Eccentric Hamstring Strength and Interlimb Asymmetry in Professional Football Players: A NordBord-Based Longitudinal Analysis of Two Professional Teams

The aim of this study was to examine temporal changes in eccentric hamstring strength, impulse-based mechanical outputs, and interlimb asymmetry in professional football players performing a football-specific eccentric hamstring training program. Forty male football players (18–25 years) from two teams competing in the Turkish Second Professional Football League participated in this longitudinal cohort study. Eccentric hamstring performance was assessed at three time points (pre-, mid-, and post-season) using the NordBord Hamstring Testing System. Mixed-design repeated-measures ANOVA revealed significant main effects of time and significant time × Team interactions for left and right maximal impulse values (p < 0.05). In contrast, maximal eccentric force variables showed no significant time effects, although significant time × Team interactions were observed for both limbs (p < 0.05). Interlimb maximal force asymmetry and mean asymmetry demonstrated significant time effects, while Team effects and interaction terms were not significant. Overall, these findings indicate that temporal changes in eccentric hamstring performance in professional football players may be more clearly reflected in force–time–dependent metrics, particularly impulse, rather than peak force outputs. Accordingly, impulse-based measures may provide additional insight into eccentric hamstring performance changes in professional football players.