Hamstring injuries affect athletes across the globe, accounting for 10% of all injuries in field-based sports. Despite their frequency, the mechanisms behind these injuries and effective prevention strategies remain unclear. Findings from two recent studies offer new insights on hamstring injury prevention.
In the first study, researchers from the University of Queensland and Stanford University, in collaboration with Stanford Athletics, explored the effects of nine weeks of Nordic hamstring exercises - a popular training method aimed at preventing hamstring strains - on the muscle structure of 12 participants. Utilizing a novel microendoscopy technique developed at Stanford, the team measured the effects of training on human hamstrings at an unprecedented level of detail. The study is published in the Journal of Sport and Health Science.
"The truly unique part of this study was our ability to examine the microscopic adaptations of the muscle in response to Nordic hamstring exercises. No other studies have been able to explore this in humans," said Scott Delp, the James H. Clark Professor in the School of Engineering at Stanford University and senior author of the study, as well as director of the Wu Tsai Human Performance Alliance at Stanford.
Delp and Mark Schnitzer, a professor of biology and of applied physics in the School of Humanities and Sciences, previously developed a microscope capable of visualizing sarcomeres - the fundamental units within muscle fibers responsible for generating muscle force - in humans.
Using the microendoscope, the researchers observed significant changes in the biceps femoris long head muscle, one of the four muscles that comprise the hamstring, following the nine weeks of Nordic hamstring exercises. Notably, sarcomeres were getting added end-to-end to existing sarcomeres. This growth pattern lengthens the muscle fibers and could decrease the amount of stretch each sarcomere experiences, protecting muscle fibers from overstretching.
Some muscle areas experienced sarcomere increases up to 49%. However, both muscle fiber lengthening and sarcomere increase began to decline after a three-week break from training.
"Our findings, along with previous work from our group, highlight the importance of long-term training to induce meaningful adaptations in the hamstrings," said Max Andrews, first author and a graduate student at the University of Queensland. "While Nordic hamstring exercises are often used by coaches in short blocks, such as three to four weeks during pre-season, this may not be enough to induce the necessary adaptations we observed. And consistent training is essential to maintain these muscle changes."
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Researchers used the low-cost, smartphone-based OpenCap technology to capture motion data during their experiment on a natural running surface. | Zach Teplin
