“It is no secret among athletes that in order to improve performance you’ve got to work hard. However, hard training breaks you down and makes you weaker. It is rest that makes you stronger. Physiologic improvement in sports only occurs during the rest period following hard training. This adaptation is in response to maximal loading of the cardiovascular and muscular systems and is accomplished by improving efficiency of the heart, increasing capillaries in the muscles, and increasing glycogen stores and mitochondrial enzyme systems within the muscle cells. During recovery periods these systems build to greater levels to compensate for the stress that you have applied. The result is that you are now at a higher level of performance.
If sufficient rest is not included in a training program then regeneration cannot occur and performance plateaus. If this imbalance between excess training and inadequate rest persists then performance will decline. Overtraining can best be defined as the state where the athlete has been repeatedly stressed by training to the point where rest is no longer adequate to allow for recovery. The “overtraining syndrome” is the name given to the collection of emotional, behavioral, and physical symptoms due to overtraining that has persisted for weeks to months. Athletes and coaches also know it as “burnout” or “staleness.” This is different from the day to day variation in performance and post exercise tiredness that is common in conditioned athletes. Overtraining is marked by cumulative exhaustion that persists even after recovery periods.”
This comes from an article written by Mark Jenkins, MD in 1998. Here’s the full article.