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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.
The most common symptom is fatigue. This may limit workouts and may be present at rest. The athlete may also become moody, easily irritated, have altered sleep patterns, become depressed, or lose the competitive desire and enthusiasm for the sport. Some will report decreased appetite and weight loss. Physical symptoms include persistent muscular soreness, increased frequency of viral illnesses, and increased incidence of injuries.
There have been several clinical studies done on athletes with the overtraining syndrome. Exercise physiologic, psychological, and biochemical laboratory testing have been done. Findings in these studies have shown decreased performance in exercise testing, decreased mood state, and, in some, increased cortisol levels -- the body's "stress" hormone. A decrease in testosterone, altered immune status, and an increase in muscular break down products have also been identified. Medically, the overtraining syndrome is classified as a neuro-endocrine disorder. The normal fine balance in the interaction between the autonomic nervous system and the hormonal system is disturbed and athletic "jet lag" results. The body now has a decreased ability to repair itself during rest. Heaping more workouts onto this unbalanced system only worsens the situation. Additional stress in the form of difficulties at work or personal life also contributes.
It appears that there are two forms of the syndrome. The sympathetic form is more common in sprint type sports and the parasympathetic form is more common in endurance sports. The results from various measurements taken during exercise physiologic testing differ between the two forms, but decreased overall performance and increased perceived fatigue are similar. In the parasympathetic form there may be a lower heart rate for a given workload. Athletes training with a heart rate monitor may notice that they cannot sustain the workout at their usual "set point." Fatigue takes over and prematurely terminates the workout. Regulation of glucose can become altered and the athlete may experience symptoms of hypoglycemia during exercise.
http://www.rice.edu/~jenky/sports/overtraining.html