PLAYER/GROUND INTERACTION ON ARTIFICIAL TURF.

Transcription

1 PLAYER/GROUND INTERACTION ON ARTIFICIAL TURF Steven Blackburn, Philippe Brachet, A.C. Nicol and C. Walker 2 Bioengineering Unit, University of Strathclyde, Glasgow, Scotland. 2 Department of Mechanical Engineering, University of Strathclyde, Glasgow, Scotland. steve.blackburn@strath.ac.uk INTRODUCTION The use of artificial sports surfaces has increased dramatically in the past thirty years due to its greater versatility, durability and decreased maintenance costs compared to natural turf. However, the increased public participation of sports such as football, rugby and hockey has lead to the over-usage of the natural turf pitches on existing sports grounds. Consequently, there is a drive for the use of artificial pitches to allow wider access to the public and which are independent of weather conditions. The introduction of artificial pitches into major sports such as American football and hockey has lead to changes in the style of play and altering the biomechanical interactions between players and the surface (Nigg and Yeadon, 987). There are conflicting reports in the literature regarding the effect of playing on artificial surfaces on injury rates. Many of the studies relating to injuries on different surfaces are difficult to compare due to methodological deficiencies and inconsistencies in the definition of injuries and injury rates. The level of friction between the athlete and the surface is a factor associated with acute, non-contact injuries (Dixon et al., 999). The introduction of artificial turf has been the suspected cause for an increase in foot-fixation injuries due to the high friction coefficient of some artificial turfs. Such injuries include anterior cruciate ligament (ACL) damage, and turf-toe : an acute injury of the first metarsophalangeal joint caused by forceful dorsiflexion of the toe as a result of increased friction between the shoe and surface. During a ten year study, Powell et al. (992) reported higher rates of ACL injuries in American Football players competing on Astroturf artificial turf compared to natural turf. In a study of American football players, 83% of those players where it could be determined on which surface an injury occurred, reported initial turf toe injury on an artificial surface (Rodeo et al., 99). In contrast, Renstrom et al. (977) found no differences in injury frequencies between playing on an artificial football surface and natural grass. In a 26-year retrospective study of injuries in players of an American football team, Nicholas et al. (988) also found no significant differences in rates of major injuries per game on either natural or artificial surfaces. Contact and non-contact injuries were, however, not differentiated in this study. The use of artificial surfaces in sports is set to increase dramatically, especially with the development of a new generation of artificial turf. FieldTurf and other third generation longer pile, rubber infill artificial turfs have been marketed as a surface that duplicates the playing conditions of real turf. The governing body of European football has recently published guidelines that may soon lead to the employment of artificial turf in professional matches (UEFA, 22). Current turf testing methods utilise mechanical rigs, which apply forces that are not representative of the actual player/ground interaction (Nigg, 99). Previous research regarding subject biomechanical loadings of artificial turf for a variety of different sporting movements is sparse and has generally been focussed on the vertical ground reaction force (GRF). (Dixon et al., 999) described a typical peak vertical GRF of 4-N for heel-toe running. Valiant (987) assessed ground reactions forces (GRF) and moments measured on Astroturf by eight athletes performing straight line running, a 9 cut turn and a 8 pivot. Mean peak vertical GRF, expressed in multiples of bodyweight (BW) were 2.46BW, 3.4BW and 3.2BW for the run, 9 cut and pivot, respectively. Shear forces during the running movement were.6bw anteroposterior (A-P) and.66bw mediolateral (M-L). For the 9 cut: -.BW (A- P),.8BW (M-L). For the 8 pivot: -.46BW (A-P),.6BW (M-L). The mean free moment (the torque developed to resist rotation during turning) was 7.2Nm for the 8 pivot movement. A cutting movement is used in team sports to produce a rapid change in running direction to get past opposition players or to catch a ball (Bencke et al., 2). Peak vertical GRF during this cutting movement have been measured to be 8-2N (Simonsen et al., 2) and 2.88 (±.74)BW (Bencke et al., 2). (Rand and Ohtsuki, 2) described the techniques used by the athletes performing turns during running. An open technique involves the athlete using the foot on the opposite side to the direction he/she wants to turn. A cross technique involves the use of the foot on the same side as the direction he/she wants to turn. The contralateral leg then crosses the plane in which the run up occurred and continues in the new direction. The former technique is recognised to be more efficient when a rapid change of direction is required. The open technique causes less speed reduction and fewer steps to complete the turn. In contrast, the greater speed reduction involved in the cross technique may allow a tighter turn to be achieved. This study examined the ground reaction forces (GRFs) and moment of rotations produced during running and turning activities on FieldTurf and two other commonly used artificial turfs to provide a better understanding of the interaction between the player and the artificial surface. This is part of a larger study aiming to produce a biomechanically valid rig for testing artificial turf that employs the relevant forces applied by the athlete on the turf during sporting movements.

2 METHODS Eight subjects (7 male, female: 2 footballers, 2 hockey players, 3 rugby players; mean age: 26.6±7 years; mean mass: 8.8±2kg; mean heights: 77.±3cm) performed 8 types of running movement on three types of artificial turf. Turf (T) was a short pile, sand infill turf with rubber underlay. This type of turf is commonly used on community -a-side football pitches. Turf 2 (T2) was a medium pile, sand dressed turf with rubber underlay, which is commonly used for synthetic hockey pitches. Turf 3 (T3) was FieldTurf, a long pile turf with sand/rubber infill. The participants recruited for this study have performed at the professional level or a high standard of competition in each of their respective sports. A Kistler 982B force plate (6mm 4mm) was used to measure the loading actions of the players performing the different activities. The laboratory was laid out with each turf to allow at least a 7m run-up from each direction to the force-plate and 7m run-off area to allow sufficient distance for the subject to slow down. Each turf was installed to manufacturers specifications. A cut-out piece of turf (and rubber underlay, if required), equal to the dimensions of the force plate was attached to the force plate with strong double-sided carpet tape. This was done to prevent any force transference along the turf, which could lead to inaccuracies in the measurement of the GRFs. Unless there was close inspection of the turf, it was difficult to identify the cut-out piece of turf and hence the force plate. A digital video camera (DVC) was placed at one end of the lab to provide a visual reference of the movement performed. sprinting along the runway, which was approximately 4m long. The subjects were instructed not to slow down until they had passed a designated marker, positioned approximately 4m past the force plate. The running-stop (RUNSTOP) movement involved the subject sprinting 7m towards to the force plate. The subject was instructed to stop on their left leg at a position level with a marker located adjacent to the forceplate (Fig. 2). A trial was defined as successful if the subject was able to stop their forward momentum within two steps of the stopping action and if the researcher assessed that the action on the left leg was the main stopping action and that the left foot landed fully on the force plate. For the turning activities, it was necessary to place markers on the turf in order to designate the position that the subjects were to perform the respective turns. This allowed the stance foot, on which the turn took place, to impact the force plate. The 4 turns involved running towards the markers and performing the turn using an open technique on the left foot and running off 4 to the right (4R) (Fig. 3). The turn was repeated using the right foot to turn and running off 4 to the left (4L). The 9 turn involved the subject running towards the markers and perform the turn using the cross technique on the left foot and running off 9 to the left (9L) (Fig. 4). The process was repeated using the right foot to turn and running off 9 to the right (9R). The 8 turn (8T) involved the subject starting from an upright standing position with their left foot positioned on the force plate (Fig. ). On command, the subject was asked to turn to their right (medial rotation of the left foot) through 8 and sprint off in a forward direction. The players wore their own preferred footwear and were instructed to perform each running movement as fast as possible. Three trials of each type of movement were performed on each of the three artificial turfs. The three turfs were tested on different days. (a) Vicon console DVC Force Plate Figure 2: RUNSTOP Figure 3: 4R Figure 4: 9L (b) Figure (a): Lab with turf laid out. (b): Schematic of lab set up The movements performed were walking, sprinting, running-stop, 4 turns (left and right), 9 turns (left and right) and 8 turns. The starting position of the subjects for all movements, with the exception of the 8 turn, was from a standing start, approximately 7m from the force plate. The sprinting (RUN) movement involved the subject Figure : 8T

3 RESULTS Figure 6 shows typical GRFs and moment of rotations measured as a percentage of the stance phase for four of the movements assessed in this study. The data is from one subject performing on turf 3 (T3). Walking and running are not included, as they have been well documented previously. These figures to demonstrate the varying time histories where peak GRFs and moments of rotation occur in each of the different movements. The STOP movement has large GRFs during the first % of the stance period. Peak M occur at approximately 3% and % stance. The 4R movement has two periods of peak GRFs. The first represents a braking phase and occurs at approximately 2% of stance and also incorporates a peak M STOP %Stance R %Stance % Stance L 8 TURN % Stance Figure 6:Typical ground reaction forces and moments on rotation for STOP, 4R, 9L, 8 turn (Nmm) (Nmm) (Nmm) (Nmm) The second period represents a propulsive phase and occurs at about % stance. The 9L turn depicts an inverted U GRF profile with a peak at approximately % of the stance period. A peak M occurs at 6% stance. The 8T movement has two GRF and M peaks. A braking peak occurs at about 2%, and a larger propulsive peak occurs at approximately % (GRF) and 7% (M ) of stance. Five outcome measures were analysed in this study: the peak vertical ground reaction force, the peak resultant shear ground reaction force (GRF), the peak moment of rotation (M ), the peak translational grip ratio (TGR), and the rotational grip ratio (RGR). The ground reaction forces were calculated from the force plate outputs and normalised to bodyweight (BW). The translational grip ratio (TGR) as a function of time (t) was defined as: GRF t TGR = The TGR gives an indication of the resistance to translational movement. The equation is the same as the calculation for a mechanically tested friction coefficient but the use of this term was deemed inappropriate due to the implication that a standard normal force is used. The rotational grip ratio (RGR) as a function of time (t) was defined as: M ' t RGR t = where M is the Free Moment (or torque) in Nmm applied to the ground by the athlete. This RGR with a unit in mm, gives an indication of the resistance to rotation by the turf during the sporting movement, taking into consideration the magnitude of the normal force. For all variables, the mean of three successful trials of each movement was calculated for each of the eight subjects. For each turf, means were calculated for the eight subjects for each movement. A within subject two-way ANOVA was conducted to assess for significant differences between turfs for each movement. Peak vertical GRF (N/BW) T-4R T2-4R T3-4R T-4L T2-4L T3-4L T-9L T2-9L T3-9L T-9R T2-9R T3-9R T-8 T2-8 T3-8 Figure 7: Mean peak (± s.d.), normalised to BW, for 8 movements on 3 artificial turfs The greatest was measured for the RUNSTOP movement. The mean peak values ranged from 3-3.BW over the 3 turfs. The smallest s were for the WALK

4 with approximately.4bw. The straight sprinting (RUN) and the cutting turns (4R, 4L) produced similar VGRFs. The RUN ranged from BW, whereas the 4L/R were slightly higher with BW. The 9 cross turns (9L, 9R) had a range of.7-.9bw and the 8 turns had a range of.9-2.bw. There was no significant difference observed for any of the movements between the turfs. Peak resultant shear GRF (N/BW) T-4R T2-4R T3-4R T-4L T2-4L T3-4L T-9L T2-9L T3-9L T-9R T2-9R T3-9R T-8 T2-8 T3-8 p<. Figure 8: Mean peak GRF (± s.d.), normalised to BW, for 8 movements on 3 artificial turfs For all movements except WALK and RUN, the peak GRF was greater than BW. Peak GRF values were greatest for the STOP, 4L and 4R movements at approximately.bw. 9L and 9R ranged between.3-.bw. It was observed that 9R turn produced slightly lower GRFs values than the 9L. The 8 turn ranged between.-.2bw across the turfs. The WALK GRF was the smallest at.3bw and running was approximately.6bw. There were no consistent significant differences in shear forces between the turfs, although T appears to produce smaller GRF values than the other turfs and was significantly smaller for the STOP and 4L movements. Moment of Rotation (Nm) T-4R T2-4R T3-4R T-4L T2-4L T3-4L T-9L T2-9L T3-9L T-9R T2-9R T3-9R T-8 T2-8 T3-8 p<. Figure 9: Mean peak Moment of Rotation (± s.d.) for 8 movements on 3 artificial turfs Figure 8 shows the absolute values for the mean peak moment of rotation (M ) for each movement. There is a relatively large variance, as the data has not been normalised to any subject related parameter. The greatest M was produced in the 9L movement: 2-3Nm. The M of 9R was less than the 9L at -9Nm. Moments of rotation for RUN, 4L/R and 8 turns ranged from -Nm, 2-7Nm and 7-2Nm, respectively. WALK produced values ranging from -Nm. Again there was no consistent significant difference between turf conditions, although T was significantly greater than the other two turfs for WALK and RUN. The TGR was greater for the movements involving a large Translational Grip Ratio T-4R T2-4R T3-4R T-4L T2-4L T3-4L T-9L T2-9L T3-9L T-9R T2-9R T3-9R T-8 T2-8 T3-8 Figure : Mean peak Translation Grip Ratio (TGR) for 8 movements on 3 artificial turfs Rotation grip ratio (mm) angle of turn. All turning movements produced a TGR of. or above. The greatest TGR was obtained during the 9 turns ( ). The 8 turns produced a TGR of approximately.6. The 4 cut produced a TGR ranging from RUN and RUNSTOP produced similar TGRs of approximately.4. The TGR for WALK was approximately.3 across the turfs. There were no significant differences in the TGR values between the turf conditions. The RGR was higher for the movements requiring a large T-4L T2-4L T3-4L T-4R T2-4R T3-4R T-9L T2-9L T3-9L T-9R T2-9R T3-9R T-8 T2-8 T3-8 p<. Figure : Mean peak Rotational Grip Ratio (RGR) (± s.d.) for 8 movements on 3 artificial turfs angle of turn, as with the absolute values of moments of rotation. The greatest RGR was for the 9L and 8 turn with RGRs ranging between 6.-8.mm and.6-8.2mm, respectively. The 9R RGR ( mm) was smaller than that for the 9L. The 4L and 4R movements had RGRs ranging from 7-.2mm. RUN and STOP were

5 similar with RGRs ranging from 6-9mm and 7-9mm, respectively. WALK produced the smallest RGR, ranging from.3-6.4mm. For the WALK and RUN movements, T was significantly greater than the other two turfs. DISCUSSION The largely indifferent results between turfs highlights the possible kinematic adaptation of athletes to the various turfs assessed in this study. This phenomenon has been described by other authors (Dixon, 999; Nigg, 99). The analysis of ground reaction forces illustrates the great range of forces applied to the ground for different movements. For running and turning activities, the mean vertical ground reaction force and mean resultant shear ground reaction force ranges from.7-3.bw and.6-.8bw, respectively. For vertical ground reaction forces, approximate GRF limits could be applied to categories of movement. For turns requiring a large angle of turn, a limit of 2BW could be applied. For faster movements or those requiring less speed reduction, the limit could be 3.BW. An important factor arising from this study is the high magnitude of shear forces observed in the different movements. This was approximately % of the in the 4 cut turns, for example. This indicates the high joint moments and joint loadings that could occur in the lower segments of the body during the execution of such dynamic movements. A resultant shear GRF was calculated in this study as it was deemed that the use of separate anteroposterior and mediolateral GRF measurements would hold no real anatomical significance. During fast movements involving a change of direction, it would have been extremely difficult to standardise the placement of the foot on the force plate that would be aligned with the force plate s measurement axes. Component forces should be reported with reference to a suitable anatomical axis system to provide anatomical significance. It was evident that during some of the turning activities, a high resistance to translational movement was produced. A TGR boundary limit could be set at approximately.3 to.8 for fast, dynamic movements performed on artificial turf. These values are similar to the peak force ratios on artificial turf, reported by Valiant (99). It was expected prior to testing that the STOP movement would produce a higher TGR, due to the abrupt reduction of forward momentum. However, the ratio of vertical to shear GRF was high and there would have been a great risk of injury if the subjects were asked to stop completely on the force plate. The moment of rotation or torque measured during the different movements remained below an approximate limit of 3Nm, although some individual subjects achieved higher moments. These values are comparable with other authors data who reported that moments of rotation kept below a limit of about 2Nm in tests with test subjects (Stucke et al., 983). The RGR values clearly show that more grip is required when performing tight-angled turns. A RGR limit of approximately 2mm could be applied for these movements, and mm for all the other movements examined in this study. It is interesting to note that the moments of rotation and RGR values for the 9L were generally higher than those for the 9R. All of the subjects were right-foot dominant. It could be speculated that the subjects had a superior proprioceptive control when their left foot is used as the stabilising stance/turning foot when performing tight turns, and therefore felt more comfortable turning to the left. The relatively low values for rotation on the 4 turns indicates that a small foot rotation occurred during this movement and the change of direction is generally obtained from the application of shear forces. The variability between subjects for the normalised measurements could be due to the different velocities that each subject could perform at a maximal effort and the individual footwear that the subjects wore. In conclusion, the outcomes of this study will enable a greater insight to the understanding of the interaction between the player and the ground during different sporting movements on artificial turfs. It demonstrates the wide range and large magnitude of forces and moments that are applied to the turf by the athlete when performing highly dynamic movements. An important aspect of this study is the identification of the large shear forces that are generated during fast turning movements. It is the combination of vertical and shear forces and rotational moments that provide possible injury mechanisms and affect the overall performance of conducting sporting movements. Current methods for the mechanical testing of artificial turf are onedimensional and apply loads that are not relevant to those experienced by athletes. It is suggested that future methods should apply biomechanically relevant, three-dimensional loads in order to characterise and test artificial turf. 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