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

This study aimed to determine whether muscle activation distribution between hamstrings is modified after 9 wk of two resistance training programs that induce selective muscle hypertrophy. Using a blinded, randomized, controlled design, 36 resistance-untrained individuals were assigned to one of the three groups: control (CON), Nordic hamstring exercise (NHE), or stiff-leg deadlift (SDL). 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 three-dimensional (3-D) 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 9 wk for the NHE (37.4 ± 13.8%) and SDL (34.0 ± 21.2%) groups compared with 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 muscle activation was not altered over time, between the groups, or by their interactions (all ≥ 0.05). Our findings demonstrate the 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 effects 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. We demonstrate that stiff-leg deadlift and Nordic hamstring exercises are effective in inducing selective hypertrophy of the semimembranosus (11.2%) and semitendinosus (24.4%), respectively. Hamstring muscle activation did not adapt to the change in the distribution of muscle volume. These resistance training exercises, commonly used in hamstring prevention and rehabilitation strategies, appear effective at increasing the force-generating potential of the targeted muscles in noninjured individuals, as their muscle volume increases without altering their activation strategies.