News

By Sarah C.P. Williams

When athletes perform heavy resistance exercise, their muscles aren’t the only tissues that get stronger. People who regularly carry out eccentric exercises also have more resilient tendons.

In eccentric exercises, muscles work to control movement as they stretch rather than shorten—like lowering yourself down from a pull-up or controlling the descent of a bicep curl. These exercises create especially high forces at the junction between muscles and tendons.

Now, research funded in part by the Wu Tsai Human Performance Alliance has helped explain how this type of exercise directly impacts tendons. The study, published in American Journal of Physiology-Cell Physiology, revealed that within just four hours of a workout, there are rapid changes to the cells within tendons and to the areas where tendons connect to muscles. The changes, which include flipping on and off certain genes, could be key to understanding how eccentric exercise ultimately strengthens tendons.

“We knew that lifelong athletes who have done lots of these exercises tend to have thicker tendons and that athletes who do this type of exercise regularly are less likely to get tendon injuries,” said cell biologist Ching-Yang Chloé Yeung of the Institute of Sports Medicine Copenhagen, who led the study. “But we really didn’t know how this was happening at a molecular and cellular level.”

To shed light on this, Yeung’s team took advantage of a unique opportunity: collecting healthy tendon tissue from twelve patients undergoing ACL reconstruction surgery at Copenhagen’s Bispebjerg Hospital. Half the patients performed an intense, supervised bout of eccentric hamstring exercises—including straight-leg deadlifts and Nordic hamstring curls—four hours before surgery, while the other four simply waited for their procedure. Then, surgeons gave a small sample of each person’s tendon and the surrounding tissue (already routinely removed during the surgery) to Yeung.

Yeung and her colleagues analyzed more than 61,000 individual nuclei from the harvested tissue, identifying which genes were turned on or off in people who did or didn’t exercise before the surgery. They then used specialized microscopy to map exactly where different cell types were located within the tissue.

Surprisingly, tendon cells themselves were not dramatically different between patients who had exercised and those who hadn’t. However, cells in two other key tissues did change: the muscle-tendon junction (MTJ) where tendons attach to muscles, and the interfascicular matrix (IFM)—a gel-like substance between tendon fibers.

In people who had carried out the exercises, IFM cells produced higher amounts of lubricin and versican—proteins that help tendon fibers slide smoothly and resist compression. At the same time, muscle fibers at the MTJ, precisely where most hamstring strain injuries occur, made more of a specialized collagen protein called COL22A1.

“The cells in both of these areas are in higher-turnover areas and, during eccentric exercise, may feel more stress and get damaged more quickly than the tendon cells,” said Yeung. “That may explain why we’re seeing immediate changes to them with exercise rather than directly to the tendon fibroblasts which are more protected.”

She also noted that both the IFM and MTJ are prone to injuries; hamstring strains, for instance, often occur right at the junction with tendons, and the gel-like IFM is known to undergo changes in people with tendinopathy—chronic pain and inflammation of a tendon.

More work is needed to fully understand the link between these rapid responses and the structural changes that build stronger, more injury-resistant tendons over time.

“There are still unanswered questions about what happens between these initial cellular changes and the long-term protection from tendon injury that exercise can convey,” said Yeung. “For now, it’s exciting to have this first detailed look at how these cells change in response to a bout of exercise.”

Understanding these cellular mechanisms could eventually help clinicians design better injury prevention and rehabilitation programs.

Yeung, with continued support from the Wu Tsai Human Performance Alliance, is now planning studies probing how the IFM cells may respond differently after exercise in people with tendinopathy. She also wants to follow the cells for longer periods of time in people who regularly carry out eccentric exercises.