The Effects of a Unilateral Strength and Power Training Intervention on Inter-Limb Asymmetry and Physical Performance in Male Amateur Soccer Players

The aims of this study were (a) to investigate the effects of a unilateral training program in reducing inter-limb asymmetry in male soccer players; (b) to explore such effects on measures of physical performance and unilateral inter-limb asymmetry. Twenty-four soccer players, randomly assigned to a 6-week unilateral strength and power training (UNI) (n = 12) or a control group (CON) (n = 12), performed single countermovement jump (SLCMJ), single leg broad jump (SLBJ), single leg drop jump (SLDJ), 10-m sprint, and 505 change of direction (COD) speed test. Raw jump scores revealed small to large improvements in SLCMJ, SLBJ, and SLDJ reactive strength index (RSI) (g = 0.46 to 1.66) in the UNI group, whereas negligible changes were found in the CON group (g =  − 0.31 to 0.33). Asymmetry indexes showed a moderate significant reduction in the SLDJ (RSI) and in the SLDJ stiffness (K) (g = 1.00 to 1.11) in the UNI group. The between-group comparison indicated a significant change in the SLDJ (RSI) and in the SLDJ (K) (g = 1.01 to 1.07) in favour of the UNI group. Thus, a unilateral training program seems to be able to reduce between-limb imbalances and foster improvements in jump performance, without any detrimental effects on linear or COD speed times. Given the importance of these physical characteristics for soccer, it is suggested that unilateral strength and power training are incorporated into strength training routines for players of all levels.