But running can come with risk of injury and that's the quickest way to derail any workout plan.
A new study by Tracy Dierks, assistant professor of physical therapy at Indiana University, found runners who continue running when they are exhausted unknowingly change their running form, which could be related to an increased risk for injury.
Testing runners
Dierks studied 20 uninjured recreational runners between the ages of 18 and 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.
Each runner ran on a treadmill until he or she either reached 85 percent of their 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.
Resulting injuries
Dierks said the changes throughout the lower extremities were subtle but more severe in the back of the foot, or rear foot (the ankle, the heel, and surrounding tissue), where there was a "complete breakdown of mechanics."
Toward the end of a normal running session, Dierks noted the runners generally displayed an increase in motion in their hips, knees and ankles.
Excessive range of motion in the joints is generally 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 ("Runner's Knee") and plantar fasciitis at the foot ("heel spur").
How much is too much?
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.
So how can injury be avoided? Dierks recommends taking things slowly.
"For all runners, I think it is pretty well known in the research world that overuse injuries are bound to happen if you 'ramp up' your mileage too fast over a short time period. A good example is the recreational runner (5-10 miles per week) who decides to train for a marathon and ramps up their mileage to 25-30 miles per week in a matter of a couple of weeks. So for new runners, my advice is to increase weekly mileage at a slow and gradual rate," said Dierks.
Dierks also recommends cross training -- substituting another physical activity for a day between runs to help the body adjust and avoid an overuse injury.
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.
