Hamstring injuries are among the most common injuries in sports, particularly in activities involving sprinting and rapid acceleration. They account for roughly 10% of injuries in field-based sports and often result in significant time away from competition. Despite their frequency, the mechanisms underlying the effectiveness of certain training programs in preventing these injuries remain poorly understood.
Researchers from The University of Queensland and the University of Southern Queensland, in collaboration with Stanford University, investigated how nine weeks of Nordic hamstring exercise training affects the hamstring muscle function. Published in the Journal of Sport and Health Science on March 19, 2026, the study examined how this widely used injury-prevention exercise alters the mechanics of the biceps femoris long head—the hamstring muscle most commonly injured during sprinting.
"Previous research has shown that exercises such as the Nordic hamstring exercise are very effective at reducing hamstring injury risk," said first author Dr. Max Andrews, who conducted the study as a visiting researcher at Stanford and is now a postdoctoral fellow at The University of Queensland. "These exercises target the lengthening phase of muscle contraction and are known to increase eccentric strength and muscle fascicle length. However, we still don't fully understand how these structural adaptations alter the muscle mechanics during exercise to produce this protective effect, which is what motivated this study."
To investigate these changes, the researchers had participants complete nine weeks of supervised Nordic hamstring exercise training. During the training, researchers measured hamstring strength and used ultrasound imaging to track the behavior of muscle fibers, while motion capture was used to estimate changes in the length of the entire muscle–tendon unit. The researchers also estimated sarcomere lengths—the microscopic contractile units responsible for producing muscle force—by combining fascicle length measurements obtained during the exercise with previously measured serial sarcomere numbers.
After nine weeks of training, eccentric knee flexor strength increased by about 40%, allowing participants to control the Nordic hamstring exercise through a greater range of motion. During the exercise, after training participants could lean further before reaching peak force. As a result, the hamstring muscle–tendon unit reached longer lengths during the movement, while the muscle fibers themselves reached lengths about 25% greater than before training. Despite these longer fiber lengths, the estimated lengths of the sarcomeres did not change.
According to the researchers, this finding is consistent with the addition of sarcomeres in series within the muscle fibers—an adaptation for which previous work from the same group has provided evidence. By adding sarcomeres end-to-end, muscle fibers become longer while each individual sarcomere continues to operate near its optimal length during contraction. This structural adaptation allows the muscle to generate force effectively even when stretched to longer lengths, such as during sprinting.
"Following training, the muscle fibers can stretch to longer lengths during the exercise without overstretching the sarcomeres," said senior author Dr. Patricio Pincheira, from the University of Southern Queensland. "This may be one reason why eccentric training is effective at reducing hamstring injury risk. By increasing fiber length through serial sarcomere addition, the hamstrings can generate high forces across a wider range of muscle lengths, which may allow them to stretch further without overstretching."
Previous findings show that hamstring strains often occur during the late swing phase of sprinting, when the muscles lengthen rapidly while producing high forces. If muscle fibers can tolerate longer lengths without overstretching the sarcomeres, they may be better able to withstand these demands.
"One of the long-standing questions has been why the Nordic hamstring exercise is effective at reducing injury risk," said Dr. Pincheira. "Our findings suggest that the muscle adapts in a way that allows it to generate force at longer lengths, which may help the hamstrings tolerate the large mechanical demands placed on them during dynamic movements."
The findings help bridge the gap between laboratory measurements of muscle structure and the real-world effectiveness of hamstring injury-prevention programs. A better understanding of muscle adaptation to eccentric training could help refine exercise prescription in practice. This insight could ultimately enable coaches and clinicians to develop more targeted and effective strategies to reduce one of the most persistent injuries in sport.
