Effects of Upper-Body Stable Plyometric Versus Unstable Strength Training on Upper-Body Strength, Proxies of Power, and Dynamic Stability in Trained Prepubertal Baseball Players

Abstract

Hammami, R, Noomen, N, Negra, Y, Selmi, W, and Granacher, U. Effects of upper-body stable plyometric versus unstable strength training on upper-body strength, proxies of power, and dynamic stability in trained prepubertal baseball players. J Strength Cond Res XX(X): 000–000, 2026—In sports such as baseball, which involve high-intensity actions like throwing, hitting, and fielding, targeted upper-body strength training (ST) is essential for developing strength, power, and dynamic stability required for optimal performance in training and competition. Moreover, the physical fitness foundations necessary for later sport-specific performance should be established during childhood and adolescence. This study compared the effects of upper-body plyometric training on stable (SPT) vs. upper-body ST on unstable surfaces (UST) on strength, power proxies, and dynamic balance in prepubertal baseball players. Twenty-eight male players aged 11 ± 1 year (tier 2, trained/developmental) completed a 6-week program (2 × 30 min·wk ⁻¹ ) integrated into regular baseball practice. Push-up exercises were performed with body-mass loading in both groups. The SPT group conducted the exercises explosively on stable surfaces, incorporating brief flight phases and minimal ground contact to engage the stretch-shortening cycle, whereas the UST group executed the same exercises on unstable devices at a controlled pace, without a flight phase, emphasizing trunk stability. Pre- and post-training, assessments included upper-quarter dynamic balance (CS-UYBT), maximal back extensor strength, one repetition maximum bench press, and seated front, back, and side medicine ball throws. Both groups showed significant changes over time ( p < 0.01) in dynamic balance ( d = 2.54), back extensor strength ( d = 2.54), bench press strength ( d = 1.73), and medicine ball throw performance ( d = 1.23–2.82). No significant group-by-time interactions were observed for all outcome measures ( d = 0.00–0.44, p > 0.05). Six weeks of SPT and UST are easy-to-implement upper-body training modalities for physical fitness development of young athletes. However, given the short intervention (i.e., 6 weeks) and subjects' prior training experience, the results reflect short-term training responses rather than evidence of equal efficacy or superiority between training methods.