Credit: iStock

“That day, for no particular reason, I decided to go for a little run. … For no particular reason I just kept on going. I ran clear to the ocean. And when I got there, I figured, since I’d gone this far, I might as well turn around, just keep on going.” – Forrest Gump

The feats of ultra-endurance athletes are remarkable and sometimes incomprehensible. There are few published data on how the body deals with the stresses of an ultra-endurance event because the fieldwork is difficult to perform without interfering with the athlete’s performance. A new study published in the Journal of Applied Physiology explores how muscle deals with prolonged exercise performed in an “unfriendly” environment (high mileage at high altitude). The primary question these researchers wanted to know: In an event that requires a lot of energy, would the muscles have enough energy to rebuild and adapt to the stress?

The researchers studied a single cyclist during a mountain bike race that spanned 497 miles from Denver to Durango, Colorado, along the high-altitude Colorado Trail. The participant rode 19 to 20 hours each day for five days. The investigators took muscle and blood samples at the beginning and end of the race and compared the changes to a period of normal exercise training.

They found that during the race, the muscles were able to make mitochondria—which are responsible for producing energy in the cells—at an extremely high rate. In addition, the mitochondria increased their ability to use fat energy sources, an important adaptation for long-term exercise. The proteins that contract the muscle also continued to build, but not at a rate fast enough to maintain the muscle’s size, which led to the muscle shrinking in size. In addition, there was significant muscle damage and inflammation. The blood samples also showed evidence of significant stress with changes that were consistent with impaired kidney and liver function.

Overall, this study suggests that when the body is performing an exceptionally, energetically challenging activity, muscle is able to rebuild at an extremely high rate, although maybe not enough, to try to adapt to its new demands. In this case, the extreme stress of the race caused significant muscle damage and organ dysfunction. Scientists hope to continue to find new approaches to study the demands of ultra-endurance athletes to better understand the limits of human performance.

Benjamin Miller, PhD, is an associate professor in the department of Health and Exercise Science at Colorado State University. He co-directs the Translational Research in Aging and Chronic Disease (TRACD) Laboratory with Karyn Hamilton, PhD.