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Original Research

The Effects of Plyometric, Tennis-Drills, and Combined Training on Reaction, Lateral and Linear Speed, Power, and Strength in Novice Tennis Players

Salonikidis, Konstantinos; Zafeiridis, Andreas

Author Information
Journal of Strength and Conditioning Research: January 2008 - Volume 22 - Issue 1 - p 182-191
doi: 10.1519/JSC.0b013e31815f57ad
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Abstract

Introduction

Acceleration is an important component of several sports. Particularly in tennis, reaction time, first-step quickness, speed over short distances, the ability to change direction quickly, and lateral (sideways) movement are important determinants of performance (22,26). In fact, tennis players spend 48% of the time moving sideways (26). Lateral first-step quickness and speed over 4-5 m are essential in returning balls not far away from the body or to get quickly into position for the upcoming play. According to Parsons and Jones, “Quick lateral movement is key when one must react immediately and change direction” (26). Thus, it is not surprising that Tennis Associations have included lateral movements in testing protocols to evaluate the fitness levels of tennis players (28). Furthermore, several investigators and tennis experts have included lateral/side movements in tennis pattern running programs (10,20,26,28,29). Thus, it is conceivable that improving the ability to quickly perform lateral movements to both sides may improve the performance of a tennis player; in contrast, the compromised ability to react (first-step quickness) and/or to perform lateral movements to one side will put the tennis player in disadvantage and may influence the overall performance.

Experts in the field have stretched the negative impact of laterality presence in tennis. Bös et al. reported that a deficit of a tennis players to quickly react, move or turn to one side compared with the other should be seriously considered in training programs and, if possible, corrected (1). Studies that investigated the presence of laterality in footwork in tennis are limited. Bös et al. reported differences in reaction time and speed of movement between the 2 sides (1). The deficit between the 2 sides was smaller in elite athletes. This finding is in line with Hotz et al., who stated that a successful athlete should be able to react and move equally well to both sides (15).

Linear and lateral drills practiced for 15-20 minutes, 2-3 times a week improve the ability of the player to react and move faster (26), which supports the concept of specificity of movement and velocity adaptations. Thus, the execution of fast lateral drills (similar to those performed during the game) towards the “slow” side will improve the side-specific lateral speed. Improvements in the speed of lateral movement also may be attributed to increases in strength (11). Hence, a focused strength-training program on the leg primarily responsible for the movement towards the “slow” side also may improve the side-specific lateral speed and correct the deficit during tennis-specific lateral movements. Plyometric training, which is used to train tennis players (5), improves the neural system coordination and muscular power of lower limbs (13,14). Furthermore, sprint-specific plyometric training is used for sprinting events for improvement of power (5) and sprint-specific resisted training has been shown to improve acceleration performance (8,41). Also, it is possible that a combined training approach incorporating both tennis-specific lateral drills and sprint-specific plyometric strength exercises may induce distinct adaptations compared to each of training programs executed alone and result in greater improvement in lateral speed movements. To the best of our knowledge, no study has searched for the optimal training program to correct the presence of laterality in tennis-specific lateral movements and to improve physical conditioning parameters, which are of great importance in tennis.

Therefore, the aims of this study were (i) to diagnose the presence of laterality in tennis sideways movements, (ii) to compare the effects of three training programs that involved either tennis-specific lateral drills, plyometric training or the combination of the two former programs on tennis-specific movements and on power/force of lower limbs, and (iii) to examine the relationships between the performances on tennis-specific linear and lateral movements and power/strength exercises. The last is of particular importance given the significance of linear and lateral sprint movements in tennis performance and the use of power/strength exercises by tennis players.

Methods

Approach to the Problem

The participants were randomly assigned to a control (C; n = 16) or to 1 of 3 exercise-training groups: plyometric training (PT; n = 16), tennis-drills training (TDT; n = 16), and combined training (CT; n = 16). Each training program was performed 3 times per week for 9 weeks. Before the commencement of the training program, all participants performed sideways shuffles to the right and left sides to determine the “slow” side. The PT used only plyometric exercises that were focused on the contralateral limb of the “slow” side because previous study showed that at fast speed trials lateral movement is initiated by a vigorous push-off of the contralateral limb (4). The TDT performed training program that consisted of only tennis-specific drills, where all lateral movements were performed towards the “slow” side. The training program of the CT group included equal number of plyometric and tennis-drills exercises. The control group did not follow a training program. Testing procedures for data collection were conducted before and after the completion of the training programs for the evaluation of performance of reaction time (single lateral step), 4 m lateral (sideways shuffles) and linear sprints, 12 m linear sprints with and without turn, reactive ability, power (drop jump), and strength (Fmax). The selection of the aforementioned fitness characteristics was based on the fact that they are important parameters for tennis performance. The training programs were selected because they are used by tennis players and have shown to increase the muscular power of lower limbs and to improve the ability to react, accelerate, and move faster.

Subjects

Sixty-four novice tennis players (mean ± SD: age, 21.1 ± 1.3 years; body weight, 71.7 ± 13.1 kg; height, 1.74 ± 0.09 m) volunteered to participate in the present study. The subjects had 2 to 3 years of tennis experience and were competing in matches at beginner's level. They also had similar previous experience time in various individual and team sports in which they performed activities like jumping, linear sprinting or change-of-direction maneuvers. All participants were healthy, had no injury of lower limbs, were not taking any medication, and were instructed to refrain from other training during the course of the study. Before participation, the experimental procedures were explained and written consent was obtained from all subjects. The study was approved by the institutional ethics committee.

Procedures

Assessments of the “Slow” Side for Lateral Displacement

The time to complete the task of 4.115-m displacement with side steps was served as criterion for detecting the “slow” side because the fact that this displacement is frequently repeated during a course of a tennis match regardless of the performance level of contestants. More specifically, each participant performed 3 trials of side-step displacement to the left and three to the right side. Then, the best performance times of each side were compared, and the side with the worse performance was defined as “slow” side.

The PT was focused on the contralateral leg of the “slow” side for 2 reasons. First, it has been suggested that the strength of lower limbs may influence the lateral speed of a tennis player (11). Second, during lateral (side) movement, the contralateral leg produces greater action (compared to ipsilatteral leg) for body displacement as indicated by the maximal and average contralateral ground reaction forces (4). The selection of one-sided lateral tennis drills was based on the premises that adaptations are directly related to the movement and velocity specificity, and that quality training can be optimized by using sport-specific exercises. The tennis-drills exercises used in this study mirror the movements that are seen during the tennis game. All training programs were applied 3 times per week for 9 weeks, and were conducted and the same time of the day (1:00-3:00 pm).

Training

All plyometric exercises were conducted with the contralateral leg of the “slow” side that was defined in the results section as a “trained” leg, whereas the other leg was defined as an “untrained” leg. Six exercises were selected for PT: single-leg hops (6 × 14 m), single-leg hopping on stairs 20-40 cm high (6 × 20 stairs), single-leg “kangaroo” jumps and knee lifting to the chest (8 reps), single-leg “jump and reach” in place (8 reps), single-leg drop jumps from 20-cm to 40-cm heights (8 reps), single-leg zig-zag lateral hops over the sideline (8 × 11 m). Each PT session consisted of 4 of the aforementioned exercises that were selected sequentially. Each exercise was repeated for two sets. The rest period was 1-2 min between repetitions, and 3-4 min between the sets.

The tennis drills were conducted with maximal speed. All starts (push-offs), changes of direction, and turns were performed with the contralateral leg of the “slow” side that was defined as a “trained leg”. All side displacement exercises were conducted towards the “slow” side to train the push-off phase of the contralateral lower limb. The following drills were performed: (i) sprint from baseline to service line (5.5 m × 8 reps) and back, (ii) while jogging the participants received a visual stimulus, made a 180° turn using the contralateral leg of the “slow” side as push off limb and sprint for 10 m (8 reps), (iii) “shuttle-run” between the long lines of tennis court. One repetition consists of 9 starts (9 runs) and 8 turns of 180° performed using the contralateral leg of the “slow” side as pivot leg (∼22 m × 6 reps), (iv) lateral speed drills executed towards the “slow” side. The participants sprint from the centre mark diagonally, towards doubles post and singles stick, perform side-steps (towards the slow side) along the net up to the opposite doubles side-line (11 m) and return to the service line with backward running (6-8 reps), (v) “fan” drill with side-steps within the “dead zone.” The participants performed from the center mark side-steps towards all the corners and the opposite side of the dead zone and back (6 reps), and (vi) The participants moved along the base lines of four tennis courts (∼60 m). At the first court they ran with side-steps (towards the slow side), using as push off leg the “trained leg”, at the second court they ran with free steps, at the third with side-steps, and at the fourth court again with free steps. In the CT, the participants performed 4 exercises in each training session: 2 exercises from the PT and 2 exercises from TDT. The exercises were replaced in every training session in sequential order.

Testing-Instrumentation

All pre- and posttesting was conducted during a 2-day period and twice a day (10:00-12:30 pm and 3:30-6:00 pm) in the Biokinetics Laboratory (Serres, Greece). The participants were advised to refrain from any kind of physical fatigue for 48 hours before testing. All tests were conducted for both lower limbs. Each subject performed half of tests starting with the left limb and the other half starting with the right limb. On the first day the participants performed the following tests: reaction time for a single side-step movement (RTSS), 4.115-m side steps (4mSS), 4.115-m forward sprint (4mFS), and 1-legged and 2-legged drop jumps from a 20-cm height (DJ). On the second day, they performed an 11.885-m forward sprint (12mFS), 11.885-m forward sprint with turn (12mFST), and 1-legged and 2-legged isometric strength tests (Fmax).

The DJ, and the reaction and sprint times in all 4.115-m tests were assessed with a load cell (AMD Co. Ltd., LC 4204 - K600, 1-D, Saitawa, Japan) positioned on the ground. The particular load cell provides accuracy of measurement ±5 N and sampling frequency of 1000 Hz. An identical dynamometer was mounted vertically on a special device for the assessment of maximal isometric force of lower limbs. The load cell was connected to a signal converter (analog to digital) that was connected to PC. For reaction and sprint times in 4.115-m tests the recording was initiated with a visual stimulus (light). Briefly, by pressing the PC mouse a white lamp, placed on photographic tripods at 1.5 m height and 2.5 m away from the participant, was turned on and at the same time a signal was given to the PC to initiate the recording of the trial. The recording was terminated by the stepping of participants on the load cell. For the DJ and knee angle assessments, a bench of 20 cm and a goniometer (CCKL-Creator, Taichung, Taiwan) were used.

The performance times of all 11.885-m sprints were assessed with (i) 2 pairs of photocells (Autonics Beam Sensor BL5M-MFR, Korea), with the respective reflectors, placed at the start and finish lines of the 11.885 m run and (ii) a digital chronometer with capacity to measure hundreds of a second (Saint Wien H5K, Tapei, Taiwan).

Measurements

Reaction Time for Side Movement

The subject was positioned sideways to the load cell facing the source of the visual stimulus. The proximal leg was placed 5 cm from the load cell. Following the visual signal the subject performed a single side-step on the load cell as fast as possible and executed 2-3 side steps to the opposite direction. The time from the visual signal to the activation of load cell was recorded in msec as “reaction time.”

Side Steps and Forward 4.115-m Performance

The distance of 4.115 m is equal to that of center mark to the singles side-line. The subject was positioned sideways with the leading leg 4.115 m away from the load cell. On visual signal the subjects performed consecutive side-steps as fast as possible stepping on the proximal half of the load cell. For the evaluation of the forward 4.115-m sprint performance, the subject executed a turn on the limb closer to the load cell and performed a free run as fast as possible. The time from the visual signal to the activation of the load cell was recorded as performance time in seconds for 4 m side steps or 4m forward sprint. The test was repeated for both sides (right and left legs being leading or turning).

Forward 11.885-m Sprint Performance Without and With Turn

The distance of 11.885 m is equal to that of the baseline to the net. The subjects were placed in the starting position facing the runway with the “trained” or “untrained” leg at the starting line. The subjects executed a maximal run without a signal to avoid the effects of reaction time. For the assessments of the 11.885-m sprint with turn the subjects were placed backwards with their feet parallel and behind the starting line. The subjects executed 2 backwards steps, a 180° turn, and then a maximal run. Both tests were repeated 3 times with both trained and untrained legs used for push-off for start and turns. The best performance was recorded in seconds.

Reactivity Coefficient, Power, and Strength Measurements

Each subject performed unilateral testing of each leg and bilateral testing. The participants performed three attempts, out of which the best was recorded and used for analysis. During DJ the subjects maintained their trunk in an upright posture with their hands on the hips and executed jumps on the load cell with one or two legs from heights of 20 cm. The subjects were instructed to jump as high as possible and as soon as they had contact with the ground. The heights of all jumps were recorded in cm. The reactivity coefficient (RC) was calculated with the equation: RC = drop jump height (cm)/contact time (s) (7). For the assessment of Fmax, the subjects were seated with their feet placed on a vertical load cell and the hips and knees joints flexed at 95° and 90°, respectively. At command, the subjects applied maximal force as quickly as possible. In one-legged press the uninvolved leg was placed on the ground.

Statistical Analyses

We used the SPSS 10.0 statistical package (SPSS Institute, Chicago, IL) to analyze all data. Paired t-tests on the entire sample size (n = 64) were used to examine the differences between the 2 sides in performances of lateral movements, power and strength. Two-way analysis of variances on “trained” and “untrained” leg/side (4 levels of “training program” and 2 levels of “time”) with repeated measures on the “time” (pre- and post-) factor were used to investigate the effects of “training program” and “time” on tennis-specific and power/strength parameters of “trained” and “untrained” leg/side. An analysis of covariance (ANCOVA) was performed to examine the differences between groups in post-training values for each leg, where the pretraining mean was used as a covariate. Tukey pair-wise comparisons were used to locate the significantly different means. Pearson correlations (r) were used to investigate the relationships among tennis-specific lateral movements, speed, and power/strength parameters. All data are presented as means ± standard deviation (SD) with a P value of ≤0.05 considered as statistically significant. All testing procedures used in this study reported a high intraclass correlation coefficient (0.82-0.98; P < 0.01).

Results

Leg-to-Leg (Side-to-Side) Differences

The leg-to-leg (side-to-side) differences were examined on the entire sample size (n = 64). The t-test for dependent groups revealed significant leg (side) differences for 4 m lateral side-steps (t = 7.976; P < 0.001), whereas the differences for 4 m forward sprint just failed to reach significance (t = 1.940; P = 0.057). For all other variables there were no significant differences between leg-to-leg (side-to-side) performances.

Reaction Time (RT), 4 m Side Steps (4mSS), 4 m Forward Sprint (4mFS)

Before and after comparisons within each group for RT and 4mSS are presented in Table 1. For the “trained” leg/side, RTtrain decreased significantly only in PT and CT groups (P < 0.001), whereas performance times for 4mSStrain and 4mFStrain decreased significantly in all 3 training groups (P < 0.001). For the “untrained” leg/side, RTuntrain and 4mSSuntrain performances did not change in all 4 groups, and 4m-FSuntrain improved significantly only in CT group (P = 0.049).

Table 1
Table 1:
Mean and ±SD for reaction time (ms), 4-m side-steps (s), 4-m forward sprint (s), 12-m forward sprint (s), 12-m forward sprint with and without turn (s) before and after plyometric, tennis-drills, combined, and control groups.

Figure 1 shows the ANCOVA results for RT, 4mSS, and 4mFS performances for both trained and untrained legs/sides. Comparisons among adjusted post-training means for reaction time revealed significantly decreased RTtrain in PT and CT vs. TDT and C groups (P < 0.05), and RTuntrain in PT vs. C group (P < 0.05). The performances for 4mSStrain and 4mFStrain, improved significantly in all 3 training groups vs. C. For the “untrained” leg/side, 4mSSuntrain decreased in PT and TDT vs. C group (P < 0.05), whereas 4m-FSuntrain decreased in PT and CT vs. C group (P < 0.05).

Figure 1
Figure 1:
The adjusted post-training means ± SD (analysis of covariance results) for performances on reaction time (single side-step), 4-m side-step sprint, 4-m forward sprint, and 12-m forward sprint with and without turn for both trained and untrained legs. #P < 0.05 vs. TDT; *P < 0.05 vs. C.

Forward 12-m Sprint Without Turn (12mFS) and With Turn (12mFST)

Before and after comparisons within each group for the “trained” leg/side revealed that 12mFStrain, and 12FSTmtrain improved significantly after the TDT and CT protocols (P < 0.05). For the “untrained” leg/side, the 12mFSuntrain and 12mFSTuntrain performances did not change (Table 1).

ANCOVAs on adjusted post-training means revealed a significant “training program” effect on 12mFStrain and 12mFSTtrain (Figure 1). With the use of Tukey post-hoc tests for analysis, we found improved 12mFStrain performance in all training groups vs. C (P < 0.05), while the performance in 12mFSTtrain was improved only in TDT and CT vs. C group (P < 0.05), and failed to reach significant differences in PT vs. C (P = 0.085). For the untrained leg/side, the performances did not change.

Reactivity Coefficient, Power, and Strength Measurements

Before and after comparisons within each group for RC, DJ, and Fmax are presented in Table 2. For the “trained” leg, the post-hoc analysis indicated that all three training programs improved DJtrain and RCtrain (P < 0.05). However, in the “untrained” leg/side, DJuntrain and RCuntrain were improved only after PT and CT (P < 0.05). Performance for Fmaxtrain was significantly greater after PT and CT (P < 0.05), and unchanged for Fmaxuntrain in all groups. DJboth improved after the three training protocols, while performances of RCboth and Fmaxboth were better only after PT and CT (P < 0.05).

Table 2
Table 2:
Mean and ±SD for drop jump (cm), reactivity coefficient (cm/s), and isometric force (N) before and after training in plyometric, tennis-drills, combined, and control groups.

All ANCOVA results for power and strength measurements in “trained” and “untrained” legs/sides are presented in Figure 2. Comparisons among post-training means adjusted for baseline values revealed significantly increased DJtrain in all three groups vs. C (P < 0.05), whereas RCtrain and Fmaxtrain improved only in PT and CT vs. C (P < 0.05). For the “untrained” leg, DJuntrain and RCuntrain were greater after PT and CT compared with C group (P < 0.05), and Fmaxuntrain in PT vs. C. Results for both legs indicated significantly greater DJboth values after all 3 training programs, whereas RCboth and Fmaxboth post-training values were greater after PT and CT vs. C (P < 0.05).

Figure 2
Figure 2:
The adjusted post-training means ± SD (analysis of covariance results) for performances on drop jump, reactivity coefficient, and isometric strength for trained leg, untrained leg, and both legs. #P < 0.05 vs. TDT; *P < 0.05 vs. C.

Correlations Among Lateral Movements, Sprint, and Power/Strength Exercises

All the relationships between the performances in tennis-specific lateral movements, sprints, and power/strength exercises are presented in Table 3. RTSS for trained and untrained leg/side was not related with any sprint and power/strength exercises (r = 0.01-0.23; P > 0.05). 4m-SS and 4m-FS for either trained or untrained leg/side showed moderate to high correlations with 12-m sprint performances (r = 0.73-0.87; P < 0.01) and power parameters (r = −0.59 to −0.67; P < 0.01), and isometric strength (r = −0.49 to −0.64; P < 0.01).

Table 3
Table 3:
Correlation matrix for all performance variables.

Discussion

The results of this study show that there is a significant difference in lateral speed (side steps) between the 2 sides in novice tennis players. This result is in line with Bös et al., who reported side differences in performance of tennis-specific lateral displacements and sprints with turn (1). Nine weeks of PT, TDT, or CT with emphasis on the contralateral (push-off) leg to the “slow” side (for plyometric) and of the “slow” side (for tennis-drills) improved the 4-m side step and 4-m forward sprint performances. PT and CT also improved the reaction time (single side step) of the “slow” side, whereas TDT and CT improved the 12-m forward sprint performances with and without turn. One-legged and 2-legged power and strength parameters improved on most tests after the PT and the CT training protocols. The relationships between the performances on tennis linear and lateral sprint movements and power/strength exercises were moderately related (r = −0.50 to −0.75).

The ability of a tennis player to quickly react is crucial to successfully answer the incoming balls from a serve or a passing shot. Reaction/response time is divided to premotor and motor time components. Motor control is directed by the central nervous system using sensory input from proprioceptors. PT has been shown to induce a favorable adaptation to the sensorymotor system and enhance proprioception (33). In this study, the performance of a lateral single-step reaction time improved significantly towards the “slow” side after PT and CT. Furthermore, both training regimens were superior compared to the TDT group. Because no improvements in the single step reaction time were observed after TDT, it is possible that the better performance after CT was a result of plyometric exercises. It is not apparent why PT improved reaction time and TDT did not, because both types of training will most likely result in favorable neuromuscular adaptations. However, it should be noted that in this study the reactivity coefficients and strength parameters of the trained leg (push off leg during lateral side step) were improved compared with the control only after PT and CT, and not after TDT, indicating possibly greater improvements in explosiveness and rate of force development. It has been suggested that the response time is a skill linked to experience and learning (6). During PT the subjects performed the subsequent jump as soon as they had contact with the ground. Therefore, they may have adopted the pattern of quickly react to a stimulus. Thus, possible differences in the acceleration/deceleration movement pattern and power development profile between PT and TDT may explain the results. Previous studies report controversial results on the effects of isokinetic, isotonic, isometric, and aerobic training on muscle reflex/reaction time, pre-motor and motor times, and speed of contraction (12,17,25,37). However, Wojtys et al. (37), who included power and sprint exercises in their training program (comparable to the CT in our study), reported significantly improved spinal reflex and cortical response times to anterior tibial translation in selected leg muscles.

The propulsive force during the first few foot contacts is important for the initial running velocity (31). Using the trained leg for push-off, the performances of 4-m side steps towards the slow side and of 4-m forward sprint improved in all 3 training programs. However, none of the 3 training protocols was superior to the others. In the PT group, the improvement in performance was most likely attributed to gains in reactive ability, power, and strength. It is also possible that the 4-m side-steps performance improved in PT because of lesser reaction time after training. The aforementioned speculations may be supported by 3 facts. First, PT resulted in greater gain in drop jump over TDT suggesting greater gain in leg power. Second, reactivity coefficient of both legs improved only in groups that performed plyometric drills. The importance of reactive ability in the propulsion phase of sprinting has been previously suggested (38). In fact, reactive strength appears essential for lateral change-of-direction speed possibly because of similar push-off actions (40). Third, reactive strength correlates with performances in short sprint (8 m) (40), and this study documented correlations between unilateral and bilateral reactivity coefficient with both 4m lateral and 4m linear sprints (r = −0.64 to −0.66; P < 0.05). In this study, the TDT group performed both 4 m forward sprints and lateral side steps during training. Thus, improvements in 4 m lateral and linear sprints in TDT group are in accordance with the concept that task-specific training may allow greater activation in a given activity and potentially improve performance (30).

Although performances in 12-m linear sprints with and without turn improved after all 3 trainings, there were significant only after TDT and CT. In fact, the improvement was greatest in CT group and least in PT (n.s.). It is possible that the CT group adopted the benefits of greater first-step push-off and power of PT and the specificity of velocity and motor task of TDT. A previous study also documented the improvements in short-sprint times after combining plyometric, resistance, and sprint training (24). PT combined with other types of resistance exercise may improve sprinting performance (21) and power (9). However, there are studies that report no significant effect on 10 to 40 m sprint performances of plyometric exercises alone (3,36) or combined with resistance training (34). Thus, it is not surprising that in our study PT did not improve the 12 m sprints since the training was conducted only on one leg. The improvement detected in TDT and CT are in accordance with the notion of specificity of movement and velocity training adaptations (13) because both groups involved 12 m sprints in their training programs. The differences in the results between the 4-m and 12-m sprints are probably attributed to the less contribution of the first-step push-off in 12-m sprint performance compared with that in the 4 m sprint.

Drop jump height improved significantly after PT than after TDT. Also, reactivity coefficient and isometric force improved compared to control only in the PT and CT groups. Thus, the results of our study clearly indicate that PT is superior to TDT to improve the reactive ability (first step), power, and strength. The improvements of the PT group are in accordance with several studies that reported improved jumping performance in stretch-shortening exercises after PT alone or in combination with other type of resistance training (9,21,34,36). However, others reported no change in the jumping ability (3) and isometric strength following PT (13,36). Studies in collegiate women tennis players also reported that resistance training using machine and free weights improved strength, jumping ability and sport specific-motor performances (18,19).

In the present study reactivity coefficient, power, and strength were significantly related to 4 m and 12 m sprint performances (r = 0.49-0.75; P < 0.05; Table 3). In contrast, reaction time was not correlated with any power or strength parameter. This finding was surprising, since someone would expect significant correlations at least between response time and reactivity coefficient. Drop jump and reactivity coefficients were the best predictors for all 4 m and 12 m sprints performed in this study (r = −0.60 to −0.75; P < 0.05). This is in line with previous studies that reported significant correlations between reactivity coefficient and 8 m performance for unilateral (r = −0.61) and bilateral (r = −0.55) drop-jumps (40), and between concentric power/force during a squat jump and 5 m (r = 0.64) (31) and 2.5-m (39) sprint performances. However, a recent study (7) reported a nonsignificant correlation between the drop jump reactivity coefficient and performance in 5 m and 10 m sprints (r = −0.35 to −0.38; P > 0.5).

Unilateral training is frequently a part of training programs for the development of strength and power. Even though, our purpose was not to examine the effects of unilateral training on the performance of the untrained limb, the results of this study showed that unilateral PT improved reaction time, drop jump height, reactivity coefficient, and isometric strength of the untrained limb vs. control. Furthermore, the 4-m lateral side step and 4 m forward sprint performances using the untrained leg for push-off were better after unilateral PT. The CT group also improved most parameters of power and strength, while the cross effects of TDT were detected only on 4 m lateral displacement, suggesting the importance of motor-task specificity in motor-learning. Thus, the results of our study confirm the notion that unilateral voluntary movements may induce a learning effect for the activation of the contralateral limb/side and result in adaptation of the untrained limb/side (23). Central neural mechanisms may be responsible for the cross-transfer effect (23).

The improvements in performance measures after PT and TDT in this study were most likely the result of neuromuscular adaptations. Neural adaptations after PT may include changes in stretch reflex, elasticity of musculotendinous system and Golgi tendons (3,5,16,35) and enhanced proprioception and kinesthesia (33). Resistance training has been also shown to increase muscle tendon stiffness that would increase the speed of force transmission (27) and the input-output properties of the corticospinal pathway (2). However, there is some evidence that explosive strength training does not change the reflex latency suggesting no changes in sensorimotor nerve conduction properties (13). The effects of sprint training on neural adaptations are not clear. Slevert et al. documented similar increases of nerve conduction velocity following sprint or strength-sprint resistance training (32). However, stretch reflexes and neural stimulus to the muscle are possibly increased after sprint training (30).

Practical Applications

The results of this study indicated that unilateral PT and TDT trainings with emphasis on the “slow” side, or CT, including both types of training improved the overall performance in tennis-specific fitness tests. PT improves fitness characteristics that rely more on reactive strength and powerful push-off of legs such as lateral reaction time, 4 m lateral side steps and forward sprint, and drop jump and maximal force. TDT improves all 4-m and 12-m sprint performances. CT appears to incorporate the advantage of both programs and improve all fitness tests performed with the trained side. It should be acknowledged that the cross-transfer adaptations to the untrained leg (or side) were most pronounced following PT. Thus, unilateral PT of the contralateral leg to “slow” side appears at least as effective as TDT in improving the lateral performance of the “slow side.” However, tennis coaches should be aware that each training regimen may induce more favorable changes to different aspects of fitness and should plan the training accordingly. The improvement of milliseconds in reaction time and in short sprints as well as in jumping ability may allow the tennis player to achieve a faster direction shift, to reach a certain point of the court more quickly and/or to answer a faster or higher (passing shot) tennis ball. Thus, the improvements achieved in fitness parameters in this study are essential for a tennis player, because they can lead to a further improvement in performance during the game. The outcomes of this study are limited to novice tennis athletes (2 to 3 years of tennis experience). It is possible that with a thorough approach of a training program they may be applied to higher-level tennis players.

Acknowledgments

We would like to thank Dr. Papadopoulos C. for his assistance as well as for providing the Biokinetics Laboratory for the implementation and completion of this study.

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Keywords:

response; side step; speed; isometric; drop jump; unilateral

© 2008 National Strength and Conditioning Association