Neuromuscular contributions to inter-limb quadriceps strength asymmetries across contraction types in healthy adults

PURPOSE: This study investigated the contribution of neuromuscular mechanisms to quadriceps strength asymmetry across isometric, concentric, and eccentric contractions by examining the covariation between strength asymmetry and asymmetries in voluntary activation and doublet-evoked torque. METHODS: Fifteen healthy participants performed maximal isometric and anisometric contractions (30°/s) of both lower limbs on an isokinetic dynamometer. Side-to-side differences in torque, electromyographic (EMG) activity, voluntary activation, and doublet-evoked torque were assessed by comparing the stronger and weaker limbs. Associations between asymmetries in torque, voluntary activation, and doublet-evoked torque were subsequently examined. RESULTS: A significant difference between the stronger and weaker limbs was observed for torque across all contraction types (162.1 and 145.6 N.m, respectively), whereas no between-limb differences were found for neuromuscular parameters. Covariation analyses indicated that asymmetries in activation level (estimate = 0.57, p < 0.001) and doublet-evoked torque (estimate = 0.29, p = 0.006) were significant predictors of torque asymmetry, together explaining 29% of its variance (Marginal R²). In contrast, these relationships were not influenced by contraction type (p = 0.670). CONCLUSION: In healthy individuals, strength asymmetries covary positively with neuromuscular asymmetries. Given the absence of contraction-type effects, findings derived from isometric conditions likely generalize to anisometric contractions.