Robustness of hamstring muscle activation strategies following selective hypertrophy induced by Nordic hamstring curl and stiff-leg deadlift exercises

This study aims to determine whether muscle activation distribution between hamstrings is modified after nine weeks of two resistance training programs that induce a selective muscle hypertrophy. Using a blinded randomized controlled design, thirty-six resistance-untrained individuals were assigned to one of three groups: control (CON), Nordic hamstring exercise (NHE), and stiff-leg deadlift (SDL) groups. Strength gain was measured as changes in one-repetition maximum (1RM). Changes in semimembranosus (SM), semitendinosus (ST), and biceps femoris (BF) muscle volume were measured using 3D freehand ultrasound. Activation of each hamstring muscle head was assessed using surface electromyography during the trained exercise (or both for CON) performed at 80% of 1RM. We found a significant increase in 1RM after nine weeks for the NHE (37.4 ± 13.8%) and SDL (34.0 ± 21.2%) groups compared to CON. This strength gain was accompanied by selective hypertrophy of ST (24.3 ± 10.8%) and SM (11.2 ± 12.7%), for the NHE and SDL groups, respectively. However, statistical parametric mapping analyses revealed that the muscle activation was not altered over the time, between the groups, or by their interactions (all p ≥ 0.05). Our findings demonstrate a robustness of muscle activation strategies over time despite training-induced selective hypertrophy. These results provide a deeper understanding of the complex interplay between neural drive and muscle mechanical characteristics. This provides additional impetus to study long term effect of activation strategies (e.g., on the development of musculoskeletal disorders), as they seem to represent a trait-like characteristic rather than a transient state.