The results of the meta-analysis indicate that when subjects can adequately follow plyometric exercises (e.g., technical ability and adequate musculoskeletal strength), the training gains are independent of the fitness level (i.e., maximal aerobic power). Some authors explain that the gains that can be made via PT are dependent on training status (6). For example, Delecluse (17) studied beginner and experienced sprinters and found greater sprint increases for the beginner and smaller gains for the experienced sprinters. Furthermore, a major part of the improvements in untrained subjects during the initial weeks in power-type strength training is probably because of adaptations of the neural system (1,14,24,28,60).
Sprint performance improvement was found to be nonsignificantly greater when plyometrics was performed combined with other types of exercises (i.e., plyometric + weight training or plyometric + electrostimulation) than when performed alone (Table 3). There is a possibility that the subjects in the combination training group were exposed to a higher volume of training than those in the others groups, that is, the total workload was not equated between groups. It would be very interesting if future studies made an attempt to equate workloads between groups when comparing different training methods. Another difference is the model used to provide the training stimulus to the subjects. Training intensity, volume, and exercise selection followed the principle of progressive overload, starting with lower intensities, single-joint exercises, and less complex exercise techniques, and progressing to higher intensities, multijoint exercises, and more complex techniques. In any case, the optimal training strategy to enhance sprint performance appears to be a specific PT. That is, sprint performance gains will be optimized by the use of training programs that incorporate greater horizontal acceleration (i.e., skipping, jumps with horizontal displacement). It is generally accepted that the more specific a training exercise is to a competitive movement, the greater is the transfer of the training effect to performance (17,49). Athletes, such as sprinters, who require power for moving in the horizontal plane engage in bounding plyometric exercises, whereas athletes, such as high jumpers and volleyball players, who require power to be exerted in the vertical direction train using vertical jumping exercises (15,59).
It can also be concluded that when plyometric exercise intensity is high during the session, there is a greater improvement in sprint performance (Table 3). Some authors (9,51,57,63) determined that plyometric or SSC loading is higher during DJs, followed by CMJs, and then during squat jumps (SJs), there is little SSC loading. This is mainly attributed to the different characteristics of movement and, thus, to the different use of SSC characteristics. For these reasons, the combination of various exercises may result in higher gains compared with the performance of each exercise alone. Furthermore, it was found that a combination of plyometric exercises (i.e., SJ + DJ, bounding + CMJ) resulted in better training effects (ESs = 0.76) compared with the use of a single type of exercise (DJ) (ESs = 0.27) (Table 3). The higher improvements in sprint performance may be because of a training specificity. It is possible that a training program incorporating more horizontal acceleration (e.g., bounding and form running) may improve sprint times. In fact, it has been reported that no significant increase in sprint acceleration and velocity has resulted from training programs involving essentially vertical plyometric exercises (2,22,61). In contrast, significant improvements in running velocity, as measured by a 40-yd sprint, have resulted from the use of form running in conjunction with weighted depth jumps (21,38). Besides, improvements in 10- and 100-m sprint times have been found after a training intervention that incorporated some sprint-specific plyometric exercises (17).
Some research studies have shown that the PT with additional weights (vests, bars on the back, etc.) resulted in greater gains in sprint performance (21,38). The results of the meta-analysis indicated no significant differences among the training conditions (Table 3). Intuitively, this would make sense because adding weights increases ground contact times, and therefore, the duration of the eccentric and concentric phases are longer. Given that the magnitude of reflex potentiation, storage, and use of elastic energy is related to movement velocity and the time between the end of the eccentric phase and beginning of the concentric phase (coupling time), once more, the use of weights would seem problematic in the plyometric exercises and the sprints (i.e., the contact times) (42). The contact times during the initial acceleration phase of a sprint are similar to the contact times of the exercises employed (34,35,62,63). Therefore, the greatest transfer of the plyometrics to sprinting likely occurred during the initial acceleration phase. This theory is supported by Young (62), who suggested that bounding may be considered a specific exercise for the development of acceleration because of the similar contact times of bounding and sprinting during the initial acceleration phase. Further research is required to test the theory that the greatest transfer of sprint-specific plyometrics to sprinting occurs during the phase of the sprint when the contact times of the sprint during that phase are the same as the contact times of the plyometric exercises. Therefore, training effects using additional weights are not guaranteed.
Volume and frequency are very important parameters to be taken into account for an optimum PT program design. The results showed that training for <10 weeks (i.e., between 6 and 8 weeks) with 3–4 sessions per week is more beneficial than similar programs of a longer duration. Similarly, treatment with more than 18 sessions increases sprint performance, whereas performance of >80 jumps per session seemed to result in the most beneficial volume (Table 4).
The finding of this study is that a sprint-specific plyometrics training program can improve sprint performance over distances down to 40 m in length. The effects of a sprint-specific plyometrics program appear to be the greatest over the initial meters (10–40 m). The results suggest that sports participants who are accustomed to performing sprints over distances up to 40 m could potentially improve sprint speed, particularly in the initial acceleration phase, by adding sprint-specific plyometric exercises to their training. Explosive speed is required in many sports and physical activities; coaches and participants should therefore consider a plyometrics training program that incorporates sprint-specific exercises as part of the overall training plan.
In conclusion, the present meta-analysis demonstrates that PT significantly improves sprint performance. The estimated improvements in velocity as a result of PT could be considered as practically relevant—for example, an improvement in sprint time of >−0.081 seconds (i.e., ES = 0.37) could be of high importance for trained athletes in sports relying on sprint performance. A training volume of <10 weeks (with >18 sessions) using high intensities (with >80 jumps per session) is the strategy that will maximize one's probability of obtaining significant improvements in performance. Another important conclusion is that sprint performance gains will be optimized by the use of training programs that incorporates greater horizontal acceleration (i.e., sprint-specific plyometric exercises, jumps with horizontal displacement). However, there are no extra benefits gained from doing plyometrics with added weight.
Plyometrics can be recommended as an effective form of physical conditioning for augmenting the sprint performance; yet, the effects of PT could vary because of a large number of variables, such us program duration, training volume, or intensity. The velocity and conditioning coach may consider taking into account the dose-response trends identified in this analysis to prescribe the appropriate level of training. Therefore, in addition to the well-known training methods such as resistance training, explosive and sprint training, strength and conditioning professionals may well incorporate PT into an overall conditioning program of athletes striving to achieve a high level of explosive leg power and dynamic athletic performance.
The authors have no professional relationships with companies or manufacturers that might benefit from the results of this study. The results of this study do not constitute endorsement of any product by the authors or the National Strength and Conditioning Association.
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