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Overtraining is the description given to a particular athletic approach to the physical development required in a sport, when the training workload or volume of the athlete exceeds their present ability to perform. This general understanding of what represents excessive training is defined more comprehensively in the recognized sports medicine condition, overtraining syndrome.

Overtraining syndrome is a neuron-endocrine disorder, when the function of the hormones, the chemical messengers that stimulate a wide array of functions within the body, are disrupted. This condition leads to a wide range of physical, emotional, and behavioral consequences for the athlete. While the syndrome most commonly affects high-performance or elite-level athletes, overtraining can result in any circumstance where the athlete's current training workload exceeds present abilities. Overtraining can affect athletes in any sport.

Simply, overtraining is excess. No athlete has ever achieved competitive success without taxing the body in training to a point very close to the physical limit. Successful athletes are often intense, driven people who ignore certain physical signs of injury or other dangers to their health in the pursuit of their competitive goal. It is this athlete who most commonly falls victim to the overtraining syndrome.

Overtraining can also impair more recreational athletes, who through inexperience, may not understand what their body is telling them as they increase their training volume in pursuit of a personal goal. Overtraining syndrome occurs most often among recreational athletes who are training for a marathon or similar event.

Athletic training is founded on the fundamental principle of workouts followed by rest; as the workouts intensify, a rest interval consistent with both the intensity and the duration of the workout becomes crucial. In this cycle, the rest period is the means by which the body becomes stronger, because while at rest, the body restores the cardiovascular system and the glycogen stores to greater levels than existed prior to the workout. While at rest, the body also enhances the enzymes utilized by the mitochondria in each cell to produce adenosine triphosphate (ATP), the ultimate energy fuel source. If the body has sustained periods of insufficient rest, the restoration processes do not occur and the body is not able to repair itself.

When the balance between workouts and rest is not sustained, the body becomes fatigued, leading to a cumulative exhausted state that is the overtraining syndrome. Overtraining is distinct from the natural and day-to-day variables that occur in training; one or two poor workouts because the athlete feels tired may be a signal as to workout practices, but are not evidence of the overtraining syndrome.

The overtraining syndrome is also referred to as staleness or "burnout." It often presents with a number of physical and emotional indicators, including:

  • decreased performance levels, including an inability to properly complete regular workout assignments
  • rise in resting heart rate to 10% or greater than usual rate
  • depressed mood
  • heightened feelings of mental stress, often arising from performance expectations, or coaching or team pressures
  • reduced immune system function and greater vulnerability to infection; lymph glands (a component of the immune system) often swell; the production of lymphocytes, the cells manufactured by the immune system to fight infection, is reduced
  • lower testosterone function and reduced sexual desires
  • generalized muscle and joint pain
  • insomnia
  • gastrointestinal problems

The treatment to resolve an overtraining syndrome is rest. The amount of rest required will usually be proportionate to the period of overtraining; as an example, if the athlete has been overtraining for two months, he or she may require a two- to three-week period of rest before resuming training. The bodily systems most affected by the syndrome, especially the immune system, must be restored to full capacity or the syndrome will likely return. The return to training must be gradual, with careful attention paid to both the intensity and the volume of the workouts. When an athlete has been afflicted with the overtraining syndrome, a training log will often assist in reminding the athlete and any support personnel to keep the workouts within the physical capabilities of the athlete.

There are no pharmaceuticals or dietary supplements that will serve to cure overtraining syndrome. If in the course of the overtraining period the athlete had not been eating a balanced diet or otherwise consuming all necessary foods, vitamins or minerals, these points can also be addressed in the rest period. A plant extract, Eleutherococcus senticosus, has been tested in Eastern Europe as a potential aid to assisting the body in overcoming the fatigue associated with overtraining syndrome, with some success; it is not marketed commercially.

see also Carbohydrate stores: Muscle glycogen, liver glycogen, and glucose; Fatigue; Muscle glycogen recovery; Recurrent stress fractures.