General Adaptations to Exercise: Acute Versus Chronic and Strength Versus Endurance Training

Abstract

Exercise can be regarded as a biological stress. The body’s reaction to the stress of exercise is similar to how it reacts to other forms of stress. Muscle contractions disturb the internal cellular milieu during rest, and this elicits a variety of homeostatic responses. Examples of these responses include altered blood flow to the active muscles; increased heart rate; increased breathing rate; increased oxygen consumption; increased rate of sweating; increased body temperature; secretion of stress hormones such as adrenocorticotropic hormone (ACTH), cortisol, and catecholamines; increased glycolytic flux; and altered recruitment of muscles. These changes are transient and return to baseline levels after exercise. If exercise is repeated on several occasions, adaptations occur. Adaptations involve either remodeling of tissue or altered regulation of the central nervous system. The outcome of exercise-induced adaptations depends on the type of exercise, but either makes the muscle more resistant to fatigue, stronger, more powerful, or better coordinated. The exact type of adaptation is dependent on the overload stimulus. For example, the muscle contractions in a training session can range from relatively low effort (submaximal) to maximal effort. More specifically, endurance training consists of several thousand submaximal contractions per training session in contrast to resistance training, which consists of 10–30 high-intensity muscle (maximal) contractions per training session. When the training stimulus is removed, the adaptations slowly regress to the form they had before training. Exercise-induced adaptations have application for sporting performance, rehabilitation after injury, and treatment of disease.