A stress fracture can occur when you ignore nagging pain symptoms. It's an overuse injury the can result when muscles become fatigued and are unable to absorb added shock. Eventually, the fatigued muscle transfers the overload of stress to the bone causing a tiny crack called a stress fracture.

In runners, stress fractures in the foot are common -- also in the lower legs. To a runner, a stress fracture means 6 to 8 weeks of non-negotiable recovery time. That means no running -- none. Period. If you run on a stress fracture, you can cause permanent damage, so adequate rest and healing time is vital.

Stress fractures are one good reason -- a very good reason -- to incorporate rest days into your fitness routine.

Read everything you ever want (and need) to know about stress fractures here.

The bottom line? Listen to your body...running on fatigued muscles can cause long-term damage. Better to take a day or two off than 8 weeks.