There are many hypothesised mechanisms by which exercise is thought to influence mental health. Several biological, psychosocial and psychological models and theories have been suggested that may help us to understand these proposed relationships. It is difficult to pinpoint any single mechanism that is likely to account adequately for the exercise and mental health relationship until further research specifically addresses each proposed explanation. Although there have been a number of suggested hypotheses for this relationship, research supporting physiological explanations has been weak, with many studies being poorly designed. Moreover, while evidence supporting psychologically based explanations are not conclusive they do seem intuitively appealing. Specifically, the mastery explanations have had consistent empirical support.
Hyperthermic model
A common explanation of affective change is a neuro-physiological model known as the hyperthermic model. This suggests that the primary stimulus for promoting affective change is the elevation in body temperature from exercise, and this alteration in physiological state is manifested in a number of responses that are orchestrated by the hypothalamus. The belief that elevating body temperature can be therapeutic is longstanding (e.g. saunas and steam room). Indeed, research has revealed that total body warming (e.g. warm showers) reduces muscle tension (deVries et al, 1968). Specifically, in relation to exercise, Horne & Staff (1983) concluded from their counterbalanced design that high intensity exercise and passive heating produced similar increases in slow wave sleep (relaxation effect) and that exercise may be a vehicle for these effects. Support for the hyperthermic explanation has been mixed and, in fact, there are many studies that have failed to demonstrate a positive relationship between exercise, increases in temperature and improved psychological states. It is now thought that there are more comprehensive models that explain the relationship between physical activity and psychological well-being more accurately.
Endorphin hypothesis
The endorphin hypothesis proposes that the effects of acute exercise on psychological well-being, in particular ‘euphoria’, is caused by the release and subsequent binding of endogenous opiods, these being -endorphins to receptor sites in the brain (Steinberg & Sykes, 1985). The endorphin hypothesis originated from early research on rat brain tissue that revealed significant increases in opiate receptor occupancy after the rats had been forced to exercise (Pert & Bowie, 1979; Wardlaw & Frantz, 1980). Christie & Chesher (1982) have also demonstrated that mice can become ‘swimming junkies' if they exercise regularly. Unfortunately, because of the inherent problems of examining -endorphin receptor site occupancy in humans, research has attempted to examine peripheral -endorphin levels (blood outside the blood–brain barrier) after exercise (see Synder, 1977). Furthermore, DeMeirlier et al (1986) reported a positive relationship between -endorphins and exercise, which they contended leads to more positive affective states.
Research by Farrell (1989) suggested that exercise does not alter the blood–brain barrier in a way that allows peripheral endorphins to act directly upon the barrier. In other words, endorphins have been measured in the circulation and not directly in the brain, and because the blood–brain barrier blocks the passages by which ‘opiate’ substances move from the blood to the brain it is difficult to test the endorphin hypothesis. Interestingly, despite very limited support, the endorphin hypothesis remains one of the most popular explanations of the psychological benefits of exercise.
Self-esteem and mastery explanations
There are a number of theoretical models that have attempted to consider the possible relationship between participation in physical activity and self-esteem. One such model is Sonstroem's (1978) psychological model for physical activity. This model assumes that involvement in physical activity increases physical ability, which positively influences one's self-estimation and consequently leads to higher levels of self-esteem. Specifically, as individuals with high self-esteem take pride in their bodies, they continue to exercise, thereby maintaining and/or increasing fitness. Additional physical activity leads to increased perceptions of physical ability and self-esteem, which results in even greater attraction to physical activity. The cycle continues in this way. The model has been reasonably successful in presenting correlational evidence associating physical activity and psychological health, although some researchers have indicated inconsistencies in the application of this model.
Related to the concept of mastery is the idea that exercise can provide proprioceptive feedback of improvements in the overall functional ability of individuals' bodies. This in turn might be effective in changing body perceptions. Related to this, the positive relationship between exercise and mental health could be explained in terms of the physical signs associated with exercise (e.g. muscle tone and weight loss) and these physical signs could be viewed as positive cues towards a sense of achievement in one's physical self. This is important for clinical patients who often have negative feelings about themselves and their bodies.
Distraction hypothesisA popular explanation for the positive relationship between exercise and mental health is the distraction hypothesis. This was originally conceptualised by Bahrke & Morgan (1978) in a study that compared the effects on state anxiety of walking on a treadmill, meditating or resting in a comfortable chair. Their findings demonstrated that all three groups showed decreased levels of state anxiety after treatment. It was concluded that exercise can serve as a useful distraction or ‘time-out’ from stressful stimuli and feelings and that this can lead to improved psychological wellness. In this respect, patients may find exercise a useful strategy to help them focus on events other than their particular life circumstances.
