Effects of physical exercise on anxiety, depression, and sensitivity to stress: A unifying theory
Section snippets
The exercise literature
Literature on psychological effects of exercise has burgeoned to the extent that even reviews of reviews are now available (Scully, Kremer, Meade, Graham, & Dudgeon, 1998). The present article addresses several limitations of the existing review literature. First, previous reviews have tended to focus on specific effects, particularly depression. In the present article, an account of relevant evidence across related areas will allow the development of a theory in which the effects of physical
Exercise, fitness and exertion
Physical exercise implies a regular, structured, leisure-time pursuit, whereas physical activity also arises in domestic or occupational tasks. Although physical activity has benefits for cardiovascular health (Paffenbarger & Hyde, 1988), its possible psychological benefits have been neglected because research has focused on formal exercise programs. In general, prior evidence of the cardiovascular benefits of exercise has shaped research into its psychological effects. For instance, the
Hedonic properties of exercise
Despite popular awareness that regular and relatively strenuous exercise improves physical health, few people exercise (Brawley & Rodgers, 1993): only around 30% of Western populations engage in significant amounts of exercise weekly and, once initiated, attrition is high (around 50% of participants being lost within 3–6 months). The exercise literature has tended to present this as paradoxical, reflecting an assumption that, as well as being beneficial, exercise is enjoyable. Accordingly,
Effects of exercise training on mood and emotional disorder
More important clinically than the short-term effects of single sessions of exercise are the enduring effects of long-term training. Although systematic research into mental health benefits has continued since Morgan's (1969) demonstration that physically unfit psychiatric patients were more depressed than their fit counterparts, claims for an antidepressive effect have tended to anticipate rather than reflect the accumulation of strong evidence Folkins & Sime 1981, Kostrubala 1976.
Psychopathology in exercisers
If exercise is a way of improving emotional state, it might be expected that adherents include many who take up exercise because of emotional problems. Reliable evidence is obviously hard to obtain although, from retrospective interviews with runners, Colt, Dunner, Hall, and Fieve (1981) reported such a finding. The gradual increase in symptoms of depression and anxiety over 2 weeks after cessation of regular running is consistent with recovery of preexisting emotional disorder (Morris et al.,
Explaining emotional effects of exercise training
Changes in aerobic fitness are probably unimportant to the effects on mood. First, although anaerobic exercise has received very little attention, the evidence that exists indicates an antidepressant effect comparable to that of aerobic exercise. However comparisons have been uncontrolled (Martinsen, Hoffart, & Solberg, 1989) or controlled by untreated subjects Doyne et al. 1987, Norvell & Belles 1993 or groups have differed in therapist supervision (Anshel & Russell, 1994). Secondly, after
Exercise training and resistance to stress
A hitherto separate research area has concerned the effect of exercise training to reduce vulnerability to stress. Reports can be distinguished according to whether differences in exercise experience have been studied cross-sectionally or experimentally, whether stress has been studied in real life or modelled in the laboratory and, finally, according to the types of stress and stress response that have been examined.
Explaining effects on stress responses
The details of, and constraints upon, the effect of exercise on stress responses remain to be clarified. Nevertheless, the balance of the evidence indicates that sensitivity to stress is reduced after exercise training. As with antidepressive and anxiolytic effects, two broad explanations should be considered. The first is the accumulation of acute effects of individual exercise sessions. In turn, two types of acute effect can be envisaged. One is to palliate responses to concurrent or recent
Exercise training as stress adaptation
Diverse explanations have been proposed for one or other psychological effects of exercise training, but many have been inconsistent with understanding of the mechanisms that control emotional state or stress responses (see Dishman, 1995), or have focused on one effect only. By contrast, the overall pattern of effects is an invitation to a broader, unifying theory. Such a theory should accommodate key features of the evidence reviewed here:
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exercise can be aversive, but also has positive hedonic
Neurochemical correlates of effects of exercise
In parallel with behavioral adaptation, stress causes physiological adaptation. Michael (1957) suggested that adaptation of the adrenal glands underlay stress-adaptation by exercise. In the intervening decades it has been appreciated that the physiological adaptations that underlie behavioral adaptation to stress are to be found, not peripherally, but in the central nervous system. Changes in several neurotransmitter systems have been causally implicated in behavioral adaptation. Review of
Implications for future research
The function of proposing that exercise is a human analogue of stress adaptation is not to provide answers, but to offer a way of asking questions about effects of exercise in future that are better integrated into psychobiological theory than hitherto. In reality, no single theory can account for the effects of such a complex stimulus as exercise. Nevertheless, although processes such as social integration, self-mastery, and distraction will, in practice, influence the effects of exercise, the
Conclusion
Claims for the emotional benefits of exercise are rooted in philosophical and religious ideas that date from at least 2,500 years ago (Dishman, 1986) and evidence is now catching up with these claims. Undoubtedly, exercise provides a vehicle for many nonspecific therapeutic processes, including physiological benefits of mobilization and psychological benefits of self-mastery and social integration. Effects related specifically to exertion include anxiolytic and antidepressant action, but also
Acknowledgements
Preparation of this review was assisted by a grant from the UK Medical Research Council. I am grateful to Lindsay Edmonds, Sam Dawber, Barbara Jones, and Sarah Peters for their expert help in producing the manuscript.
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