Abstract
Aim: The Nordic hamstring curl appears effective in reducing the incidence of injury in physically active young adults, likely through its capacity as an eccentric exercise to increase muscle stiffness. Although eccentric exercises have been shown to increase muscle stiffness, medium- and long-term Nordic hamstring curl training programs have not demonstrated an effect on muscle stiffness. This study examined the acute effects of a single session of Nordic hamstring curls on the stiffness of the biceps femoris, semitendinosus, and semimembranosus muscles using ultrasound shear wave elastography, an accepted method for measuring passive muscle stiffness. Material and methods: Twenty physically active adults (ages 19-27 years) were randomly assigned to either the Nordic hamstring curl group (n = 10) or the control group (n = 10). Shear wave elastography was performed on the dominant kicking leg for both groups. The exact location of the probe was marked to ensure the same area was assessed during post-testing. Both groups performed a 5-minute cycle ergometer warm-up followed by three 30-second standing static stretches. The Nordic hamstring curl group then performed three sets of six repetitions of the eccentric phase of the Nordic hamstring curl with 1-minute rest intervals between sets. All subjects then rested for five minutes before shear wave elastography was performed. Results: Repeated measures ANOVA revealed no significant main effects or interactions for the biceps femoris or semitendinosus (p >0.05). However, analysis of the semimembranosus was inconclusive due to variability of measurement values. Conclusions: These results are in agreement with findings indicating that long- and short-term Nordic hamstring curl training has no impact on hamstring stiffness, although the effects of Nordic hamstring curl on reducing the probability of hamstring injury are still valid.