Acute Effects of Nordic Hamstring Exercise on Hamstring Stiffness: A Randomised Controlled Trial

<b>Background</b>: This study aimed to examine the immediate effects of the Nordic Hamstring Exercise (NHE) on the stiffness of the biceps femoris (BF) and semitendinosus (ST) muscles. <b>Methods</b>: This parallel-group randomised controlled trial followed CONSORT 2025 guidelines. Twenty-four physically active adults (16 females, 8 males) were randomly assigned to an NHE group (<i>n</i> = 12) or a control group (<i>n</i> = 12) using 1:1 gender-stratified randomisation. The NHE group performed 3 sets of 10 repetitions of the NHE, while the control group remained inactive. Muscle stiffness of the BF and ST was assessed pre- and post-intervention using the MyotonPro device. <b>Results</b>: No significant overall changes in hamstring stiffness were observed within or between groups (<i>p</i> > 0.05). Within the NHE group, the ST tended to increase in stiffness (11.25 N/m, <i>p</i> = 0.057), while the BF showed a small, non-significant reduction (-12.00 N/m, <i>p</i> = 0.696). The difference in changes between BF and ST was significant (<i>p</i> = 0.039). Independent of group allocation, males demonstrated significantly higher baseline stiffness than females for BF (258.13 vs. 195.81 N/m, <i>p</i> < 0.001) and for ST (247.88 vs. 174.00 N/m, <i>p</i> = 0.003). Regression analysis showed that only height predicted the change in ST stiffness after NHE (R = 0.625, R² = 0.39, <i>p</i> = 0.030). <b>Conclusions</b>: A single NHE session did not alter overall hamstring stiffness but produced opposite, muscle-specific effects. More research with larger, uniform samples is needed to confirm these findings.