Hamstring strain injuries (HSI) represent a significant burden in soccer. High-speed running is one of the most common HSI mechanism, in particular during match congested periods. Peak force and rate of force development (RFD) of the hamstring muscles tested at long muscle length have shown reductions following fatiguing tasks. However, no study has used a meticulous fatiguing protocol nor reliability scores have been provided. Hamstring peak force, RFD₅₀₋₁₀₀ and RFD_{100 −150} were assessed at long muscle length in 19 soccer players (26.0 ± 4.1 years) before and after the repeated sprint ability (RSA) test. We aimed to calculate reliability scores for both limbs before and after the fatiguing task, and to compare peak force, RFD₅₀₋₁₀₀ and RFD_100−150 following the RSA test to baseline values. Peak force displayed “excellent” reliability scores before and after the RSA test, whereas RFD ICC showed “good” values in both time points, but CV scores were not acceptable (i.e. > 10%). Significant moderate to large decreases were found in peak force (g = − 1.11 to − 0.90), RFD₅₀₋₁₀₀ (g = − 1.37 to − 1.11) and RFD_100−150 (g = − 0.84 to − 0.69) in both dominant and non-dominant limbs. Maximal isometric peak force, RFD₅₀₋₁₀₀ and RFD_100−150 of the hamstrings tested at long muscle length reduced following the RSA test. However, only peak force displayed “excellent” reliability scores, whereas RFD measures could not be considered acceptable owing to their lower reliability scores. Thus, practitioners can be confident about peak force changes, whilst caution should be used when examining such changes in RFD.

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.