Neuromechanical Determinants of Muscle-Specific Hamstring Damage and Hypertrophy across Exercises: An EMG-Informed Modeling Approach.

PURPOSE: Even under identical training conditions, synergistic muscles such as the hamstrings can adapt differently, with the most affected muscle varying across exercises. The aim of this study was to determine how differences in muscle alterations and adaptations (muscle damage in the acute experiment and muscle hypertrophy in the chronic experiment) are governed by neuromechanical features of exercises. METHODS: Neuromusculoskeletal modelling was used to estimate semimembranosus (SM), semitendinosus (ST) and biceps femoris long head (BFlh) activation, operating length, and force during the isokinetic seated leg curl (ISO, only in acute experiment), Nordic hamstring exercise (NHE) and stiff-leg deadlift (SLD). Muscle damage and hypertrophy were quantified from changes in shear modulus and muscle volume, respectively. RESULTS: In the acute experiment, the ST exhibited the largest shear modulus increase after ISO (+62.2%) and NHE (+33.0%). No significant difference in shear modulus was found between SM (+16.0%), ST (-9.0%) and BFlh (+11.4%) after SLD. In the chronic experiment, hypertrophy was rather localized to the ST in NHE (+24.4%) and to the SM in SLD (+11.2%). For ISO (acute) and NHE (acute and chronic), exercise-induced adaptations were consistent and correlated with greater modeled operating length and force of the ST. However, for SLD, the slightly greater SM adaptation was not explained by between-muscle differences in force. Predictive models identified the most damaged muscle in 82% of cases and the most hypertrophied muscle in 75%. CONCLUSIONS: This study indicates that differences between hamstring heads in modeled muscle length, and to a lower extent muscle force, explain muscle-specific damage and hypertrophy induced by knee flexion exercises, but not during SLD. Overall, EMG-informed musculoskeletal modeling provides a promising framework to better under the role of neuromechanical parameters in muscle-specific adaptations.