural frequency characteristics of the force platform, the material mounted on the platform surface and other important facts were not reported. The three bowlers in the study of Mason et al. (1989) who produced the greatest vertical ground reaction forces had, before front foot strike, raised a fully extended knee to or above the horizontal. This resulted in a vigorous foot strike made with a fully extended knee. Implications have mostly been drawn from the forces recorded in terms of injury potential (see later), with few relationships to ball release speed or the bowling technique being reported. Saunders and Coleman (1991) found no significant correlations between peak force values and any kinematic parameter they studied, and Elliott and Foster (1984) and Elliott et al. (1992) found no differences between peak forces for different bowling techniques. Stride length and alignment. Different values have been reported for the length of the delivery stride and these are summarized in Table 1. As these various studies analysed groups of subjects of differing age and height, a more objective and accurate comparison can be made between studies by expressing the stride length in relation to the subject's standing height. Such relative figures for stride length were provided by the analyses of Elliott et al. (1986), John (1989), Elliott et al. (1992) and Stockill (1994), who reported mean values of 70%, Table 2 Summary of relevant published literature on ground reaction forces in the delivery stride (in BW means±s.d.) Authors Elliott and Foster (1984) Foster and Elliott (1985) Elliott et al. (1986) Foster et al. (1989) Mason et al. (1989) Saunders and Coleman (1991) Elliott et al. (1992) Elliott et al. (1993) Subjects 4 international seniors 1 international senior 15 elite seniors 82 representative juniors (age 16.8) 15 medium fast juniors 7 medium fast 20 representative juniors (age 17.9) 24 juniors (age 13.7) Group 1 Group 2 Max. vertical force Back foot strike Max. horizontal (braking) force ± ±0.2 Max. vertical force 4.7± ± ± ± Front foot strike Max. horizontal (braking) force 1.7± ± ± ± ±0.7

9 The biomechanics of fast bowling in men's cricket %, 86% and 77% (juniors)/86% (seniors) respectively. Elliott et al. (1992) recommended a delivery stride of approximately 75-85% of the bowler's standing height. The MCC (1976) and Elliott et al. (1986) recognized that the length of the delivery stride is dependent on the speed of approach into the delivery stride and also, the physique of the bowler. Elliott and Foster (1984) also found this to be the case, with the bowler with the slowest approach speed (3.8 m s" 1 ) having the smallest delivery stride (1.34 m) and the one with the fastest approach speed (4.6 m s" 1 ) having the largest delivery stride (1.67 m). Elliott and Foster (1989) warned that bowlers who approach the crease with excessive speed will often have a reduced delivery stride and this 'uncontrolled' approach may inhibit the ability to master a side-on delivery. There are too few data available to substantiate any general conclusion at present. Elliott and Foster (1989) suggested that the back foot, the front foot and the stumps at the batsman's end should form a straight line during the delivery stride. Elliott et al. (1986) reported a mean displacement to the off side of the front foot relative to the back foot of 3.2 cm, reflecting a slight tendency towards a more front-on delivery position of the feet. This was expected, as those bowlers used a front-on delivery. In a more recent study, Elliott et al. (1992) reported a mean displacement to the on side of 10.9 ± 13.3 cm, reflecting a more predominant tendency to a side-on foot alignment. Although the large standard deviation (13.3 cm) suggests a high inter-group variability, the results are still somewhat surprising, considering the extremely low percentage of side-on bowlers (20%). It seems that a number of the front-on and mixed bowlers adopted a non-recommended stride alignment. Elliott et al. (1992) suggested that such a stride alignment is a sound technique for side-on bowlers but not for fronton bowlers, as it would lead to a rotated and hyperextended position of the lumbar spine. The direction in which the front foot points in relation to the wicket has received limited attention in the literature. Tyson (1976) stated that it should point down the wicket or even slightly towards fine leg, but offered no explanation as to why this should be the case. It may be that the direction in which the front foot is pointing as foot strike approaches may determine where it is planted in relation to the back foot, but little empirical evidence is available. Front knee angle. The angle of the front knee during the delivery stride has received much scrutiny over the years. This is true with regard not only to its effect upon ball release speed but also to its role in the attenuation of impact forces, although no force data have been produced to substantiate this latter role. A variety of front knee angles at front foot strike have been identified in the literature. These front knee actions are categorized into three main types. Some bowlers land with a fully or almost fully extended front limb at front foot strike and remain at, or near to, this angle at ball release. This technique (referred to as 'the straight leg' technique) is thought to be advantageous in terms of maximizing ball release speed as it provides a stable lower body which the bowler may use as an effective lever (Elliott et al., 1986). These authors suggested that an angle of greater than 150 would be sufficient to provide these benefits. Burden (1990) suggested that the term 'effective lever' presumably meant that extending the front leg will increase the radial distance between the point of front foot strike and the point of ball release, thus leading to a greater tangential ball release velocity if all other ball release parameters remain unchanged. Davis and Blanksby (1976b) and Elliott and Foster (1984) also recognized the use of the front foot as a fulcrum over which to pivot the partially or fully extended front leg. It should be noted that some of the differences in front knee angle in the literature (see below) may be caused by experimental errors. This applies in particular to two-dimensional measurements of the angle, in which the angle of the knee observed by a side-on camera will not correspond to the true joint angle, because the movement does not occur exactly in the plane perpendicular to the camera axis. Similarly, studies conducted in competition encounter problems with the clothing worn by the players. However effective the straight leg technique is in terms of maximizing ball release, it may be potentially injurious as the joint does not then play an effective role in the attenuation of impact forces. Examples of the straight leg technique include the analyses of two bowlers (Elliott and Foster, 1984) who landed with an almost straight leg (173 ) and then extended to a fully straight position (180 ). Mason et al. (1989) observed that 8 of the 15 junior bowlers they analysed landed and bowled over a straight front leg (no values of knee angle reported), with the remaining 7 flexing to absorb the impact forces. Given the high ground reaction forces experienced on front foot strike, it would appear likely that flexion of the knee on impact would reduce the forces in the skeletal structures of the knee and hip joints (e.g. Nigg, 1983), but might increase the forces in the muscles and tendons. Despite a number of studies investigating this aspect of the skill, there is little conclusive evidence. Elliott and Foster (1989) suggested that it seems desirable that some knee flexion occurs following front foot strike to assist in the absorption of the force when the front foot strikes the ground.

10 412 Bartlett et al. The second type of front knee activity has been observed in a number of bowlers who land with a flexed knee (approximately 150 ) and either maintain this angle, or flex the knee still further following foot strike. In both cases, the knee fails to extend following front foot strike. Knee flexion on impact provides apparent benefits in terms of force attenuation, but the lack of subsequent knee extension fails to provide the beneficial aspects of bowling over a straight front leg. Elliott et al. (1986) reported slightly flexed knee angles, with a group mean (± S.D.) of 168 ± 18 at front foot strike. The knee angles at release (159 ± 29 ) showed that the group as a whole tended to 'collapse'. It is obvious from the standard deviations that there was a large degree of variability in the results; this cannot be expanded upon, as the individual values were not presented. The third type of front knee action involves the knee flexing slightly on landing (thus attenuating the impact forces) and subsequently extending to a near straight or straight front leg, thus providing the benefits of bowling over a straight front leg. Although this technique is considered the optimal front knee activity, it is quite rare in fast bowling. Burden (1990) identified 2 of 9 bowlers and Stockill and Bartlett (1992a) 2 of 17 bowlers who flexed on impact and subsequently extended to a knee angle of greater than 150. Javelin throwing requires similar movement patterns to fast bowling and provides a number of examples of this front knee action. For example, Komi and Mero (1985) analysed 11 Olympic finalists (5 males, 6 females) at the 1984 Games using two-dimensional cinematography and found that knee flexions of 17 and 13 following impact for the men and women respectively, were followed by extensions of 12 and 8 to produce knee angles at release of 153 and 151. Three individuals (notably all males) flexed the knee in excess of 10 and yet still managed a knee angle at release of greater than 170. Maximov (1976) suggested that on front foot strike, if the front leg does not withstand the load (up to kn), then there is a loss of both approach run energy and additional energy created by the active extension of the right leg in the final throwing stride by the time of left knee joint flexion. Burden and Bartlett (1990b) provided evidence of a relationship between ball release speed and front knee angle at ball release, based on the results of a twodimensional analysis of 17 elite senior bowlers. They reported a low but significant correlation (r = 0.41) between ball speed and knee angle. The bowlers who did not flex the front limb after front foot strike released the ball significantly faster (P < 0.02) than those who did. Similarly, Davis and Blanksby (1976b) found that the knee angle for their six fastest bowlers was 25 closer to full extension (180 ) than their six slowest bowlers. In a comparative study between elite fast bowlers and college fast-medium bowlers, Burden and Bartlett (1990a) reported that the greatest difference between the two groups was the behaviour of the front knee between front foot contact and ball release. Their group mean data suggested that the elite bowlers (n = 7) landed with an almost straight front knee (173 ± 3.2 ), flexed the knee by 6.6, then extended again to show an angle of 173 ± 11.2 at release. Four of the seven bowlers did not flex upon impact and three released the ball over a hyperextended front knee. In contrast, all but one of the college bowlers flexed the knee by an average of 31.8 following foot strike with a mean value of 166 ± 4.3. Only four of the nine bowlers were able to extend the knee following such large amounts of flexion, as evidenced by the mean knee angle at release (135 ± 20.2 ). Although the knee angle at release was considered by Burden and Bartlett (1990b) to be a discriminator between the elite and college groups, Stockill (1994) found no such difference between his two groups of 12 International standard junior and senior fast bowlers. There is, therefore, no conclusive agreement at present on the importance of the front knee action for ball release speed. Shoulder and hip orientation. The orientation of the shoulders during the delivery stride is largely dependent upon the type of action (side-on, front-on or mixed) adopted by each bowler. Figure 4 shows the system used for calculating the angle of the back foot, hips and shoulders. The results from previous studies are discussed on the basis of this system of specification. Table 1 includes the orientations of the shoulders at back foot strike, front foot strike and at ball release. The shoulder alignment angles of Elliott et al. (1986, 1992), Stockill and Bartlett (1992a) and Burnett et al. (1995) have suggested that in all cases the bowlers tended to a more front-on shoulder alignment at back foot strike than is recommended. Elliott et al. (1992, 1993) presented data that showed this movement of the shoulders between back foot and front foot strikes to be of prime importance in predisposing the lumbar spine to injury, but made no mention of its relationship to ball release speed. Elliott et al. (1986) considered that the counter-rotation of the shoulders occurred owing to an attempt to remedy earlier flaws in their technique, that is to become more side-on. Maximum shoulder counter-rotation generally occurs after the hips have initiated rotation towards the batsman and hence the prime movers for subsequent rotation, flexion and lateral flexion are placed on stretch. It might be speculated that the stored elastic energy resulting from such a pre-stretch is used in the subsequent movement towards the batsman if the occurrence of the subsequent shoulder rotation, and

11 The biomechanics of fast bowling in men's cricket 413 hence re-use of the stored energy, occurs as soon as possible. The proposed 'lifetime' of this stored energy varies from 14 to 120 ms (Curtin et al, 1974). Bosco et al. (1981) suggested that a relatively long transient period ( > 120 ms) will result in sarcomere slipping and hence a loss of energy. To the authors' knowledge, no data are available for the stretch-shortening cycle of the trunk rotator muscles. The extent to which the shoulders counter-rotate can only be measured accurately if the minimum angle of the shoulders is reported. However, for studies in which this minimum angle was not reported, the angle of the shoulders at front foot strike will provide a guide to the degree of counter-rotation. The four bowlers analysed by Elliott and Foster (1984) only counterrotated by an angle of 3 ± 10 (no minimum angle quoted) and therefore retained their side-on or front-on orientation between back foot and front foot strikes. In stark contrast to these data are the figures for counterrotation angles: from back foot to front foot strike, 14.8 ± 12.7 (Elliott etal, 1986), 16.3 (Foster et al., 1989); from back foot strike to minimum, 180 ± 12.7 (Elliott et al., 1992), 28.2 ± 13.2 (Stockill and Bartlett, 1992a), 31.0 ± 16.0 (Burnett et al., 1995). It is also interesting to note from these results how the counterrotation angle may have been underestimated by quoting the position of the shoulders at front foot strike as opposed to the minimum angle. For the three latest studies, both values are available and show that the mean counter-rotation between back foot strike and front foot strike was 13 (Elliott et al, 1992), 9.2 (Stockill and Bartlett, 1992a) and 10 (Burnett et al, 1995) less than when using the minimum angle. This questions the validity of these specific data from the earlier studies, and suggests that the angles of counterrotation may have been underestimated. Foster et al. (1989) reported that 17 of 82 male junior bowlers (mean age = 16.8 years) counterrotated in excess of 40. Stockill and Bartlett (1992a) found that the counter-rotation (mean ± S.D.) for a group of 17 elite seniors was 28.2 ± 13.2 (range 0-47 ). Of these bowlers, three counter-rotated by more than 40, but three others were in the range This prompted Stockill and Bartlett (1992a) to suggest the inadvisability of stating an absolute figure to classify the mixed action that is deemed applicable to all bowlers. These authors also considered that it may not be appropriate to view the shoulder orientation alone, as the hips may be rotating away from the batsman to the same degree as the shoulders, thus reducing the counter-rotation. The analysis of hip to shoulder separation angles may provide more conclusive and informative results than simply viewing the shoulder angle alone. The degree of shoulder rotation towards the batsman was reported by Elliott and Foster (1984) to be greater for side-on bowlers than front-on bowlers. Elliott et al. (1986) found an angle of shoulder rotation of 83.0 ± 3.1, confirming suggestions that their 15 highperformance (A-grade) fast bowlers were generally front-on (angle of shoulders at back foot strike: 232 ± 17.6 ). The increased amount of shoulder rotation displayed by the bowlers with a side-on delivery style seems to support the views of Penrose et al. (1976) and Elliott and Foster (1984) that the adoption of the sideon technique allows for a more effective summation of segmental velocities than does a more front-on approach. The faster run-up speeds and hence increased contribution to ball speed for the front-on bowlers must however always be considered in this summation. In the vast majority of cinematographical studies of fast bowling, little attention has been paid to the accurate determination of the orientation of the hips. Previous studies have either simply viewed the bowling action as essentially a two-dimensional skill and only used one, laterally positioned camera (Davis and Blanksby, 1976b; Penrose et al., 1976; Burden and Bartlett, 1990b; Burden, 1990), or have added an independent overhead camera to provide information about back foot and shoulder orientation (Elliott and Foster, 1984; Foster and Elliott, 1985; Elliott et al, 1986, 1992, 1993; Foster et al, 1989). Mason et al (1989) used two phase-locked cameras but did not attempt to analyse the bowlers three-dimensionally, and hence produced a limited set of results. The front-on camera provides additional data regarding the stride alignment, hyperextension and lateral flexion, and the overhead camera is useful in providing back foot and shoulder orientations, but neither provides accurate, objective information pertaining to the orientation of the hips. This is essential information if biomechanists are to estimate the true position of the spine at various instances during the delivery stride. Mason et al. (1989) attempted to determine the timing of hip rotation towards the batsman during the delivery stride and concluded that it occurred following back foot strike but before front foot strike when the ball was at its lowest point (when aligned with the right hip). The method of determination of hip rotation occurrence was somewhat rudimentary, as initiation of hip rotation was identified by the first movement of the hips from the front-on perspective of the video stick figure sequence. This further emphasizes the need for accurate, objective and valid three-dimensional analyses. Stockill and Bartlett (1992a) reported hip alignments of 209 ± 15.5 and 225 ± 10.3 and Burnett et al. (1995) reported values of 205 ± 16 and 222 ± 14 at back foot and front foot strikes respectively, clearly

12 414 Bartlett et al. showing that initiation of hip rotation occurred before front foot strike. The exact times of initiation of hip rotation were not reported. Using Pearson product-moment correlations, Stockill and Bartlett (1992a) investigated the relationship between ball release speed and the orientations of the back foot, hips and shoulders along with the degree of counter-rotation. No significant relationships were found, suggesting that the type of action used (fronton, side-on or mixed) was not, in itself, a valid predictor of the speed at which the ball will be released. While further investigation is required into this area, results to date suggest that there is no reason, in terms of performance maximization, why the mixed action should be adopted, especially considering the inherent injury risks (Foster et al, 1989; Elliott et al., 1992, 1993). Non-bowling arm and trunk. Elliott and Foster (1989) stated that the non-bowling arm should be almost vertical and placed such that the bowler can look over the outside of the arm at the batsman before front foot strike for a side-on technique and inside the front arm for a front-on technique. The MCC (1976) recommended the use of the front arm as an aiming device and that at front foot strike the elbow of the front arm should be accelerated into the side to assist the rotation of the bowling limb. This limb then continues to rotate backwards as part of the follow-through. The action of the front, non-bowling arm is essential to a smooth and effective execution of bowling. The rapid adduction and extension of the non-bowling arm, which occurs before and during trunk rotation, also aids in the summation of segmental velocities, especially in the side-on action (Burden, 1990). Davis and Blanksby (1976b) suggested that the non-bowling arm also plays a role in aiding lateral flexion and hyperextension in the coil position (back foot strike). Thus by raising the front arm (elbow) high, the body will automatically lean back to facilitate the movement. Elliott and Foster (1989) considered that the role of the front arm is modified if the bowler adopts a fronton action. Elliott and Foster (1984) showed that the front-on bowlers exhibited smaller non-bowling arm vertical speeds (-2.4 ± 0.1 m s" 1 ) than the side-on group (-3.2 ± 0.0 m s" 1 ). In support of these results, Elliott et al. (1986) showed that the group of highperformance, mainly front-on bowlers exhibited a mean non-bowling arm speed of 2.8 ± 0.8 m s" 1. The importance of correct timing of extension and adduction of the front elbow must also be noted. Tyson (1976) and Elliott and Foster (1989) both emphasized that the arm and the front leg must be thrust down together, which in turn brings about the flexion and rotation of the trunk and rotation of the bowling arm. As stated above, the trunk flexes from its extended position at back foot strike to enable the body to prepare for the rotation of the bowling arm. The role of trunk flexion is not limited to the facilitation of bowling arm rotation, as it also contributes to the rhythm and fluidity of the bowling action. Trunk flexion has also been found to provide a significant contribution to the speed of the ball. Davis and Blanksby (1976b) and Elliott et al. (1986) calculated that trunk flexion contributed 11% and 13% respectively to final ball release speed. Burden and Bartlett (1990a) found differences in trunk kinematics for a group of nine college bowlers compared to a group of seven county and International (C+I) bowlers. Although the trunk angles were similar at back foot strike (C+I, 106 ± 7 ; college, 104 ± 8 ) and front foot strike (C+I, 86 ± 4 ; college, 89 ± 5 ), a difference occurred between front foot strike and ball release. The C+I bowlers exhibited higher maximum trunk angular velocities (529 ± 80 s" 1 ) than the college bowlers (355 ± 59 s~') and were in a more flexed position at ball release (C+I, 49 ± 4 ; college, ). This higher rate of trunk flexion was reflected in the slightly greater difference between maximum linear velocities of the hip joint centre and the seventh cervical vertebra in the elite fast bowlers. Sequence of segmental movements. The sequencing of segment movements has not been widely reported. Davis and Blanksby (1976a) calculated percentage contributions of the run-up, the hips, the shoulders, the bowling arm and the hand in an investigation which used restraints designed to prevent the movement of these segments. This procedure assumes that during bowling the actions of body segments are unaffected by the actions of more proximal and distal segments and hence that each segment relies on muscular activity alone, which is not the case (Burden, 1990). Stockill and Bartlett (1993, 1994) investigated the peak linear speeds of important joints in the kinematic chain from right hip to right (bowling) hand in two groups of 12 International bowlers with significantly different release speeds (juniors, 32.1 ± 1.9 m s" 1 ; seniors, 38.1 ± 1.4 m s" 1 ; P < 0.005; Stockill, 1994). The peak linear speeds were significantly (P < 0.005) greater for the seniors, for whom these peaks, except that for the right hip, occurred at a time significantly (P < 0.01) closer to ball release. When these times were normalized by the duration of the delivery stride, then the temporal differences became non-significant. Peak trunk and upper arm angular velocities failed to discriminate between the groups. More research is needed into segmental contributions to ball release speed,

13 The biomechanics of fast bowling in men's cricket 415 including energy transfers between segments and aspects of segment kinetics. Ball release This section outlines the literature pertinent to the delivery of the ball following front foot strike. The official note accompanying Law 26 (MCC, 1976) states that the ball is deemed to be thrown if either umpire considers that any part of the process of straightening the bowling arm took place during that portion of the delivery swing which directly precedes the ball leaving the hand. This law limits the action of the bowling arm to circumduction of the upper arm about the glenohumeral (shoulder) joint and the extension and flexion of the wrist and finger joints (though it is recognized that the wrist could also abduct and adduct, the radioulnar joints could supinate/pronate and the carpal joints can move). The action of the arm has been reported to contribute 41% (Davis and Blanksby, 1976a) and 50% (Elliott et al., 1986) to the final ball release speed. The circumduction of the upper arm with the elbow either fully extended (elbow angle = 180 ) or at least at a constant angle starts from a position close to the hip joint. Initiation of upper arm circumduction usually occurs between back foot and front foot strikes. The literature suggests that the degree of circumduction between front foot strike and ball release varies and that it is dependent not only on the position of the arm at release, but also on its position as the front foot lands. The position of the arm at front foot strike has been suggested as being a good predictor of ball release speed, with faster bowlers delaying the onset of upper arm circumduction for as long as possible (Tyson, 1976), but there is no scientific evidence to support this. Elliott and Foster (1984) and Elliott et al. (1986) reported mean angles of upper arm rotation with respect to the right horizontal of 73 ± 14 and 87 ±26 respectively, showing shoulder angles of 299 ± 21 and 282 ± 26 (at front foot strike) and 226 ± 5 and 195 ± 25 (at ball release) respectively. Davis and Blanksby (1976b) found that, at release, five of the six fastest and one of the slowest bowlers released the ball with the arm in front of the line of the trunk (158 ). Elliott and Foster (1989) disagreed with this finding and suggested that the arm should be almost vertical at release and the angle between the trunk and the arm approximately 200. Davis and Blanksby (1976b) had argued that if the arm was behind the trunk at release, then an effective summation of segmental velocities would not occur and much of the effective swing of the arm would be negated. This observation is not supported by the majority of findings (Elliott et al., 1986; Burden and Bartlett, 1989; Foster et al., 1989; Burden, 1990), and may have resulted from an inaccurate identification of ball release owing to a relatively slow frame rate of 64 Hz. Using such a low frame rate may have resulted in large changes in angular displacement of the humerus between frames close to release. If angular velocities of the humerus were in the region of 1700 ± 123 s" 1, as reported by Burden (1990) for seven elite senior players, this would have led to an angular rotation of 27 between frames, which represents an obvious source of error. The wrist and fingers are the most distal joints of the body that have been included in previous analyses. They are the last ones to add velocity to the ball and yet the contribution of wrist and finger flexion to the final ball release speed is not clear from the literature. Davis and Blanksby (1976b) reported that the wrist flexed 24 for the six fastest bowlers and 17 for the six slowest in the s before release. The fastest group flexed their wrists at a greater angular velocity during this period, but no angular velocities or percentage contributions were reported. Davis and Blanksby (1976b) suggested that their findings supported the subjective opinion of Lindwall (1957) that the fastest bowling was associated with extending the hand at the wrist as far as possible and then rapidly flexing the fingers and hand just before the release of the ball. Elliott and Foster (1984) reported a flexion of 17.7 ± 8.5 to an extended position (180 ± 8.7 ), whereas Elliott et al. (1986) reported that the wrist flexed by 7.0 ± 19.8 to reach almost full extension of 177 ± 30. The large standard deviations indicate a large degree or inter-subject variability. There is a lack of data relating the degree of wrist and finger flexion to ball release speeds. Burden and Bartlett (1990a) reported similar amounts of wrist flexion, and at similar angular velocities, for elite and college standard bowlers. The authors suggested that finger flexion may have been responsible for the larger discrepancy between right knuckle and ball release speed in the elite group compared with the college players. This prompted Burden (1990) to recommend that the actions of the fingers immediately before ball release should be analysed in detail to discover the effect that the finger movements have on the ball release speed. Davis and Blanksby (1976a) and Elliott et al. (1986) reported percentage contributions to ball release speed by the hand of 5% and 22% respectively. The low percentage contribution reported by Davis and Blanksby may be explained, as above, by the low frame rate used. As suggested earlier, there are inherent problems with using the above method of determining percentage contributions of different segments to ball release speed. However, wrist and finger flexion may play a role, if only minor, in increasing ball release speed. Elliott and Foster (1989) were sceptical of the role of wrist and finger flexion in increasing ball release speed

14 416 Bartlett et al. Table 3 Classification of bowlers by ball release speed (adapted from Abernethy, 1981) Ball release speed (m s" 1 ) > Bowler classification Express Fast Fast-medium Slow-medium Time to travel m (ms) < ^ and considered that wrist movement is not as pronounced as is sometimes recommended in coaching manuals. However, even if the lowest figure (5%) is correct, it may still make the difference between the bowler delivering the ball fast (40.2 m s" 1 ) or express (42.3 m s" 1 ), based on the classification by Abernethy (1981) (see Table 3) Foster et al. (1989) reported that a high release point in relation to the bowler's standing height was significantly related to the occurrence of stress fractures of the lower back, but little has been reported regarding its effect upon ball release speed. The height of ball release relative to standing height ranged from 114% (Elliott et al, 1992) to 116% (Elliott and Foster, 1984) to as high as 118% (Foster and Elliott, 1985). The height of release is likely to be related to the length of the delivery stride, the knee angle at release, and the extent of trunk flexion and lateral flexion, though no results have been reported as to the relationships between these variables. Willis (1978) suggested that it was important to bowl from a high point as it increases the lift off the pitch, but failed to mention how, in terms of technique, a bowler should effect this. Table 1 includes the ball release speed of bowlers analysed in the published studies mentioned in this review. Penrose et al. (1976) reported one bowler's ball release speed to be 44.3 m s" 1. This is similar to the fastest recorded baseball pitch of 44.1 m s" 1 cited in Atwater (1979). As Burden (1990) suggested, because of the unrealistic run-up speeds reported for the same bowler (see above), the figure must be viewed with scepticism. Follow-through Limited data are available on the follow-through, as most analyses stop shortly after ball release. Elliott and Foster (1989) suggested that the bowler should ensure that the bowling arm follows through down the outside of the left thigh (for a right-handed bowler), almost brushing the ground and allowing a gradual reduction in the bowler's speed. Tyson (1976) suggested that the first stride of the follow-through should be behind the line of the ball, before running off the wicket for a further 2-3 strides. The mechanics of swing bowling It is the asymmetrical disposition of the seam of a cricket ball which accounts for the lateral movement of the ball through the air known as 'swing'. Because of the geometrical simplicity of the cricket ball seam in comparison with seams on some other sports balls, the behaviour of the cricket ball is relatively well understood and well researched (e.g. Cooke, 1955; Lyttleton, 1957; Horlock, 1973; Mehta and Wood, 1980; Barton, 1982; Mehta et al, 1983). The asymmetrical disposition of the seam with respect to the relative air velocity (Fig. 5a) causes the seam to trip the boundary layer (the layer of air close to the ball surface) to become turbulent on one side.of the ball (at T), while the boundary layer remains in its undisturbed laminar state on the other hemisphere. The turbulent boundary layer, characterized by high-energy eddies, will separate from the ball surface (at separation point S T in Fig. 5a) further from the front of the ball than will the smooth, low-energy laminar boundary layer (SL). The resulting wake behind the ball is displaced to the side with the laminar boundary layer (Fig. 5a). For this to happen, the ball speed must be sufficiently slow for the boundary layer to remain laminar on one side of the ball. This requires a bowling speed below the critical speed at which the laminar boundary layer becomes turbulent, even on a smooth ball surface (almost 40 m s" 1 ). The speed must be high enough, however, to facilitate the tripping of the boundary layer (probably around m s- 1 ). A second crucial factor is that the ball surface must remain sufficiently smooth so as not to reduce the critical speed below the bowler's release speed. This is not helped by the quarter seams on many cricket balls which can cause boundary layer tripping, especially at the higher end of the range of swing bowling speeds (although this might facilitate reverse swing; see below). A further problem is the progressive roughening of the ball during the game owing to repetitive abrasive impacts with the ground. This is minimized by the fact that, for the ball to swing, the seam must be aligned in a vertical plane, so that the ball-ground impact should generally take place at the seam. This alignment has an additional benefit, as it increases the coefficient of friction between the ball and the ground, thus facilitating movement off the ground (seaming). A humid atmosphere might also help to minimize roughening of the ball, as the surface of the pitch will be more moist. A full explanation of the reported link between balls swinging and humid conditions has not

15 The biomechanics of fast bowling in men's cricket been provided. Moore and Needes (1973), in one of the few investigations of humidity and ball swing, found no systematic effects of humidity. The theory of a condensation shock offered by Binnie (1976) has received no empirical support. A humid atmosphere is less dense than a dry one at the same temperature and pressure and thus is not 'heavy'. This reduced density diminishes rather than aids any swing force. The water vapour in a humid atmosphere might cause the stitches of the seam to swell, although this was not found by Mehta et al. (1983), helping the seam to trip the boundary layer. The greater kinematic viscosity of moist air will lower the Reynolds number of the ball and this might enable laminar flow to be more easily maintained on the smooth side of the ball, although the changes in Reynolds number are small (Mehta et al., 1983). It would appear logical that the lower free (a) (c) velocity vector (direction of ball travel) swing velocity vector lateral lorce.vertical plane through velocity vector lateral lateral force little or no swing wake wake Figure 5 Cricket ball swing: (a) normal swing from above; (b) effect of non-vertical seam as seen from around first slip (about 5-10 to the offside of the direction of ball travel); (c) reverse swing from above. stream turbulence (the turbulence in the general atmosphere beyond the ball's boundary layer) associated with humid days will also help to maintain laminar flow on the smooth side of the ball, but this has been refuted by Mehta (1985). Some balls will swing while others will not, a phenomenon which is probably primarily due to the geometry and surface roughness of the ball. The roughness of two balls may differ, for example, because of quarter seams and manufacturers' logos, and this might affect the swing characteristics. A more important phenomenon might be a non-coincidence between the centre of mass and centre of volume of the ball. This will generate a couple, owing to the buoyancy force on the ball. Although this buoyancy force is small (roughly 0.15% of the ball's weight), the couple will tend to cause the seam of the ball to rotate. If the angle through which it rotates is large enough to move part of the seam to the other side of the vertical plane of the ball's direction of travel, the swing force will be reduced or even negated (Fig. 5b). Nearly all balls have a measurable misalignment of the centres of mass and volume. The magnitude and direction of this misalignment may be important factors in determining whether or not a ball will swing, as may general ball asymmetry. None of these factors, nor the possible stabilizing effects of the backspin naturally imparted to the ball at release, has been systematically investigated. It has been suggested that the phenomenon of late swing is caused by an initially supercritical ball speed slowing to a subcritical speed in flight. This seems unlikely given the high critical speed for a smooth ball (almost 40 m s" 1 ; few bowlers achieve such speeds), a small deceleration of less than 5 m s" 1 during flight, and the reluctance of a turbulent boundary layer to revert to a laminar one. The explanation is simpler, because even if the swing force was constant, this would result in a parabolic swing path with 75% of the deviation in the second half of flight (Daish, 1972). In addition, the swing force depends on the angle of the seam to the direction of travel (as in Fig. 6) and can reach values as high as 0.6 N, about 40% of the weight of the ball (e.g. Mehta et al., 1983). Most good swing bowlers use an initially small seam angle which increases in flight owing to the greater skin friction drag on the seam side of the ball. The swing force will then tend to increase with flight causing even more of the swing to occur late in flight. Reverse swing (Fig. 5c) has been explained by Bown and Mehta (1993) and Lewis (1993). For this effect to occur, the ball must be released above the critical speed for the smooth side of the ball, and this can only be done by the fastest bowlers. The boundary layer then becomes turbulent (at T) on both hemispheres before separation (Fig 5c). On the rough side, the turbulent All

16 418 Bartlett et al. Fast bowling and injury seam angle ( ) Figure 6 Schematic representation of the effect of seam angle on lateral force causing swing. boundary layer thickens more rapidly and then separates (at S T(early) ) earlier than on the smooth side (S T(late) ). This may be facilitated by the quarter seam on the rough side of the ball and, possibly, by ball tampering involving illegal gouging of the surface or lifting of the seam. The result is the reversal of the direction of wake displacement and swing. Lewis (1993) found that of five balls tested, with varying degrees of wear, all ekhibited a reversal of the lateral force at some ball speed. This was normally in excess of 30 m s" 1, although for worn balls it occurred at a lower speed, as the wear of the ball had caused roughening of the ball surface. Tampering with the quarter seam on one side of the ball altered its aerodynamics, but not in a way which would always favour the bowler. In contrast to the research into the aerodynamics of ball swing, there is a dearth of research on the relationship between bowler's technique and the swing of the ball, whether out-swing (away from the batsman) or inswing (towards the batsman). The crucial points, from the above, are for the bowler to release the ball with the seam lying in a stable configuration in a vertical plane, with the seam pointing in the intended direction of swing (opposite for reverse swing) and with the more shiny side appropriately disposed. The good coaching literature (e.g. MCC, 1976) supports these findings and identifies the grip required for the two types of swing. However, recommended techniques such as an exaggerated rotation of the shoulders in a side-on position and a 'stiffer' wrist for out-swing and a less sideon, more open chested position for in-swing are largely anecdotal. Although such features of the technique have been supported and elaborated upon by Foster and Elliott (1989), they have not, to date, been substantiated by any quantified, systematic research into the association between aspects of the bowler's technique and success in bowling in-swing or out-swing. It is widely considered that fast bowling in cricket is one of the non-contact sporting activities which has a very large risk of injury (Fitch, 1989), and that injuries to bowlers (42%) are more frequent than those to fielders (41%) or batsmen (17%) (Stretch, 1993). Fast bowling has been implicated in a multitude of injuries, which include a few to the upper extremity, such as rotator cuff sprains and impingement (Crisp and King, 1994), olecranon bursitis and stress fractures (Payne et al., 1987) and phalangeal stress fractures (Payne et al., 1987). Lower extremity injuries are far more common, and often associated with front foot strike in the delivery stride or overuse (Crisp and King, 1994). These lower extremity injuries include: groin strains (Payne et al., 1987), hamstring strains (Tucker, 1990), wear and tear of the front knee articular cartilage, especially in straight leg bowlers (Crisp and King, 1994), patellar tendinitis, chondromalacia patellae, intrapatellar bursitis and intra-articular osteochondral stress fractures (Payne et al., 1987), compartment syndromes and periostitis (Crisp and King, 1994), tibial stress fractures, shin splints and calf strains (Payne et al., 1987), Achilles tendinitis and adjacent bursitis (Crisp and King, 1994), ankle ligament sprains (Fitch, 1989; Crisp and King, 1994), stress fractures of the calcaneus and talus (Payne et al., 1987) and metatarsals and sesamoids (Fitch, 1989; Crisp and King, 1994), sesamoiditis (Crisp and King, 1994), plantar fasciitis (Fitch, 1989) and chronic bruising of the big toe, its nail and the heel (Crisp and King, 1994). The evidence suggests that the lower back is particularly prone to injury, as reported for example by Foster et al. (1984) and Annear et al. (1992). Many elite fast bowlers have been reported to have had serious lower back injuries, including stress fractures of the third, fourth or fifth lumbar vertebrae (Elliott and Foster, 1984; Foster and Elliott, 1985; Foster et al, 1989). Half of the 12 fast bowlers evaluated by Payne et al. (1987) over a 5 year period had a history of spondylolysis, a cleft in the neural arch of a vertebra at the level of isthmus (Rossi, 1978), a problem also addressed by Fitch (1987). A research team at the University of Western Australia has identified spondylolisthesis, pedicle sclerosis, disc degeneration and bulging as serious and common problems for the fast bowler. Other trunk injuries include 'rib-tip syndrome' to the tenth or eleventh rib on the non-bowling side (Fitch, 1989), quadratus lumborum strains from sidebending (Tucker, 1990), chronic abdominal muscle strains (Payne et al., 1987) and injuries to the facet joints of the lumbar spine and their ligaments (Crisp and King, 1994). Iillee (1977), Elliott et al. (1986) and Foster et al. (1989) suggested that the trend towards a

17 The biomechanics of fast bowling in men's cricket 419 more front-on alignment of the shoulders was a contributory factor to the apparent increase in lower back injuries. More recent work by Hardcastle et al. (1992) and Elliott et al. (1992, 1993) has pointed to the more serious, potentially harmful counter-rotation of the shoulders during the delivery stride, as an important factor in the aetiology of lower back problems. There is strong evidence to suggest that these injuries are not found exclusively in senior players (e.g. Hardcastle, 1993). Foster et al. (1989) studied a group of 82 high-performance young male fast bowlers over one season. They found that 11% sustained a stress fracture of a lumbar vertebra (four of these nine bowlers were later found to be genetically predisposed to such injury) and 27% sustained soft tissue injuries to the back causing them to miss at least one match. To emphasize the dangers of the different modes of bowling, Foster et al. (1989) stated that of the 9 side-on bowlers only one sustained a back injury, of the 15 excessively front-on bowlers 8 sustained some form of back injury, while of the 17 mixed-technique bowlers 6 (35.3%) sustained a stress fracture and 7 (41.2%) sustained a soft tissue injury during the season. The bowlers who were not injured during the season only counter-rotated their trunk by 16 during the delivery stride. Therefore, this study clearly linked a marked rotation of the shoulders (greater than 40 ) during the delivery stride with an increased incidence of lower back injury. Lumbar disc problems were also identified in bowlers as young as years of age by Elliott et al. (1993). Mason et al. (1989) found that 5 of 15 juniors tested had a history of back trouble. Elliott et al. (1992) suggested that a possible reason for the high incidence of injuries was that young athletes were being forced to train longer, harder and earlier in life to excel in their chosen sports, and that the hours of repetitious practice may produce gradual deterioration in specific parts of the body. It is not surprising, then, that physicians are diagnosing an increasing number of overuse injuries among bowlers (Elliott et al., 1992). In addition to the problems of overuse, a number of researchers have also recognized that to bowl at high speed the bowler's spine must flex laterally, extend and rotate all in a short period, while body tissue and footwear must absorb forces which result from impact with the ground. All these factors may predispose the bowler to injury. Bell (1992) added support for the multifactorial nature of the cause of injuries, suggesting that they were a combination of incorrect technique, poor preparation, overuse and clinical features. The study carried out by Elliott et al. (1992) represents one of the most comprehensive investigations of the relationship between fast bowling and injury. The 20 members of the Western Australian fast bowling development squad (mean age 17.9 years) underwent computerized tomography (CT) and magnetic resonance imaging (MRI) scans to detect the presence of bony and intervertebral disc abnormalities. The players were also filmed both laterally (200 Hz) and from directly above (100 Hz) as their front foot impacted a forceplate during the delivery stride of the fast bowling action. Kinetic data were also recorded for the impact of the back foot on a subsequent trial. In order to complete the screening process, the bowlers performed selected physical capacity tests. The occurrences of abnormal radiological data were then used to group the bowlers as follows: Group 1, no abnormal radiological features; Group 2, disc degeneration or bulging; Group 3, spondylolysis (Fig. 7a), spondylolisthesis, or pedicle sclerosis (Fig. 7b). The groups were compared for each of the dependent variables. The authors reported that pars interarticularis and intervertebral disc abnormalities were commonly identified in the sample (55% and 65% respectively) and all players who had experienced back pain showed evidence of a radiological abnormality. This, and the fact that no player with a normal diagnosis complained of pain, suggests that pain is an extremely useful predictor of potentially serious lower back problems. Elliott et al. (1992) further recognized the multifactorial nature of injuries; poor hamstring or lower back flexibility was linked with the occurrence of disc abnormalities, although this may be as a result rather than the cause of the injury. Bowlers who had a high ball release (114% of standing height), and players who had bowled over several seasons during their growth period, were predisposed to a bony abnormality. Finally, bowlers who used a technique that combined a front-on back foot placement and a side-on shoulder alignment (the mixed technique) were more likely to show abnormal radiological features in the lumbar spine. These findings provide the most conclusive evidence that the mixed technique is dangerous and places the spine in an unnecessarily awkward and potentially injurious position at a time (front foot strike) when ground reaction forces are at their greatest (6.4 ± 1.1 times body weight in this study). Elliott et al. (1993), in a study of 24 male fast bowlers who bowled competitively at school and club level in Western Australia, reported that 21% had signs of disc degeneration or bulging (Fig. 8) as revealed by MRI at various levels from Lj-I^ to Lg-Sj. The only statistically significant (P < 0.1) differentiating factor between the normal and abnormal groups was found to be the counter-rotation of the trunk between back and front foot delivery stride impacts. Elliott et al. (1993), based on this and their earlier strides, considered the counter-rotation of the trunk, which characterizes the mixed technique, to be the major predisposing factor to

18 420 Bartlett et al. lumbar spine injury in fast bowlers. It is somewhat surprising that Crisp and King (1994) appeared to implicate the front-on, rather than the mixed, technique as the problem in such injuries. There is no universal agreement about the magnitude of impact forces experienced between the bowler and the ground (Table 2), although the peak vertical forces are, as expected, greater than the horizontal ones and those for front foot strike are greater than for rear foot strike. No study which has measured ground reaction forces has reported a significant correlation between peak forces and lumbar spine defects or different bowling techniques. However, even a low peak vertical force at front foot strike of four to five times body weight, when repeated six times an over for a large number of overs per day and several (sometimes seven) days a week during the season, represents a potential cause of injury (Foster et al., 1989). Even if the loads are within the threshold that can be tolerated by the joints and tissues for a single impact, such repeated loadings can result in damage in the form of an overuse injury (Radin et al., 1979). The high demands on bowlers during the 'off season', either playing on tour or merely practising in indoor nets, may also play a significant role in the aetiology of back problems. (b) Figure 7 Injuries to young fast bowlers: (a) spondylolysis; Figure 8 Disc degeneration and a bulging annulus fibrosus (b) pedicle sclerosis (from Elliott et al, 1992). (from Elliott et al, 1993).

19 The biomechanics of fast bowling in men's cricket 421 As discussed earlier, there seems to be some link between the forces experienced and front leg activity. Mason et al. (1989) found that of the five back-injured bowlers in their study, the three that recorded the greatest vertical ground reaction forces (up to 12.3 times body weight) lifted their fully extended front leg to, or well above, the horizontal. The study also revealed that all but one of these five back-injured bowlers maintained a full extension of the delivery leg. Elliott et al. (1989) found that the knee angle at ball release was significantly more extended for bowlers who sustained a stress fracture of the lower back than for those who reported only minor or no injuries. These findings prompted Elliott et al. to suggest that bowlers who had a more flexed leg at ball release were less likely to be injured than bowlers who released the ball over a more extended front leg. Further support for this was proposed by Mason et al. (1989), who found that the vertical and horizontal ground reaction forces may be reduced to 2.0 and 0.3 times body weight respectively at ball release if the front knee is flexed. Evidence of over-bowling being a contributory factor to injury was provided by Foster et al. (1989), who revealed that of 32 players who bowled in more than the mean number of matches for the group, 19 sustained a lower back injury. Additionally, of the 45 bowlers who bowled in excess of 10 overs in a single spell, 27 reported back pain the following day. In this context, the fast bowling guidelines proposed by Foster et al. (1984) are worthy of implementation despite their somewhat arbitrary nature. Other aspects of lower back injury in fast bowling are less well documented. A number of authors have hypothesized about the relationships between the impact loads and the movements of the spine in the lumbar, region, but without linking the two systematically. No applications of the various lumbar spine models in the literature to fast bowling have yet been reported, although finite element models (e.g. Shirazi- Adl, 1994) appear the most promising. The input data for a fast bowling study using finite element models are lacking at present and it has yet to be established whether measurable differences exist in the vertebral kinematics of the three fast bowling techniques. Elliott et al. (1986) postulated that back injuries may occur because of muscular activity in the lumbar region during the delivery stride, but did not provide any supporting electromyographic evidence. The only EMG study of fast or fast-medium bowling reported to date is that of Burden and Bartlett (1990c), the objectives of which were to determine the sequential and temporal patterns of muscular activity during the fast-medium bowling action and to discover any intra- and interbowler differences between these patterns. This study did not focus on any relationship between muscle activity and tissue loads or injury. Although Foster and John (1989), Fitch (1989) and Crisp and King (1994) all mentioned boots as a factor in foot injuries for fast bowlers, there have been no systematic studies reported of the effects that footwear may have on injury to fast bowlers. The same is true for surfaces, touched on by Fitch (1989) and Crisp and King (1994). This dearth of research is in contrast to the large body of literature on similar effects in other activities, especially running. None of the studies reviewed here has reported the peak rate of change of the external force acting on the bowler (the peak load rate), although it has often been associated with injury (e.g. Nigg, 1986). Conclusions and recommendations for future research There is clear agreement on the importance of ball release speed in fast bowling, but no consensus in the scientific literature on the elements of the bowling technique which contribute most to this. Further research in this area, using frame rates of at least 100 Hz, must be of a three-dimensional nature and use sufficiently large subject groups to allow generalization. While studies during matches appear to have ecological validity, the clothing worn by the players contributes to errors in the reported data. There is also a need for intra-player studies to establish the bowler-specific factors which contribute to fast ball release. Future studies should also report features of body segment dynamics, such as energy transfers between segments and net joint muscle moments using a recommended coordinate convention. All future kinematically based studies also need to address the estimation of experimental errors far more rigorously than most previous studies. There is general agreement that the phenomenon of differential boundary layer separation is the reason for normal and reverse cricket ball swing. While wind tunnel tests to establish the effect of humidity would be of interest, such research would not, biomechanically, be of high priority. There is a need for systematic research to establish the essential features of the bowling technique which contribute to successful swing bowling. This research should also include studies of the behaviour of the ball in games, rather than in wind tunnels. This would allow investigation of the effects of ball asymmetries, such as non-alignment of the centres of mass and volume, and could require a consideration of the adoption of panning (and perhaps tilting) camera

20 422 Bartlett et al. techniques to obtain a sufficiently large image size to obtain accurate data. There is sufficient evidence in the literature to establish a very strong link between injury to the lower back and the use of the mixed technique, even for bowlers in their early teens. Although it is difficult to prove cause and effect in such cases, MRI screening of young bowlers who are potentially at risk should continue and should, wherever ethical and data protection considerations allow, be published in the scientific and coaching literature. Sports scientists and coaches should avoid young bowlers acquiring this technique and should seek to eliminate it when it is found in a bowler. The same collaborators should ensure that the cricket coaching manuals and literature are rewritten to feature the front-on technique as an acceptable alternative to the side-on one, but to caution very strongly against the mixed technique and provide clear guidelines on this. There also needs to be a far more widespread reduction of the number of overs bowled by fast bowlers of all ages, but especially by children. When such intervention strategies as those recommended here are adopted, follow-up research should be conducted to establish their efficacy or otherwise. There is less conclusive evidence to link the use of the straight front leg technique to injury. The association is probably sufficiently strong to recommend a training regimen for fast bowlers to develop enough quadriceps femoris strengdi to be able to flex slightly to cushion the shock of front foot strike and then to extend vigorously to ball release. Further research into the most appropriate ways to do this at various ages is recommended. There is also a need for the cricket equipment industry to carry out, or commission, research into optimum footwear and surfaces for net practice or, at the very least, to seek to adapt footwear and surfaces on the basis of injury-related research from relevant activities such as running. There is a clear need for fundamental research into the loads imposed on the biological structures of the body, especially the lower back, by fast bowling to provide an objective assessment of injury risk. This will require carefully controlled studies, using motion recording techniques in conjunction with accelerometry, electromyography and dynamometry. The data obtained from such studies will then serve as inputs to models of the appropriate body structures. Such models may need to be adapted from existing ones or be developed specifically for this activity. Such research is by no means of a routine nature and will need to bring together groups of scientists with the relevant research skills. 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