• Increased torque capacity enabled peak torque at more extended knee angles and longer muscle–tendon unit lengths during the Nordic hamstring exercise after training. • Eccentric training enabled the biceps femoris long-head to operate at longer active fascicle lengths throughout the exercise. • Despite longer fascicle lengths at peak torque after training, estimated active sarcomere lengths during the exercise were unchanged due to previously reported serial sarcomere addition. Prolonged Nordic hamstring exercise (NHE) training reduces hamstring injury risk, but the underlying mechanisms remain unclear. This study examined how NHE training alters biceps femoris long-head (BFlh) muscle–tendon unit (MTU) mechanics during the exercise, focusing on how muscle adaptations influence fascicle and sarcomere lengths during the NHE after training. Twelve recreationally active participants completed 9 weeks of supervised NHE training and 3 weeks of detraining. Knee flexor torque was measured using ankle-mounted load cells and estimated from the knee joint moment arm. BFlh fascicle lengths were assessed during the NHE using dual-transducer ultrasound, and MTU length was estimated from joint angles using markerless motion capture. Sarcomere lengths during the NHE were estimated using instantaneous BFlh fascicle length and previously calculated serial sarcomere numbers. Linear mixed-effects models analyzed changes between pre-training, post-training, and post-detraining. Nine weeks of NHE training increased peak knee flexor torque by 40% (previously reported), enabling 37% greater knee extension ( p = 0.012) and 4% longer MTU lengths at peak torque during the NHE ( p = 0.001). BFlh fascicles reached 25% longer lengths during the NHE at peak torque after training ( p < 0.001), while estimated active sarcomere lengths at peak torque were unchanged ( p = 0.266). Following 3 weeks of detraining, knee flexor torque decreased by 6% (previously reported) and fascicle lengths at peak torque decreased by 14% ( p < 0.001), while MTU length ( p = 0.578) and estimated active sarcomere length ( p = 0.753) at peak torque during the NHE did not significantly change compared to post-training. Despite the longer fascicle lengths during the NHE after training, estimated active sarcomere length during the NHE relative to optimal sarcomere length remained similar, consistent with the addition of sarcomeres in series, which may contribute to the protective effects of NHE training.
Andrews et al. (Sun,) studied this question.