Runners who continue running when they are exhausted unknowingly change their running form, which could be related to an increased risk for injury. A study by Tracy Dierks, assistant professor of physical therapy at Indiana University, found that toward the end of a normal running session, runners generally displayed an increase in motion in their hips, knees and ankles.

"Our study showed that at the end of a normal run, when they were getting tired, their mechanics were beginning to change," Dierks said. "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 said an excessive range of motion in the joints generally is associated with overuse injuries. The extra motion makes it harder for the muscles, tendons and ligaments to handle the strain forces related to running.

Common overuse injuries in runners are patellofemoral pain syndrome and iliotibial band syndrome at the knee and plantar fasciitis at the foot. Dierks' study, "The effects of running in an exerted state on lower extremity kinematics and joint timing," was published in the November Journal of Biomechanics. Co-authors include Irene S. Davis, University of Delaware and the Drayer Physical Therapy Institute; and Joseph Hamill, University of Massachusetts.

About the study:

The study involved 20 uninjured recreational runners ages 18-45. None wore orthotics and each ran at least 10 miles per week. The women and men were fitted with neutral running shoes and tracking markers were placed around their pelvises and along a single leg and foot on each runner.

The runners ran on a treadmill until they either reached 85 percent of the subject's heart rate maximum or a score of 17 (out of 20) on the rating of perceived exertion (RPE). By the end of their runs, all of the runners reported an RPE of at least 15 -- studies have shown that RPEs between 13-15 indicate fatigue. Dierks said the changes throughout the lower extremities were subtle but more severe in the rearfoot, where there was a "complete breakdown of mechanics.

"Dierks said runners and scientists for years have pondered when runners should stop running in order to avoid injuries. Runners' RPEs could provide some answers, with RPEs of 15-17 indicating runners' have reached a point where their mechanics have likely begun to change in an undesirable way.

Source: Indiana University