Exercise and training are usually only considered in a positive manner, but acute exercise and chronic training are both stressors.
A stressor is defined as “…any activity, event, or impingement that causes stress.” (Plowman & Smith, 2011, p.20). External stress leads to stimulation of the adrenal gland to release cortisol and adrenaline. Together these increase heart rate, blood pressure and the availability of energy to the muscles.
This response disrupts normal body homeostasis and this elevation of processes is observed during a period of exercise. The degree and duration of elevation is dependent upon the intensity and duration of the exercise. After the stressor is removed or the period of exercise is over the body attempts to regain homeostasis.
Typically, at this stage, individuals experience some degree of fatigue or reduced capacity to respond to the stimulation (i.e. exercise), accompanied by a feeling of tiredness. This fatigue is temporary and is readily reversed with proper rest and nutrition.
Training programmes are made up of a series of acute periods of exercise organised in such a way as to provide an overload on the body leading to a stress response, followed by recovery processes that not only restore homeostasis but also encourage supercompensation or adaptation (changes that occur in response to an overload) (Kenttä & Hassmén, 1998; Kuipers, 1998; O’Toole, 1998).
In this sense, the goal of a training programme is to alternate the exerciser between overload and adaptation and to avoid plateau, retrogression and/or reversibility. Progress in fitness is achieved by the cyclical interaction between adaptation and progression (change in overload in response to adaptation).
Theoretically, each progression of the overload should allow for adaptation, however, this is not always accomplished.
 The ability of the body, or a cell, to seek and maintain a condition of equilibrium or stability within its internal environment when dealing with external changes
Plowman, S.A. & Smith, D.L. (2011) Exercise Physiology for Health, Fitness, and Performance. 3rd ed. London: Lippincott, Williams and Wilkins.