Abstract
Background: Although previous research in pediatric populations has reported performance enhancements following long-term plyometric training, the acute effects of plyometric exercises on measures of balance, vertical jump, reactive strength, and leg stiffness remain unclear. Knowledge on the acute effects of plyometric exercises (i.e., maximal versus submaximal hurdle jumps) help to better plan and program warm-up sessions before training or competition. Objectives: To determine the acute effects of maximal vs. submaximal hurdle jump exercise protocols executed during one training session on balance, vertical jump, reactive strength, and leg stiffness in young volleyball players. Materials and methods: Thirty male youth volleyball players, aged 12-13 years, performed two plyometric exercise protocols in randomized order. In a within-subject design, the protocols were conducted under maximal (MHJ; 3 sets of 6 repetitions of 30-cm hurdle jumps) and submaximal (SHJ; 3 sets of 6 repetitions of 20-cm hurdle jumps) hurdle jump conditions. Pre- and post-exercise, balance was tested in bipedal stance on stable (firm) and unstable surfaces (foam), using two variables [center of pressure surface area (CoP SA) and velocity (CoP V)]. In addition, the reactive strength index (RSI) was assessed during countermovement maximal jumping and leg stiffness during side-to-side submaximal jumping. Testing comprised maximal countermovement jumps (CMJ). Results: Significant time-by-condition interactions were found for CoP SA firm (p < .0001; d = 0.80), CoP SA foam (p < .0001; d = 0.82), CoP V firm (p < .0001; d = 0.85), and CoP V foam (p < .0001; d = 0.83). Post-hoc analyses showed significant improvements for all balance variables from pretest to posttest for MHJ but not SHJ. All power tests displayed significant time-by-group interactions for countermovement jumps (p < .05; d = 0.42), RSI (p < .0001; d = 1.58), and leg stiffness (p < .001; d = 0.78). Post-hoc analyses showed significant pre-post CMJ (p < .001, d = 1.95) and RSI (p < .001, d = 5.12) improvements for MHJ but not SHJ. SHJ showed larger pre-post improvements compared with MHJ for leg stiffness (p < .001; d = 3.09). Conclusion: While the MHJ protocol is more effective to induce acute performance improvements in balance, reactive strength index, and vertical jump performance, SHJ has a greater effect on leg stiffness. Due to the importance of postural control and muscle strength/power for overall competitive performance in volleyball, these results suggest that young volleyball players should implement dynamic plyometric protocols involving maximal and submaximal hurdle jump exercises during warm-up to improve subsequent balance performance and muscle strength/power.