Dose-response characteristics of the Nordic hamstring exercise: Total impulse versus total repetitions.

Abstract

We clarified the relationship between the bilateral Nordic hamstring exercise (NHE) training volume and adaptation by incorporating total impulse, the time-integrated force across sessions. Twenty-one healthy males (42 legs) were randomly assigned to twice-weekly (TRA2), once-weekly (TRA1), or control groups for six weeks. NHE-specific (peak torque, break-point angle, eccentric torque) and non-specific outcomes (isometric torque, isotonic power, total work during fatigue task, jump height, muscle architecture and stiffness) were assessed pre- and post-intervention. Dose-response relationships were examined using linear mixed models with total impulse as a continuous covariate, alongside group-based comparisons. Group-based analyses showed significant improvements in all NHE-specific measures in both training groups, with greater gains in TRA2 than in TRA1 (d = 0.50-0.77). Several non-specific variables showed similar improvements without group differences (e.g. isotonic power: d = -0.06). There were significant time × total impulse interactions across both NHE-specific and non-specific outcomes. Combined with group-based findings of a larger biceps femoris (BF) stiffness reduction in TRA1 than in TRA2, these results suggest a non-linear relationship between total impulse and BF stiffness changes. Our results demonstrated that total impulse was a superior metric for quantifying the dose-response relationship of the NHE training compared to total repetitions.