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Original Research

Effects of Plyometric Training on Achilles Tendon Properties and Shuttle Running During a Simulated Cricket Batting Innings

Houghton, Laurence A.; Dawson, Brian T.; Rubenson, Jonas

Author Information
Journal of Strength and Conditioning Research: April 2013 - Volume 27 - Issue 4 - p 1036-1046
doi: 10.1519/JSC.0b013e3182651e7a
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Abstract

Introduction

The advent of multimillion dollar team franchises in the shorter, more intense formats of Twenty20 cricket (29) has spurred a need for evidence-based strength and conditioning programs for cricketers (3). Cricket is a sport played around the world and requires the skills of batting, bowling and fielding. When batting, points (‘runs’) can be scored by hitting the ball and running shuttles between 2 lines (separated by 17.68 m) before the fielder returns the ball. As such, cricket batting is typical of many team games in that it requires intermittent running with change of direction.

Cricket batting has traditionally been portrayed as physically undemanding because it requires intermittent shuttle running interspersed with long periods of standing and walking (10). Nevertheless, successful batting innings are often prolonged (e.g., typically >2.5 hours when scoring 100 runs in One-Day cricket) and require frequent accelerations, decelerations and changes of direction (10,29). These actions, repeated over time, contribute to physical fatigue in batting (16,27). Recent findings demonstrated a significant decrease (∼5%) in squat jump (SJ) height, an increase in straight-line sprint times and an increase in change-of-direction times across a prolonged, simulated, high-intensity, One-Day hundred (here-in referred to as BATEX; [15,16]). Although physical fatigue has been observed when batting, there has been no research into strength and conditioning programs that aim to improve intermittent shuttle running (running between the wickets) times during a prolonged batting innings (3,27).

It is possible that plyometric training may benefit performance during a cricket batting innings because shuttle running requires short, high-intensity accelerations and decelerations. Plyometric training involves short, high-speed movements that use the stretch-shortening cycle (30). Previously, the benefits of plyometric training have been demonstrated in team sports (basketball and soccer) and individual sports (swimming), but no investigations have been carried out on cricket-specific performance (6,19,31). A recent review concluded that plyometric training may improve change-of-direction and straight-line running times (typical in cricket batting), particularly if horizontal and lateral jump exercises are included (4). However, this review did not report any research on the effects of plyometric training on prolonged, intermittent shuttle running times as required in a successful batting innings.

The theoretical benefit of plyometric training on intermittent, shuttle running performance is further supported by considering previous research on the Achilles tendon. The mechanical properties of the Achilles tendon significantly contribute to ensuring efficient human locomotion (23). Previous research demonstrated that individuals with faster running-between-the-wicket times had stronger plantar flexors and stiffer Achilles tendons (16). It was suggested that stiffer tendons may allow better force transmission when accelerating and changing direction (16). Additionally, research has demonstrated increased Achilles tendon stiffness and increased lower-limb strength after plyometric training (5,12,22,33). Coupled together, these prior observations further suggest that, in theory, a plyometric training program might improve shuttle-running times. Also, the effects of plyometric training on other Achilles tendon properties has been observed, including increased elastic energy and muscle-tendon-junction displacement (22) and no change in cross-sectional area (12,22).

Methods

Experimental Approach to the Problem

There is no published research on the effects of strength and conditioning programs on shuttle-running performance during cricket batting. The primary aim of this study was to determine whether the addition of plyometric sessions to normal preseason training affected shuttle running times (straight-line sprints and turns) during a prolonged, simulated cricket batting innings (BATEX). A secondary aim was to assess whether there were accompanied changes in Achilles tendon properties because Achilles stiffness has been reported to correlate with change-of-direction time in cricketers (16). Previous research has demonstrated that at least 2–3 plyometric sessions per week over at least 6 weeks are required to improve strength and power (26,33). Therefore, running-between-the-wickets performance was assessed during a simulated batting innings (BATEX) both before and after 8 weeks (15 sessions) of plyometric training.

To assess running-between-the-wicket performance, mean 5-m sprint times (seconds; intraclass correlation coefficient [ICC] = 0.83; typical error [TE] = 0.01) and mean 5-0-5-m turn times (seconds, ICC = 0.76; TE = 0.04) were recorded (dependent variables). Also, countermovement jump (CMJ) heights (centimeters, ICC = 0.94; TE = 1.76), and SJ heights (centimeters, ICC = 0.81; TE = 1.88) were assessed to compare the efficacy of the plyometric program with previous research. In addition, the following Achilles tendon properties (independent variables) were assessed: force (newtons), low force-range stiffness (Stiffness0–40%), high force-range stiffness (Stiffness50–90%, newtons per millimeter), elastic energy (joules), peak strain (percent), peak stress (megapascals), Young's modulus (gigapascals) and cross-sectional area (millimeter square).

At the start of preseason (late-June, local winter), the participants were allocated to either an 8-week plyometric training group (PLYO, n = 9) or a control group (CON, n = 10). The participants were familiarized to BATEX and to the Achilles tendon test approximately 4 weeks before the training program started. Two weeks before training commenced, the Achilles tendon test was completed. One week before the training program, BATEX was performed. Outside of their involvement with the study, both groups completed their normal preseason preparation. The Achilles tendon test and BATEX were both repeated within 2 weeks after the training program. Before and after the training program, testing sessions were performed at a similar time of day and a 24-hour food diary was used to replicate dietary intake before BATEX.

Subjects

Nineteen players were recruited from local district-grade cricket clubs. All the participants were without injury, had experienced only minor lower limb sprains in the previous 2 years, regularly batted in the top 7 of the batting order, and had not completed a plyometric training program before. Typically, maximum aerobic speeds of approximately 15 km·h−1 have been recorded in our laboratory (unpublished work). The participants were informed of the experimental risks and signed an informed consent form before the study commenced. The study was approved by the University Human Research Ethics Board for use of human participants. Two participants withdrew from PLYO (one because of sporting commitments and the other without giving reason). Two CON participants were not included in final analyses because they were unable to attend the final BATEX test. As a result, 15 participants were included in the final analyses (21 ± 4 years). Stature was marginally higher in CONT (2.8%, p = 0.009, Table 1). Conversely, body mass was similar in PLYO and CON but increased in both groups from pretraining to posttraining (1.7%, p = 0.029, Table 1). In both PLYO and CON, a typical week of training (determined using diaries) included 0.5 hours of walking or running, 0.5 hours of upper body weights, 0.75 hours of cricket training, and 1.5 hours of recreational sport. This training regime was similar to that reported by the participants before the study.

Table 1
Table 1:
Anthropometric variables pretraining and posttraining.*

Procedures

The mechanical properties of the right Achilles tendon were determined using the protocol described in detail in Houghton et al. (16). In brief, the participants lay prone with ankle held in neutral (90°) and performed five, 5-second ramp isometric plantar flexions to maximum voluntary contraction (MVC) (21). The ramp contractions allowed Achilles tendon mechanical properties to be determined by plotting a graph of muscle-tendon-junction displacement at 10% force increments of the day's MVC. Displacement of the medial gastrocnemius muscle-tendon-junction was visualized using ultrasonography (6–7 MHz, 60–70 frames per second, 60-mm scanning length, Echoblaster 128 EXT-1Z, Telemed Ltd., Lithuania) and measured in ImageJ software (v1.43, NIH, USA). In each 5-second ramp contraction, displacements of the muscle-tendon junction were digitized 3 times, and the mean was used in final analyses. Achilles tendon force was estimated using an inverse-dynamics protocol using a dynamometer (Biodex System 3, 835-220, Shirley, NY, USA), electromyography (EMG; Telemyo 2400R G2, Noraxon, AZ, USA) and tendon moment arm (28). For the EMG, electrodes were placed on the surface of the tibialis anterior and medial gastrocnemius. Inevitably, some heel slip occurred (7.8 ± 2.5° plantar flexion) during the ramp contractions, and so, muscle-tendon-junction displacement was corrected for change in ankle angle by determining muscle-tendon-junction displacement at different degrees of plantar flexion during passive rotation of the ankle (16,24). In contrast to Houghton et al. (16), ankle angle was recorded using a motion-capture system (Tracking tools, v 2.3.1, OptiTrack, OR, USA). Continuous monitoring of ankle angle also enabled an estimation of the misalignment of the medial malleolus with the dynamometer's axis of rotation that occurred during heel slip (1). Moment arm of the Achilles tendon was determined using the tendon excursion method (11,24).

To measure tendon cross-section, 3 transverse ultrasound images (10 MHz, 40-mm scanning length, Echoblaster 128 EXT-1Z, Telemed Ltd.) of the right Achilles tendon were taken 2 cm superior to the line joining the lateral and medial malleoli of the right ankle (4.9 ± 0.7 cm superior to the inferior margin of the calcaneus). During this procedure, the participants were in normal stance with ankle at a neutral angle (90°). Care was taken to ensure the probe was kept perpendicular to the line of the Achilles with minimal compression of soft tissue. Using ImageJ software (v1.43, NIH, USA), cross-sectional area was measured 3 times for each the images (Figure 1), and the mean was calculated. Resting Achilles tendon length was measured from the insertion at the calcaneus to the muscle-tendon junction of the medial gastrocnemius (using ultrasonography). At the start and end of the study, lower leg pain was subjectively rated on a visual-analog scale after performing 10 calf raises. The visual-analog scale was from ‘no pain at all’ (0 mm) to ‘extremely painful’ (100 mm).

Figure 1
Figure 1:
Cross-sectional area of the Achilles tendon using ultrasonography (white arrows denote the tendon area).

Linear regression equations (R2 = 0.94 ± 0.06) were fitted to the plot of tendon force against muscle-tendon-junction displacement between 0–40 and 50–90% MVC with gradient used to determine stiffness in the low (Stiffness0–40%) and high force range (Stiffness50–90%), respectively. The total area underneath the Achilles tendon force-displacement curve was used as a measure of elastic energy. Muscle-tendon-junction displacement was expressed as a percentage of resting Achilles tendon length (strain) to standardize between participants. Tendon force was standardized by dividing by cross-sectional area of the Achilles tendon to determine stress (megapascals). Finally, by plotting stress against strain, Young's elastic modulus was determined from the gradient of the linear regression (R2 = 0.97 ± 0.03) fitted between 50 and 90% of peak stress on the day of testing. If a change in tendon stiffness is accompanied by a change in Young's modulus, it might suggest that alterations in Achilles tendon material properties or biochemistry rather than tendon hypertrophy. All mechanical properties were determined as the mean of three of the ramp isometric contractions.

The physical demands of a high intensity, prolonged (2 hours 20 minutes) cricket batting innings were simulated using the BATEX protocol at an indoor net facility. Overall, BATEX lasted 2 hours 20 minutes, required 49 straight-line runs, 30 shuttle runs (total of 34 changes of direction) and approximately 5 km was covered (17). Details of BATEX (including standardization of food and fluid intake) are available in Houghton et al. (15). Briefly, BATEX required the completion of six, 21-minute stages. During each stage the batsman played shots against balls delivered by a bowling machine. To replicate the timings of a match, balls were delivered every 35 seconds but with 80-second rest after every 6 balls (an ‘over’). Before each ball, audio cues instructed the batsman the number of shuttles (e.g., 1, 1.5, 2, or 3) to be made after each shot. In stages 1, 3, and 5 all shuttle runs (running-between-the-wickets) were at a self-selected ‘cruise’ pace, whereas during stages 2, 4, and 6 all shuttle runs were at full speed. In the full-speed stages, mean 5-m straight-line, sprint time was assessed in the single shuttles and a 5-0-5-m turn time was assessed in the 1.5, 2, and 3 shuttle runs (9). Running-between-the-wicket times were measured using electronic timing gates (Swift, Australia). Because of the limited space available in the indoor net facility, the pitch length was 1 m shorter compared to previous studies (15) and so the 5-m sprint time was sampled between 6.68 and 11.68 m (rather than 7.68 and 12.68 m) from the start ‘crease’. Further, in contrast to previous studies (15), BATEX was adapted to allow completion in pairs rather than by individuals. Therefore, each batsman received half the deliveries (i.e., 90 rather than 180 balls) but still completed the same shuttle running demands as in previous studies. Leg-guard design can affect running performance (35), and so, the participants were required to wear the same protective equipment before and after the training intervention.

Immediately before BATEX, jump heights were assessed using a contact mat (Innervations, Kinematic Measurement System, v 2009.1.0). After a standardized warm-up (light shuttle running and dynamic stretches), the participants performed 3 CMJs and 3 SJs at maximal effort (each separated by 1-minute rest). The highest CMJ and SJ flight times were used in later analysis. The jump-test battery was repeated approximately 2 minutes after BATEX.

The 8-week plyometric training program was based on previous literature and included horizontal and lateral (unilateral and bilateral) exercises of varying intensity (12,30,31). Details of the plyometric program are presented in Table 2. All the sessions were carried out on rubber gym matting with at least 2 days between sessions. Each session began with 4-minute cycling at moderate intensity and 5–6 minutes of dynamic stretches.

Table 2-a
Table 2-a:
Details of the plyometric training program.*
Table 2-b
Table 2-b:
Details of the plyometric training program.*

Statistical Analyses

All values are mean ± SD. Statistical significance was accepted at p ≤ 0.050. Statistical analysis was performed using PASW statistics v18.0.0 and Microsoft Excel (2003). Two-way, mixed-design factorial analyses of variance (Time: pretraining/posttraining × Condition: PLYO/CON) were used to analyze the effects of training on all variables (Achilles tendon properties, for example, Stiffness0–40%; anthropometric properties, for example cross-sectional area; BATEX performance variables, for example, 5-m sprint times; and, jump heights). Levene's test was used to confirm equality of variances. If significant interaction effects were found, t-tests were used to test for differences within (pretraining vs. posttraining) and between groups (PLYO vs. CON). Also, inferences from the smallest worthwhile change were determined for performance data (5-m sprint, 5-0-5-m turn, CMJ, SJ) using coefficients of variation (spreadsheet developed by Hopkins available at www.sportsci.org/resource/stats/). Inferences were made based on threshold chances of 5% for substantial magnitudes of change (14). Before and after training, the percentage change of variables was compared using independent t-tests. Effect sizes were considered small (0.2–0.59), moderate (0.6–1.19), or large (1.2–1.99) (7,13). Significant correlations (p < 0.050) between Achilles tendon properties and BATEX performance variables were reported as moderate (0.300–0.499) or strong (>0.500) (7).

Results

The mean 5-m sprint time and decrement in 5-m sprint time (from stages 2–6 of BATEX) were both similar between groups and from pretraining to posttraining (p > 0.050, Table 3). The effect size (−0.43) suggested that the 0.01-second decrease in mean 5-m time in PLYO was small, possibly (72%) practically beneficial and the overall inference was ‘unclear.’ Likewise, mean 5-0-5-m turn time was similar between groups and from pretraining to posttraining (p > 0.050, Table 3). The effect size (−0.21) suggested that the 0.03-second decrease in mean 5-0-5-m turn time in PLYO was small, possibly (51%) practically beneficial and the overall inference was ‘unclear.’ Although the decrement in 5-0-5-m turn time (from stages 2–6) did not differ in either group from pretraining to posttraining (p = 0.665), the mean decrement (both pretraining and posttraining) was greater in PLYO compared with CON (−0.08 ± 0.06 vs. −0.03 ± 0.05, p = 0.010, Effect size = −0.91, Table 3).

Table 3
Table 3:
Mean sprint times and mean turn times during the high-intensity stages of the batting simulation and jump heights both pretraining and posttraining.*

Self-selected running times (stages 1, 3, and 5 of BATEX) for both 5-m (1.42 ± 0.28 seconds) and 5-0-5-m (2.95 ± 0.41 seconds) were similar between groups and from pretraining to posttraining (p > 0.050).

In PLYO the percent increases in CMJ (11.0 ± 7.1%), and SJ (9.1 ± 5.5%) heights from pretraining to posttraining were all greater than CON (−0.7 ± 8.2, 1.5 ± 3.5, and 0.4 ± 12.6%, respectively; p < 0.050). These findings mirrored absolute jump height because CMJ and SJ only increased in PLYO (p < 0.050, Table 3). The effect size statistics suggested that the increases in CMJ and SJ were moderate (0.82 and 0.65), very likely (98 and 96%) practically beneficial and with the overall inferences being ‘very likely positive.’

Achilles tendon properties (resting length, Stiffness0–40%, Stiffness50–90%, elastic energy, peak Achilles tendon force, muscle-tendon-junction displacement, strain and Young's modulus) did not statistically differ from pretraining to posttraining or between PLYO and CON (p > 0.050, Table 4). The main effect of time for both Young's modulus (p = 0.067) and peak Achilles tendon force (p = 0.082) approached significance but the effect sizes were small (0.19–0.48).

Table 4
Table 4:
Achilles tendon properties pretraining and posttraining.*

There was no change in pain rating in either PLYO or CON from pretraining to posttraining (pre to post: 27.5 ± 17.8 to 31.6 ± 29.3 mm and 19.4 ± 15.4 to 25.8 ± 9.4 mm, respectively; p = 0.402).

There was an interaction effect (time × condition) for Achilles tendon cross-sectional area (p = 0.007) such that the pretraining cross-section was 9.8% lower in PLYO vs. CON (p = 0.029). Moreover, from pretraining to posttraining, cross-sectional area only increased in PLYO (%[INCREMENT] PLYO vs. CON: 12.8 ± 7.5 vs. 0.3 ± 6.9%, p = 0.010, moderate effect size of 1.02; Table 4).

Peak and submaximal Achilles tendon stress were statistically similar between PLYO and CON (main effect of condition: p = 0.749–958) but decreased from pretraining to post training in PLYO only (main effect of time: p = 0.002–0.004, Figure 2). However, there was a moderate trend for peak stress to decrease more in PLYO compared with CON (%[INCREMENT], PLYO vs. CON: −15.2 ± 12.3 vs. −5.5 ± 8.3%, p = 0.093, moderate effect size of 0.87; Table 4). Also, at all submaximal points in PLYO, the interaction effects (time × condition) approached significance (p = 0.054–0.073) and there were large effect sizes (1.21–1.24), together highlighting a strong trend for decreased stress from pretraining to posttraining in PLYO (Figure 2).

Figure 2
Figure 2:
Achilles tendon stress-strain curves both pretraining (closed symbols) and posttraining (open symbols) in control and plyometric groups. Note that data points are matched at 10% increments of peak pretraining or posttraining tendon force but point of absolute maximal stress (at maximal tendon force) has also been included on the graph. For clarity, error bars (SD) have only been included for stress. †Large effect size: 1.21–1.24.

Pretraining, maximum muscle-tendon-junction displacement (r = 0.548), strain (r = 0.577, e.g., Figure 3), Stiffness50–90% (r = −0.535) and modulus (r = −0.522) were all strongly correlated with 5-0-5-m turn time (p = 0.024–0.046). Pretraining, correlations between 5-m sprint and 5-0-5-m turn times and all other Achilles tendon properties were nonsignificant (p > 0.050). Furthermore, there were no significant correlations between change in mean 5-m time, change in mean 5-0-5-m time and change in all other Achilles tendon properties after the training program.

Figure 3
Figure 3:
Relationship between Achilles tendon strain and mean 5-0-5-m time during the batting simulation,r = 0.577, p = 0.004.

Discussion

The aim of this study was to explore whether the addition of plyometric sessions to normal preseason training affected: (a) Shuttle running times (straight-line sprints and turns) during a prolonged, simulated cricket batting innings (BATEX), and (b) whether there were accompanied changes in Achilles tendon mechanical properties. Inferences from the smallest worthwhile effect suggested that plyometric training may have possible benefits on shuttle running times and only a small chance of a detrimental effect. Achilles tendon stiffness0–40%, Stiffness50–90%, elastic energy, force, muscle-tendon-junction displacement, strain and Young's modulus were unaffected by plyometric training. Interestingly, cross-sectional area of the Achilles tendon increased after plyometric training.

In PLYO, although there were no statistically significant changes in mean 5-m and 5-0-5 m times from pretraining to posttraining, there were possible benefits on shuttle run times and the chance of detrimental effects were small (<15%). Moreover, the decrease in mean 5-m sprint (0.01-second) and 5-0-5 m turn (0.03-second) times from pretraining to posttraining are equivalent to completing a shuttle run approximately 6–13 cm quicker. It is emphasized that these are mean improvements over the course of a 2-hour 20-minute batting simulation and therefore, if consistently replicated in a match, might assist a batsman in getting his or her bat back across the crease-line before the ball is thrown in by a fielder. If a batter does not ‘ground’ his or her bat across the crease line before the ball is thrown in by a fielder then their innings is terminated (‘run-out’).

Despite no statistically significant changes in running times, the efficacy of the plyometric program was confirmed by increased jump heights. The increases in CMJ (+11%) and SJ (+9%) were comparable with previous plyometric training studies (∼+10%: 12,26). Plyometric training may improve jump height through various mechanisms including, improvements in coordination, balance, muscle activation, neural drive, muscle strength, muscle fiber composition and tendon mechanical properties (26). Previously, 5–6% increases in isometric plantar flexion torque, 19–60% increases in Achilles tendon stiffness and approximately 20% increase in Achilles tendon elastic energy were observed after plyometric training (12,22). However, Achilles tendon force, Stiffness0–40%, Stiffness50–90%, elastic energy and Young's modulus were unchanged in PLYO, suggesting increased jump heights were not related to improved plantar flexion strength or a change in Achilles tendon properties. It is possible that the increased jump heights in PLYO may have been because of improved strength, power and tendon mechanics in other muscle-tendon units of the lower limb (e.g., quadriceps femoris). Moreover, jumping, sprinting, and change of direction have been reported as specific motor abilities (4,34) and so the increased jump heights may be due to improved coordination and neuromuscular recruitment patterns (26). If improved coordination was the reason for increased jump height, it might explain why increases in jump heights were not translated to faster sprinting and change-of-direction times. Therefore, to significantly improve shuttle running times, batting-specific sprint and change-of-direction training may need to be combined with plyometric training (4). In this study, specific running-between-the-wicket practice was not reported in the training diaries of PLYO or CON.

Before training, faster turn times were correlated to lower strain and to higher Stiffness50–90% and modulus in PLYO and CON. Previously, it was demonstrated that higher Achilles tendon stiffness and force were correlated to faster 5-m sprint times and 5-0-5-m turn times during BATEX (16). It is possible that tendons with lower strain and higher stiffness allow more control of movement when decelerating and accelerating during change of direction (16). However, Stiffness0–40%, Stiffness50–90%, strain, and peak Achilles tendon force were all unchanged after plyometric training and so may provide a further explanation for the lack of statistically significant changes in 5-m and 5-0-5 m turn times.

The lack of changes in Achilles tendon stiffness may have been because of the relatively high pretraining stiffness. Stiffness values in the high force range (800–900 N·mm−1) were higher than that previously reported (∼400 N·mm−1) (16). However, peak muscle-tendon displacement and peak tendon force were similar to that previously reported (16), and so, the higher stiffness in this study may be explained by a difference in the shape of the force-elongation curve. Further, training-induced increases in stiffness may have been prevented to ensure maximum muscle efficiency (23). Previously, Achilles tendon stiffness has been reported to increase after plyometric training (5,12). Also, Kubo et al. (22) observed increased Achilles tendon elastic energy and muscle-tendon-junction displacement after plyometric training. In the current study, no changes in Achilles tendon stiffness, elastic energy, muscle-tendon-junction displacement, force, Young's modulus or strain were observed after PLYO. These findings might also be explained by the shorter duration and less intense training program used in PLYO. Previously, Kubo et al. (22) used a 12-week program with 4 sessions per week; Foure et al. (12) used a 14-week program with 2–3 sessions per week and 200–600 jumps per session (total of ∼6,800 jumps); and, although Burgess et al. (5) only used a 6-week program, participants were trained using high-intensity, single-leg, maximum-effort drop jumps. However, in this study, the plyometric program lasted for 8 weeks with a total of 1,785 jumps and 82–158 jumps per session of varied intensity (Table 2). A less intense workload was chosen to reduce injury risk in the cricketers because they had no experience of plyometric training and were also committed to their normal preseason training preparations. Nevertheless, it is possible that changes in mechanical properties occurred in different locations along the Achilles tendon. For example, in this study, determination of muscle-tendon-junction displacement was limited longitudinally along the Achilles tendon, but it is possible changes in occurred transversely or in the aponeurosis (18).

The pretraining cross-sectional area of the Achilles tendon was similar to that in previous studies (∼60–80 mm2) (20,25). The cross-sectional area of the Achilles tendon was higher in CON than PLYO, possibly explained by the higher stature in CON compared to PLYO. Posttraining, the cross-sectional area of the Achilles tendon increased in PLYO (+12.8%) but not in CON (+0.03%). However, there were no significant changes in Achilles tendon force in PLYO or CON, and so, this explained the strong trends for Achilles tendon stress (maximal and submaximal) to decrease more in PLYO than CON. Conversely, previous plyometric studies have not observed changes in cross-sectional area or stress of the Achilles tendon despite including longer and more intense training programs (12,22).

The increase in cross-sectional area in PLYO may have been due to reactive-tendinopathy or permanent tendon hypertrophy (8). Ayra and Kulig (2) observed higher cross-sectional area of the Achilles tendon in individuals with tendinopathy (93 mm2) compared with healthy controls (56 mm2), possibly because of water accumulation related to increased proteoglycan production (8). In this study, the Achilles tendon test was completed 1 week after the last plyometric training session, so it is possible that a training-induced ‘reactive-tendinopathy’ remained (8). However, the training load was tapered in the last 2 weeks to allow recovery (Table 2). Moreover, if reactive tendinopathy was present, increased Achilles tendon stiffness and pain rating (using the visual-analog scale) might have been expected alongside the increase in cross-sectional area, but this was not the case (8). Alternatively, permanent Achilles tendon hypertrophy might have occurred in PLYO because of increased type I collagen fibers (20). Findings from cross-sectional studies suggest that a history of repetitive lower-limb loading is associated with increased Achilles tendon cross-sectional area, particularly in the distal region (20,25,32). For example, in runners (>80 km·wk−1) the cross-sectional area of the Achilles tendon was approximately 85 mm2 but approximately 70 mm2 in nonrunners (4 cm above the inferior margin of the calcaneus) (25). Increased cross-sectional area may decrease stress placed on the tendon during locomotion.

Interestingly, Kongsgaard et al. (20) observed lower Achilles tendon stress in runners (72.4 MPa) compared with those with limited loading of the Achilles tendon (57.4 MPa in kayakers). It has been reported that the Achilles tendon has a failure stress of 100 MPa (25). In PLYO, Achilles tendon stress during maximum contraction was 84 MPa but was 69 MPa posttraining, and so, there was an increased ‘safety margin’ because peak stress was reduced further below Achilles tendon failure stress (assuming a failure stress of 100 MPa that did not change as a result of training) (25). Therefore, in PLYO, it is possible that the moderate to strong trends for decreased Achilles tendon stress might translate to reduced injury risk. However, further research is required to examine the relationships between Achilles tendon cross-sectional area, stress and injury risk.

It is acknowledged that a relatively small sample size was used in this study and so may limit translation of findings to a wider population. First, the small sample size was because of the difficulty of recruiting trained individuals. Previously, studies that observe the effects of plyometrics on performance have largely been carried out on nonathletes (26). In part, the preference of recruiting untrained volunteers may be because of the tendency of athletes to be hesitant at introducing novel training methods to their programs. The reality of small athlete sample sizes in scientific investigations was acknowledged by Hopkins et al. (14) when he recommended making population inferences from the smallest worthwhile change. The recommendations of Hopkins et al. (14) were followed in the current study. Second, the relatively small sample size of this study was due to the high time demands of the BATEX and Achilles tendon protocols. It could be argued that a less time-demanding protocol should have been used to allow higher sample size, but the BATEX protocol has been shown to reflect the physical demands of a prolonged, batting innings in a match (17). Therefore, the relevance of the current findings to real match performance is greater than if short duration agility tests (<5 minutes) were used. Previous studies commonly observe the effects of plyometric training on single jump height, sprint time, or agility time (26). In summary, findings presented in this paper are highly applicable to strength and conditioning coaches, particularly in the underresearched area of cricket.

In conclusion, plyometric training resulted in no statistically significant changes in shuttle run times nor decrements in shuttle run times during simulated cricket batting but jump heights increased. However, inferences made from the smallest worthwhile change suggested possible benefits (with minimal chance of detrimental affects) of plyometric training on prolonged, intermittent shuttle run times. Moreover, the change in mean sprint and change-of-direction times in PLYO might translate to practical differences (∼6–13 cm) in a cricket match. Achilles tendon mechanical properties were unaffected by training, but Achilles tendon cross-sectional area increased. The increase in the cross-sectional area of the Achilles tendon might have been because of reactive-tendinopathy (temporary changes) or reflect permanent Achilles tendon hypertrophy. Future research should investigate methods to better differentiation between reactive tendinopathy and permanent adaptations to training. In addition, research should further investigate the relationship between tendon cross-sectional area and injury risk.

Practical Applications

Addition of a lower-limb plyometric program to the preseason training program of club-standard, cricket batsmen may result in practically significant improvements (and minimal detrimental effects) in shuttle run performance (running-between the wickets) during a prolonged batting innings. This application may translate to other sports since intermittent running with change of direction is typical of many team-games. Plyometric training may also led to increased jump heights. Although increased jump heights may not be of direct advantage when batting, such improvements may benefit batsmen when fielding (e.g., when jumping to catch a ball).

Acknowledgments

This research was carried out while L.A.H. was in receipt of a University Postgraduate Reward and an Endeavor International Postgraduate Research Scholarship. The authors acknowledge the West Coast Eagles Football Club for loan of the timing gates; MiSport Ltd. for the loan of PitchVision to enhance the batting simulation; and Revolution Sports for the use of their indoor facility. They also wish to acknowledge the invaluable efforts of all the participants. The authors declare no conflict of interest in the findings of this research. The results of this study do not constitute endorsement of any equipment by the authors or the National Strength and Conditioning Association.

References

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Keywords:

change of direction; cross-sectional area; jump; modulus; stiffness; stress

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