If you are exercising for health and fitness, and not training for a specific sport, you may be more susceptible to overuse injuries or stress-related or repetitive-motion injuries. These injuries usually result when joints or soft tissue (muscles, tendons, ligaments or bursae) become irritated and inflamed. Another common and bothersome overuse injury is a stress fracture, a tiny crack in a bone that has been repeatedly stressed during exercise.
Overuse injuries commonly occur when a person does an unusually large amount of exercise, stressing tendons and joints that haven't had a chance to adapt to the new workload. Whereas your muscles can adjust to new demands fairly easily, the fibrous tissue in tendons and joints adapts more slowly, increasing in size and strength gradually over weeks and months. To avoid overuse injury, you must give these soft tissues time to toughen, even if your muscles are urging you to go farther and faster.
Avoid the temptation to "work through" joint or tendon pain. The effort of exercise may mask the pain temporarily, but overuse injuries will only get worse over time if you ignore them.