On a general level, deconditioning is a gradual physiological process whereby the level of physical conditioning and corresponding physical performance ability of the individual is reduced. "Out of shape" is the common expression used to convey this meaning. On a more sport-specific level, deconditioning is sometimes used as a planned period of inactivity to achieve a specific physiological end.
Two vastly different circumstances are the best examples of the deconditioning process. When astronauts and scientists work for extended periods in the manned stations that function in the weightless environment of space, the most common observable physical phenomenon in the space station inhabitants is a pronounced loss of muscle mass, reduced cardiovascular function, and reductions in bone density. The space stations have no facility where the residents can use their muscles against resistance, the counterforce required to build muscle. The decline in cardiovascular function stems from the same reason. The loss of physical fitness is unrelated to the otherwise sound nutritional practices observed by these space travelers.
Persons who have been bedridden with an illness for a significant time are subject to the same types of physiological deterioration as the astronauts. Muscles that are inactive and that are not required to perform the usual baseline functions associated with posture and support of the musculoskeletal system will decline over time. The process of deconditioning as it applies to overall general fitness ranges from a decline from peak fitness to a loss of competitive fitness, to a sedentary, non-athletic life, to a physically reduced condition where previously healthy body functions are at risk in the face of stress of any kind.
The natural athlete, who is not required to train to compete on a high level, is more myth than reality. There are individuals who are fortunate enough to have either superior genetics, such as a higher-than-normal base metabolic rates (BMR), or tend to stay more naturally slim than other persons. Certain individuals have other natural qualities, such as balance and hand-eye coordination, which are available for use in what are otherwise deconditioned bodies. The rules regarding the preservation of muscle structure, bone density, and cardiovascular fitness are dependent on the exercise of each of these essential bodily systems to maintain strength and fitness.
Deconditioning often occurs as the result of a cycle that begins with an injury, when the athlete does not maintain a level of physical activity through the period of recovery. Although the period within which deconditioning will occur varies from athlete to athlete, a measurable loss of muscular and cardiovascular fitness will be observed in most athletes after a period of inactivity of 14 days or longer. After the inactive period, it is common for the athlete to return to training at a level similar to that which he or she was accustomed at the peak level. When this approach is adopted on the athlete's return to training, the athlete is exposed to a greater risk of injury due to the reduced physical capabilities created by deconditioning. The new injury will often trigger a further spiral of inactivity and physical reduction. In such circumstances, the athlete must be firmly directed to return to training at a very reduced level in comparison to any previous training standards, to undertake a slow and steady rebuilding process.
In bodybuilding and strength training, deconditioning is used in a strategic manner. Strength training principles with respect to the building of muscle mass are founded on the principle of overload, in which the muscles are sufficiently stressed to produce micro-tears in the muscle fiber as a result of regular workouts. The body will naturally repair this damage with the production of myoblasts, the muscle cells that form in the areas of cellular damage. The repair cells ultimately produce a bigger and stronger muscle.
Strength training research suggests that when the athlete has worked in accordance with a focused strength program, a structured downtime period at the end of the training cycle, which may extend from approximately five to ten days, will prove beneficial to ultimate muscle gain. The muscles at the end of the scheduled deconditioning will retain all of the myoblasts produced by the body to repair the micro-tear tissue, while preserving the ability to sustain further tears to generate new myoblasts.
Deconditioning has also proven beneficial in the control of heart arrhythmia in athletes. Arrhythmia is a condition that can cause heart rate to accelerate out of control, leading to heart failure and death if first aid—in the form of defibrillation—is not immediately available. Arrhythmia is a condition that is both genetic in origin as well as presenting as a factor contributing to the overdeveloped heart muscle wall observed in some elite-level athletes. Deconditioning periods of approximately nine weeks, followed by a gradual return to athletic training, have proven to reduce the incidence of arrhythmia reoccurrence.