Study: When to end a run

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.