The state of exhaustion is one that is a common occurrence in all forms of athletic performance. It is a description that is intended to reflect a final, often dramatic, result of one or more bodily processes on the brink of failure. Where there is exhaustion, there must be an extreme level of fatigue, to the point where relief must be sought by the athlete or a catastrophe will invariably follow.

Exhaustion is a term employed in three distinct contexts in sports science. Physical exhaustion is the expression used to describe either musculoskeletal fatigue or a general inability to physically continue to perform at the desired level due to all energy stores having been consumed. Physical exhaustion is most common in those sports where the activity occurs over a longer period of time, as in distance events of all types; it may also arise through prolonged training for shorter duration events. Mental exhaustion is the loss of mental keenness. Mental fatigue can occur during an event, such as an endurance race, but more commonly this state occurs in a cumulative fashion, due to factors such as the pressure of high level competition or the stress imposed upon the athlete through daily training sessions. Terms such as "burnout," "staleness," and "brain-fag" are expressions of mental exhaustion. Heat exhaustion is a subset of physical exhaustion, but as it arises in specific environmental circumstances, it has a separate and well-developed set of physical indicators.

Reggie Evans of the Seattle Supersonics flops on the ground in exhaustion during a game.

Physical exhaustion is a condition that is most commonly revealed by extreme fatigue on the part of athletes, where they are no longer physically capable of performing at their accustomed level. As physical exhaustion typically occurs in endurance sports, it is the aerobic energy system that is central to an examination of the mechanics of this condition. When the body requires energy for activities lasting longer than approximately 90 seconds, it will fuel itself through the production of the energy source adenosine triphosphate (ATP), using available stores of glucose. ATP is produced as the culmination of a process whereby the bodily carbohydrate stores, glycogen, are converted to glucose and transported through the red blood cells of the bloodstream to the muscles where the ATP conversion occurs. The red blood cells also transport the oxygen required to metabolize, or burn, this fuel; the blood also removes the waste products and carbon dioxide produced in this process.

The simplest and most common form of physical fatigue is when the body simply runs out of the primary sources of carbohydrate required to manufacture energy in the form of ATP. When the body determines that it has no more glycogen available to it (the liver regulates the level of these sugars present in the bloodstream), it will revert to the consumption of stored fats to convert into energy sources. Fats are a comparatively lesser, more inefficient fuel for energy production. As with any machine, when the fuel sources are spent, the body cannot continue to perform. An inability to produce energy does not only affect the muscles and other working components of the body, but also the functioning of the brain and the central nervous system; a depletion of physical energy stores will cause significant reductions in concentration and mental function.

Absent any other physical factors contributing to the physical exhaustion, such as extreme cold or altitude, this circumstance will be corrected through rest and the ingestion of appropriate carbohydrate-rich foods to redress the bodily balance. The other most common potential causes of physical exhaustion in an athlete, occurring either singly or in combination with other factors, include: illness (such as cancer); poor long-term nutritional habits (such as lacking vitamins or minerals necessary to the function of the energy systems); mental stress; environmental condition (e.g., air pollution); and dehydration (when the fluid level of the body is reduced, the volume of fluid in the bloodstream is correspondingly less).

Physical exhaustion is also an expression used to describe the testing processes used to calculate performance measures such as VO2max, the maximum amount of oxygen that an athlete can process, which is a powerful indicator of endurance sport fitness. Physical exhaustion is also the stated limit to carbohydrate depletion tests and interval training of all types. The immediate, short-term athletic goal in each of these mechanisms is to train to physical exhaustion; the long-term objective is to extend the prior physical limits.

Mental exhaustion can arise in a number of circumstances in relation to both training and competitive circumstances. Professional team sport athletes who are required to play a number of games over a period of weeks will often complain of a lethargy and lack of motivation. Hard training, especially when the individual components are repetitive, can occasionally result in a similar mental fatigue.

Heat exhaustion a progression in the overheating of the body known as hyperthermia. When the body is working, especially in warm or humid conditions, it cools itself by forcing warm blood to the surface of the skin, which results in the production of perspiration, which in turn both dehydrates the system and depletes the body of the mineral sodium. The symptoms of heat exhaustion are severe thirst, generalized weakness, and a loss of coordination (due to reduced mineral levels, which aid in the transmission of nerve impulses to the muscles). The next stage in this progressive heat illness is a heat stroke, which may result in cardiac arrest and death. A notable fatality due to heat stroke was that of Korey Stringer, National Football League (NFL) lineman, in 2001 during a hot weather training camp session.

SEE ALSO Acclimatization; Exercise recovery; Fatigue; Muscle glycogen recovery.