(RxWiki News) When runners continue to run when they are exhausted, their running form changes, possibly increasing the risk for injury, according to findings published in the November issue of the Journal of Biomechanics.
The study was led by Tracy Dierks, assistant professor of physical therapy at Indiana University.
Dierks's research shows that runners generally demonstrate increased range of motion in the joints near the end of a running session. "Our study showed that at the end of a normal run, when they were getting tired, their mechanics were beginning to change," says Dierks. "When you notice fatigue, you're most likely putting yourself at increased risk for injuries if you continue because it's more difficult to control the motion ranges."
Dierks notes that excessive hip, knee, and ankle motion is often associated with overuse injuries. It becomes harder for the muscles, tendons, and ligaments to handle the strain forces related to running because of the extra motion. Knee injuries such as patellofemoral pain syndrome and iliotibial band syndrome, as well as plantar fasciitis in the foot, can result of the extra motion and stress.
The study, which involved 20 uninjured recreational runners between 18 and 45 years of age, fitted each subject with neutral running shoes and tracking markers around the pelvis, along one leg, and on a single foot. Each runner ran at least 10 miles per week and none wore orthotics. The subjects ran on a treadmill until they either reached 85 percent of their maximum heart rate or scored 17 out of 20 on the rating of perceived exertion (RPE). Other studies have shown that fatigue is indicated by RPEs of 13 to 15. In this study, all runners reported a minimum RPE of 15 by the end of their runs.
Dierks found subtle changes in the runners' lower extremities. However, the change was more severe in the rearfoot, where there was a "complete breakdown of mechanics."
For some time, researchers and runners alike have asked when runners should stop running in order to avoid injuries. Dierks' study provides some insight into this question by showing that RPEs of 15 or higher indicate an undesirable change of body mechanics.