618 - Inter-Limb Differences in Hamstring Muscle Activation During Eccentric Exercises in Elite Soccer Players Following High-Grade Injury

Objectives: High-grade hamstring injuries in elite male soccer players have high recurrence rates, particularly with tendon involvement. Persistent neuromuscular inhibition after rehabilitation may increase reinjury risk, but inter-limb differences in hamstring activation remain poorly understood. This study compared peak hamstring muscle activation between previously injured and non-injured limbs during common eccentric exercises in elite male soccer players.

Design:

Sixteen elite (Premier League/Championship) male soccer players (mean age 22 ± 3.3 years) with a history of unilateral high-grade hamstring injuries (BAMIC 3c) participated in a retrospective cross-sectional study. Surface electromyography (sEMG) was recorded bilaterally from the long head of biceps femoris (BFlh) and semitendinosus (ST) during the Nordic Hamstring Exercise, Glider, and Diver. sEMG signals were normalized to maximal voluntary isometric contraction. A three-way repeated-measures ANOVA examined effects of limb, muscle, and exercise, with Bonferroni-corrected post-hoc tests.

Results:

The previously injured limb showed significantly lower peak sEMG than the non-injured limb (F(1,15) = 41.085, p < 0.001, η² = 0.733). Inter-limb asymmetry was greater in BFlh than ST (p = 0.016) and varied across exercises (p = 0.037), with the largest deficits during the Nordic Hamstring Exercise, followed by the Glider and Diver. BFlh activation was reduced across all exercises (p ≤ 0.004), while ST activation was reduced during the Nordic Hamstring Exercise (p = 0.031) and Glider (p = 0.015), but not the Diver (p = 0.732).

Conclusions:

Elite male soccer players exhibit persistent inter-limb hamstring activation deficits after high-grade injury, despite return-to-sport clearance. Deficits are muscle- and exercise-specific, particularly affecting BFlh during high-demand eccentric exercises. These findings suggest current rehabilitation strategies may not fully restore neuromuscular function and support the potential future value of incorporating sEMG assessment to guide individualized return-to-sport decisions. Longitudinal studies are required to determine whether inter-limb activation deficits increase reinjury risk and to establish clinically meaningful inter-limb asymmetry thresholds.