Eccentric Hamstring Strength and Instrumented Anterior Knee Laxity in Professional Male Football Players: A Cross-Sectional Study

Background/Objectives: Eccentric knee-flexor capacity and passive anterior knee behavior are commonly assessed as distinct dimensions of knee function, but their coexistence in professional football players remains incompletely characterized. This study aimed to describe anatomical right-to-left differences in eccentric hamstring force and torque, characterize load-dependent anterior knee laxity, and examine within-limb relationships among GNRB-derived variables. Methods: This cross-sectional study included 25 male professional football players from two Super League clubs. Participants performed three maximal Nordic hamstring exercise trials using the NordBord system, followed immediately by bilateral GNRB Rotab arthrometry at 20° of knee flexion with three measurements at 134 N and three at 200 N. Results: We were unable to detect statistically significant right-to-left differences in maximal eccentric hamstring torque (mean difference 6.55 N·m, 95% CI −9.65 to 22.75; p = 0.412; dz = 0.17) or force (−11.92 N, 95% CI −29.67 to 5.83; p = 0.179; dz = −0.28). A post hoc sensitivity analysis indicated that n = 25 provided 80% power only for paired effects of approximately |dz| ≥ 0.58; smaller between-side differences remain compatible with the reported confidence intervals. Among the 24 participants with complete arthrometry data, separate within-limb analyses showed that TMAX was higher at 200 N than at 134 N in the left knee (p = 0.014), whereas the device-exported ACL output, SLOPE, and TMAX were higher at 200 N in the right knee (all p < 0.001). These separate tests do not establish a Side × Load interaction. At 200 N, GNRB-derived variables were strongly intercorrelated within each limb (r = 0.854–0.992). Conclusions: Anatomical right-to-left comparisons did not identify statistically significant mean differences in eccentric hamstring measures, but the small sample and absence of limb-dominance data preclude conclusions about equivalence or individual-level symmetry. Post-NHE GNRB-derived measures showed expected load dependence and substantial overlap. These findings are descriptive and do not establish a direct relationship between hamstring strength and ACL laxity or predict ACL injury.