Free radicals have been linked to health problems including cancer, heart disease, premature aging and other maladies. On a more mundane level, it's long been known that overuse can cause wear and tear to the muscles, joints and connective tissue. Unfortunately, they pay for it in the long run with continuing aches and pains that may cut their athletic careers short and plague them for the rest of their lives. People, of course, differ in their tolerance to exercise. Some require more rest than others for recovery to occur.
Adequate rest means allowing exercised tissue to recover.
Again, the key is to listen to your body. Don't push beyond your individual tolerance, and then rest until you are fully recovered. In addition, the authors of Exercise and Health recommended strength training as a preventive measure. The advice offered in Exercise and Health is still as good as gold, but we've learned more about the potential risks of training hard too often. The latest information on the physiological, biomedical and psychological aspects of overreaching is collected in Overtraining in Sport , published by Human Kinetics in 1998.