Baseball is played across a variety of age ranges and

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1 Differences in the Kinematics of the Baseball Swing between Hitters of Varying Skill BRENDAN INKSTER, ARON MURPHY, ROB BOWER, and MARK WATSFORD Human Performance Laboratory, University of Technology Sydney, AUSTRALIA ABSTRACT INKSTER, B., A. MURPHY, R. BOWER, and M. WATSFORD. Differences in the Kinematics of the Baseball Swing between Hitters of Varying Skill. Med. Sci. Sports Exerc., Vol. 43, No. 6, pp , Purpose: The aim of this study was to determine differences in bat swing kinematics in baseball hitters of varying ability. Methods: Kinematic data for the upper and lower body were collected from 20 trained male baseball players (22.3 T 5.3 yr, 1.82 T 0.07 m, 83.5 T 10.9 kg), using three-dimensional computerized motion-analysis techniques. Participants were ranked before testing based on a novel coach s rating scale and seasonal batting average. They were subsequently separated into a relatively high-caliber group of hitters (n = 10) and a relatively low-caliber group of hitters (n = 10) for comparison. Importantly, the two groups were significantly different in terms of coach s rating (P G 0.01) and batting average (P G 0.05). Results: The results showed a significant difference in maximum bat swing velocity (P G 0.05) with high-caliber hitters having a higher velocity (36.8 mis j1 ) in comparison with relatively low-caliber hitters (33.8 mis j1 ). Lead elbow maximum angular velocity was significantly higher (35.9%) among relatively high-caliber hitters (P G 0.05). Angular velocity of the hip segment approached significance between the groups (P = 0.056). High-caliber hitters also had a right knee angle of 106- at ball contact, which was significantly (P G 0.05) higher than that of relatively low-caliber hitters (100-). There were no between-group differences for wrist and linear hip joint velocities at ball contact. Conclusions: It was established that bat swing velocity is a key characteristic of the baseball swing when identifying skill level and performance between hitters. In addition, high-caliber hitters display greater lead elbow maximum angular velocity possibly because of achieving a higher angular hip segment velocity early in the swing. It is noted that although these attributes differentiate hitters of varying skill level, future research should examine whether developing these characteristics in players of lower ability improves batting performance. Key Words: BATTING, BIOMECHANICS, VELOCITY, PERFORMANCE Baseball is played across a variety of age ranges and leagues by athletes of varying ability, which leads to a wide range of skill level throughout the game. Such fluctuations in skill can occur even within a specific league. One key aspect of successful coaching is the capacity to identify swing kinematics that are critical to superior performance, with the intent of improving them through feedback and training. Swing characteristics of elite level hitters have been previously documented (2,12,17). Race (12) reported that professional batters displayed clear evidence of linear hip velocity (2.42 mis j1 average over a 90- arc) along with definitive wrist action (4.89 mis j1 average over a 90- arc). In addition, Breen (2) suggested that an elite hitter s leading forearm tends to straighten immediately during the swing resulting in higher bat speed. Welch et al. (17) proposed that sequential Address for correspondence: Brendan Inkster, B.(H.M.S.), Human Performance Laboratory, University of Technology Sydney, P.O. Box 222, Lindfield NSW 2070, Australia; brendan.inkster@uts.edu.au. Submitted for publication March Accepted for publication October /11/ /0 MEDICINE & SCIENCE IN SPORTS & EXERCISE Ò Copyright Ó 2011 by the American College of Sports Medicine DOI: /MSS.0b013e a acceleration and deceleration of proximal and distal segments allows for higher linear and rotational velocities that enable a successful swing. Recent research by Escamilla et al. (5) compared the swing kinematics between youth (14.7 yr old) and adult (22.2 yr old) hitters. The results showed that peak lead elbow extension angular velocity and bat linear velocity at bat/ ball contact discriminated between the hitters of varying age and performance. Although the study by Escamilla et al. (5) was the first to compare the swing between two populations, the authors did note that differences might have been attributed to body mass, body height, and bat mass and length. Despite the popularity of baseball and the importance of hitting in the game, sparse research has been directed toward a biomechanical understanding of the baseball swing, particularly between hitters of varying skill levels. Therefore, the purpose of the current study was to identify swing kinematics that differed between relatively high-caliber and relatively low-caliber hitters within a specific, well-trained baseball population. METHODS Data collection. Twenty subelite baseball players participatedinthisstudy(22.3t5.3 yr, 1.82 T 0.07 m, 83.5 T 10.9 kg). The sample s playing experience was, on average, 1050

2 12.3 T 5.3 yr. All subjects were right-handed hitters, currently playing first or second gradeinanaustralianmajor League competition. Before testing, all participants were rated based on two criteria. The first was the qualitative ratings of three qualified coaches (Level 2 issued by the Australian Baseball Federation, approved by the Australian Sporting Commission) who observed their technique and subsequently scored the subjects based on bat ball contact, swing power, and hitting ability. The second considered the seasonal batting averages of each player. These data in combination were used to distinguish relatively high-caliber and low-caliber hitters within the group. Using these criteria, it transpired that the high-caliber group consisted of players solely from first grade, whereas the lower-caliber group consisted of second-grade players. Institutional approval of the protocol and written informed consent from each participant were obtained before data collection. Participants were required to attend testing on one day for a period of approximately 60 min. The testing protocol was explained to each participant, and they were then encouraged to ask questions if they did not understand any of the procedures. Participants had their height recorded on a stadiometer (Holtain, Crosswell, United Kingdom) and their body mass recorded using a set of electronic, calibrated scales (A & D Mercury, Adelaide, Australia). Age, body mass and height, coaches rating, and batting average are shown in Table 1. Before data collection, a system of 16 reflective markers was placed on the participant, bat, and ball. The markers were firmly attached to each subject using rigid sports strapping tape (Beiersdorf, North Ryde, Australia). To assist in marker application, subjects were instructed to wear tight-fitting clothing. Markers for the bat were attached using double-sided adhesive tape (3M, St. Paul, MN). The ball was partially covered in reflective tape (Reflexite, Darra, Australia) and represented one marker. A spatial model was created for three-dimensional kinematic analysis, using similar locations to those of previous research (5,14,17). Marker placement included the bat end, bat knob, ball, proximal midline of the subject s forehead, and both sides of the body for the coracoid process, lateral aspect of the head of the radius, distal radioulnar joint, iliac crest, proximal superior patella, and posterior aspect of the calcaneus. The bat used for this study was made from white ash (Phoenix Bat Company, Columbus, OH) and had a mass of kg and a length of m. After the application of markers, each participant undertook a standardized warm-up routine. A batting tee (Ez Tee; PIK Products, Norwalk, CT) was placed in a hitting area and TABLE 1. Participant anthropometric and batting characteristics (mean T SD). High Caliber (n = 10) Low Caliber (n = 10) Age (yr) 22.3 T T 3.5 Mass (kg) 83.8 T T 9.9 Height (cm) T T 6.6 Coaches rating (/75) 40.9 T 8.0* 19.8 T 5.1 Batting average (/1.000) T 0.031** T * Significantly different from low-caliber hitters (P G 0.01). * adjusted to the participant s preferred height. The average tee height for all subjects was 40% T 2% of body height. Subjects were then instructed to hit 10 baseballs (A1010S; Wilson, Chicago, IL) off the tee and into a net with the intent of hitting a line drive up the middle. A 45-s rest period was allowed between each swing. Criteria for determining a successful swing included verbal feedback from the participant, clean solid contact, and flight path of the ball. Variables were processed for the subject s best five swings with each bat. These swings were determined by a feedback rating given by the subject after each swing. Where two swings were ranked evenly, the swing with a higher resultant ball speed was used. During data collection, the motion of the reflective markers in space was simultaneously captured by four infrared cameras at a rate of 240 Hz (Qualysis AB, Gothenburg, Sweden). Each camera was mounted on a tripod set at a height of approximately 2.6 m and placed around the batting tee. These heights were determined from pilot studies, which maximized the field of view and allow all markers to be detected during the swing. The cameras were calibrated using a calibration frame and wand before each period of testing. They were positioned to include the trajectory of the bat through a normal swing as well as the batted baseball for a distance of approximately 3 m (7). The hitter was instructed to swing in response to a computer-generated auditory signal, which automatically triggered a 2-s data collection period. The three-dimensional position of each marker was collected by each of the four cameras and mathematically processed using digitizing software (Qualysis AB). The paths of the bat and ball were defined using the automated algorithms and user interaction in Qualysis motion capture software. A single data file containing three-dimensional coordinates of all markers for each swing was generated and exported from Qualysis. The Z direction was defined as vertically upward. The X direction was defined as a vector pointing from the tee to the net, perpendicular to the Z direction. The Y direction was defined as the cross-product of Z and X and referred to any lateral displacement of the intended ball path. To ensure accuracy of measurement in the current study, technical error of measurement (TEM) and interclass correlation (ICC) were calculated for both angle and distance. Angle TEM was less than 1% with an ICC of 1.00, and distance TEM was 1.09% with an ICC of Data analysis. Data files were processed using QTools (Innovision, Inc., Warren, MI) to calculate bat swing kinematics. To smooth the raw data for each digitized point, data were filtered using a fourth-order, zero phase shift, lowpass Butterworth digital filter with a cutoff frequency of 13.3 Hz (4,17). Swing initiation was determined as the instance bat swing velocity reached 1 mis j1 and continued to increase rapidly within the proceeding five frames. Bat ball contact was defined as the frame before ball contact. Swing duration was the total time between swing initiation and bat ball contact. Maximum angles and velocities presented were taken from KINEMATICS OF THE BASEBALL SWING Medicine & Science in Sports & Exercise d 1051

3 TABLE 2. Bat and ball swing kinematics for high- and low-caliber baseball hitters (mean T SD). Bat Maximum linear velocity (mis j1 ) 36.8 T 3.1* 33.8 T 2.1 Swing duration (s) T T 0.05 Ball Maximum linear velocity (mis j1 ) 38.0 T 2.4* 35.6 T 1.4 this period. Bat and ball velocities presented in the current study are the resultant linear velocities. Knee and elbow velocities are angular velocities. The lead elbow angle was defined as the intersection of the shoulder to the elbow vector and elbow to the wrist vector. The knee angle was defined as the intersection of the hip to the knee and knee to the ankle vectors. Elbow and knee angles were defined as 180- when full extension was achieved. Linear velocity of the hips and rotational (angular) velocity of the segment joining the hips was calculated. The rotation of the hips was determined from angles projected on the transverse plane for the segment joining the left and right hip markers. Linear translation of the hip markers was assessed as the maximum displacement of the left and right hip markers in the X direction. Head movement was noted as the head marker s maximum change in Z direction throughout the swing duration. Stride length was calculated by taking the difference between the left ankle marker X position before the foot leaving the ground and the left ankle marker X position when the foot returned to the ground and presented as a percentage of hip width. Hip width was the distance between the two hip markers. Lead foot height was defined as the distance between the left ankle marker Z position when the foot was in contact with the ground and the maximum Z position when the foot was in the air. Before statistical analysis, subjects were ranked according to their combined coaching and batting score and split into two even groups. The 10 highest ranked hitters were labeled the high-caliber group and the 10 lowest ranked hitters were labeled the low-caliber group. The terms high-caliber and low-caliber are relative terms and only describe a group s hitting performance (coaching and batting average rank) relative to the other group. The use of these subjective and objective ranking criteria ensured the formation of two groups of distinctly different hitting ability. To assess whether the biomechanics of the swing differed between relatively high-caliber and low-caliber hitters, all data were entered into SPSS (Version 17; Chicago, IL) for statistical analysis. Initial analysis involved the calculation of descriptive statistics for all variables. Each set of data was tested for normality using the Shapiro Wilk test and for homogeneity of variance using the Levene test. Because the data were normally distributed, differences in the biomechanics of the swing between groups were assessed using one-way ANOVA. A significance level of 0.05 was set a priori. Bonferroni corrections were considered for the current study but were not used. This was because of the ongoing debate as to their usefulness in producing accurate differences (1,11). The argument against the use of Bonferroni corrections suggests that the procedure is overly stringent, and while reducing the chance of making a type I error, the chances of producing a type II error are increased, thus increasing the possibility of incorrectly determining that no difference exists. RESULTS The anthropometric and coaching scores for each group are presented in Table 1. The data clearly show no differences in age, mass, or height between each group. However, there was a significant difference between groups in both the coaches rating and the batting average. Thus, we were confident that each group consisted of players with a significantly different level of batting skill. Bat swing and ball kinematics are shown in Table 2. The high-caliber group displayed significantly higher maximum bat swing velocity (9%) and batted ball velocity (7%) in comparison with the low-caliber group. No further significant differences were found for swing duration. Upper-body swing kinematics are presented in Table 3. The high-caliber group produced greater lead elbow maximum angular velocity (36%) when compared with the lowcaliber group. No significant differences in any of the other upper-body variables were found. Lower-body swing kinematics are shown in Table 4. Compared with low-caliber hitters, right knee angle at contact in high-caliber hitters was significantly greater (106- vs 100-). Angular velocity of the hip segment between groups approached significance (P = 0.056). There were no significant differences in normalized stride length or lead foot height off the ground between high-caliber and lowcaliber hitters. DISCUSSION Previous researchers have noted that bat swing velocity is of great importance for baseball hitters (3,8,10,13,14,16). A greater bat speed will allow the batter to see the pitch for longer (15) and produce greater resultant ball velocity (6,9). The current study has shown that high-caliber hitters TABLE 3. Upper body kinematics for high- and low-caliber baseball hitters (mean T SD). Head Vertical head movement down (m) T T 0.09 Left elbow Maximum angle (-) 149 T T 12 Angle at contact (-) 148 T T 14 Maximum angular velocity (-Is j1 ) 991 T 230* 729 T 248 Time from maximum angular velocity T T to ball contact (s) Right elbow Maximum angle (-) 132 T T 11 Angle at contact (-) 131 T T 11 Maximum angular velocity (-Is j1 ) 1907 T T 330 The left side of the body was the lead side for all participants Official Journal of the American College of Sports Medicine

4 TABLE 4. Lower body kinematics for high- and low-caliber baseball hitters (mean T SD). Left knee Maximum angle (-) 136 T T 10 Angle at contact (-) 136 T T 9 Maximum angular velocity (-Is j1 ) 386 T T 150 Right knee Maximum angle (-) 115 T T 7 Angle at contact (-) 106 T 4* 100 T 5 Maximum angular velocity (-Is j1 ) 474 T T 368 Hip segment rotational velocity Maximum velocity (-Is j1 ) T 72.4** T 57.2 Time from maximum to ball contact (s) T T Left side of the hip Linear velocity at contact (mis j1 ) 1.29 T T 0.72 Maximum linear velocity (mis j1 ) 4.98 T T 1.5 Linear displacement in direction of ball 0.16 T T 0.02 travel (m) Right side of the hip Linear velocity at contact (mis j1 ) 1.94 T T 0.57 Maximum linear velocity (mis j1 ) 6.19 T T 2.0 Linear displacement in direction of ball 0.26 T T 0.08 travel (m) Stride Stride length as a percentage of hip T T 14.2 width (%) Lead foot height as a percentage of subject height (%) 7.22 T T 3.5 ** Approached statistical significance when compared with low-caliber hitters (P = 0.056). The left side of the body was the lead side for all participants. produce higher maximum bat swing velocities compared with relatively low-caliber hitters, thus supporting the notion that bat speed is a characteristic that is crucial to the baseball swing. Furthermore, the resultant ball velocity was significantly higher among high-caliber hitters, resulting in part from the increased bat swing velocity. The higher ball velocity has the potential to make fielding more difficult, thus increasing the batter s chance of reaching base. Sergo and Boatwright (13) and DeRenne et al. (3) reported that bat swing velocity can be increased through weighted implement training, although the specific kinematic mechanisms that lead to such change were not reported. Further research should use similar training regimens, with additional examination of the kinematic changes within each hitter that occur during training. It has been suggested that if front arm extension occurs immediately after swing initiation, it will allow for greater bat swing velocity (2). This is due to the bat end moving immediately in response to the sudden straightening of the arms, presumably enabling greater transfer of momentum via summation of speed. Poorer hitters tend to pull the bat first with a relatively bent elbow, and this inhibits the speed of the bat end by reducing movement derived from rotation (2). The results of the current study showed that highcaliber hitters produced higher maximum lead elbow angular velocity during the swing (991-Is j1 vs 729-Is j1 ). Breen (2) stated that a higher angular velocity of the front arm would lead to the hitter attaining maximum front arm extension earlier in the swing. However, further analysis of our data showed that the spatial characteristics of peak angular velocity in the lead elbow did not differ between the two groups. More recently, Escamilla et al. (5) showed differences in lead elbow extension angular velocity between youth and adult hitters, with adult hitters achieving higher velocities (752-Is j1 vs 598-Is j1 ). It is plausible in this research that differences between the adult and youth hitters may have been due to variance in strength, power, and possibly technique. In the current study, muscular strength was not assessed; however, both groups were of similar body mass and height (Table 1). Therefore, it is postulated that the difference in the maximal angular velocity of the lead elbow may be due to technique differences between the groups, such as the capacity to develop a high angular velocity of the hip segment early in the swing. The importance of hip velocity in developing bat swing velocity is discussed in detail below. Supporting this reasoning, further analysis of the current data showed a significant correlation between maximum bat swing velocity and lead elbow maximum angular velocity (r = 0.5), which indicates that this facet accounts for 25% of the variance of maximum bat swing velocity. Collectively, the results of the current and previous research indicate that the characteristics of the lead elbow portrayed in baseball hitting are crucial to the generation of high bat swing velocity and successful hitting. Future research should identify whether various training regimens, in particular those focusing on lead arm kinematics, are able to improve a hitter s bat swing velocity and batting performance. This research should also examine what other factors make up the remaining 75% of the variance in maximum bat swing velocity. Biomechanical data presented by Welch et al. (17) revealed that during bat ball contact, the rear leg of professional hitters was 135- and loaded with approximately 16% of body weight. In the current study, high-caliber hitters had a larger knee angle compared with low-caliber hitters, indicating a straighter leg at bat contact (106- vs 100-). The preceding actions of the rear leg, before bat ball contact, seem important to the success of the overall swing. It is hypothesized that the straighter back leg at ball contact reflects the ability of a hitter to transfer momentum from the lower body effectively. Specifically, a straighter back leg may lead to increased hip rotation, thus permitting a greater force production through the trunk, arms, and bat, potentially leading to a higher bat swing velocity. Early cinematographic analysis of the baseball swing in professional hitters indicated that hip linear velocity was a key characteristic to effective hitting (12). However, in the current study, there was no difference in left or right linear hip velocity between the high- and low-caliber hitters. Furthermore, the translation of the hips (in the X direction) was statistically equivalent between groups (Table 4). In contrast, the 7% difference in angular hip segment velocity between groups in the current study approached significance (P = 0.056; Table 4). These findings suggest that highercaliber hitters generate a larger angular velocity at the hip, which presumably enables a higher bat swing velocity. Thus, we postulate that it is the magnitude of the angular hip segment velocity, and not linear hip velocity, that is a requirement for a high bat swing velocity. Indeed, in contrast to the peak KINEMATICS OF THE BASEBALL SWING Medicine & Science in Sports & Exercise d 1053

5 angular velocity data, there were no differences in the timing (relative to bat contact) of either maximum elbow or hip segment angular velocity (Tables 3 and 4), indicating that the spatial characteristics of the swing were similar between groups. Welch et al. (17) reported that hip rotation is an important component of the swing, allowing for the development of trunk preload such that the musculature of the trunk and upper extremity can contribute effectively to bat swing velocity. No differences were found between levels of hitters for swing duration and head movement. Escamilla et al. (5) reported significant differences in swing duration between hitters of varying age, with the stride phase providing the greatest effect on total swing time. The authors did note that the maturation of the older hitters may have resulted in size and strength advantages and thus were able to generate greater velocities. It is noted that a further breakdown (into phases) of the current study s swing may have found differences between high-caliber and low-caliber hitters; however, such an analysis was beyond the scope of this research. Race (12) reported that the movements of the head are vital to effective hitting. Specifically, among 17 elite hitters, only one raised his head before contact, whereas the others lowered theirs. The current study found no differences in head movement between hitters of varying skill (Table 3), although it should be noted that all hitters tended to lower their head before contact, supporting the notion of Race (12). The current study used subelite baseball players and used a novel coach s rating method to distinguish between relatively high-caliber and low-caliber hitters. However, for the purposes of further investigation into the kinematic differences between hitters of varying skill and the determination of the characteristics of relatively high-caliber hitting, it REFERENCES 1. Bland J, Altman D. Multiple significance tests: the Bonferroni method. BMJ. 1995;310(6973): Breen J. What makes a good hitter? J Health Phys Educ Recreat. 1967;38: DeRenne C, Buxton B, Hetzler R, Ho K. Effects of weighted bat implement training on bat swing velocity. J Strength Cond Res. 1995;9(4): Dun S, Fleisig GS, Loftice J, Kingsley D, Andrews JR. The relationship between age and baseball pitching kinematics in professional baseball pitchers. J Biomech. 2007;40(2): Escamilla RF, Fleisig GS, DeRenne C, et al. A comparison of age level on baseball hitting kinematics. J Appl Biomech. 2009;25(3): Fleisig G, Zheng N, Stodden D, Andrews J. Relationship between bat mass properties and bat velocity. Sports Eng. 2002;5: Greenwald R, Penna L, Crisco J. Differences in batted ball speed with wood and aluminium baseball bats: a batting cage study. JAppl Biomech. 2001;17: Hughes S, Lyons B, Mayo J. Effect of grip strength and grip strengthening exercises on instantaneous bat velocity of collegiate baseball players. J Strength Cond Res. 2004;18(2): Lund R, Heefner D. Training the baseball hitter. J Phys Educ Recreat Dance. 2005;76(3): is recommended that two different populations (preferably elite and subelite hitters) are used with a greater number of subjects. Thus, the current study s results are limited to male subelite hitters and the current spatial model that was selected for analysis. Moreover, the subjects in this study hit the ball from a hitting tee and were instructed to hit a line drive. Therefore, it must be noted that swing kinematics may be different when swinging against live pitching and/or trying to produce different swing outcomes such as a fly ball or ground ball. In conclusion, for the selected kinematic variables in the current study, both similarities and differences occurred between high-caliber and low-caliber hitters. Linear parameters such as swing duration, head movement, stride length, and hip velocity were all similar. In contrast, differences were found between groups for maximum bat swing velocity, ball velocity, lead elbow and hip segment maximum angular velocity, and right knee angle at ball contact. Identifying bat swing characteristics crucial to successful hitting is essential and should be encouraged in future research, as it will allow both biomechanists and coaches to enhance baseball training methods to improve the baseball swing and further help in the discrimination between elite and subelite hitters. It is suggested that future research also incorporate more sophisticated analysis of segment coordination and other variables such as range of motion and vision. 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