These classifications and results are shown in figure 1. below.
Figure 1: Changes in physical activity status and subsequent depression
(Camacho et al . 1991. Table adapted from Biddle et al 2002)
As shown in the table above, those who increased their physical activity levels from 1965 to 1974 were no more likely to develop depression than those who had been active at both times. The odds ratio was computed and showed that those who decreased their activity levels from 1966 to 1974 were 1.6 times more likely to develop depression than those who maintained a high level. Although these results was not found to be significant the authors did note that it was relatively unaffected by other variables such as age, gender, physical health, life events and weight. This is an important consideration to make since certain factors, such as physical disability may have hindered the participants activity levels. These two studies do show an association between activity and depression as well as showing that there is a predictive nature of depression from baseline to follow up.
The following diagrams are taken from the work of Lawlor & Hopker, 2001 who conducted a review study of exercise as an intervention for reducing depression. Studies that were included in the review were randomised controlled trials in which the participants had been diagnosed as having depression, by any measure, although the severity may have been different. There were two main categories for the studies and the results:
- Exercise compared with placebo intervention or as adjunct to standard treatment.
- Exercise compared to standard treatments for depression.
Figure 2: Exercise compared with placebo intervention or as adjunct to standard treatment
Taken from Lawlor et al (2001) p765
The above diagram shows the effect sizes when exercise was compared to a placebo intervention. The pooled standardised mean difference in effect size in the above studies was -1.1. That is, people who exercise are 1.1 standard deviations less depressed than non exercisers. The effect size found here is similar to other meta analytic studies (North, McCullagh and Tran 1990; Carlson 1991; Craft and Landers 1998) The authors of this study have critiqued their findings and state that results that give the standard mean difference can be difficult to understand in clinical terms and what would have been a better measure would be a dichotomous outcome; the likelihood of suffering from depression after the intervention.
-
Figure 3: Exercise compared to standard treatments for depression
Taken from Lawlor et al (2001) p765
The four studies shown in the above table consisted of standardised mean differences in effect size between exercise and cognitive therapy. These results are very interesting as it does not support a sustained effect of exercise after the intervention period, however, the results were not found to be significant.
Other review studies have been conducted in the area which provides useful summaries of data and similar results to above. Byrne & Byrne, (1993) reviewed literature that had been published in referred journals relating to exercise and depression. The authors found that 90% of studies supported the effectiveness of exercise in reducing depression. However, this review was criticised for including patients who were depressed following myocardial-infraction (MI) rather than purely symptoms of clinical depression. This can lead to problems of generalisability. (Craft and Landers 1998). As stated earlier, there are different strains of depression and this is a limitation of some of the reviews as they include the different types.(Martinsen, 1990) conducted a series of reviews, however, that considered the evidence dealing with clinically anxious and depressed individuals. The latest of the reviews (Martinsen, 1994) consisted of 10 studies and the researcher came to 5 concluding points:
- Fitness levels are generally lower for clinically depressed individuals.
- Exercise is more effective than no treatment for clinically depressed.
- Exercise alongside other therapies and treatments were found to be advantageous.
- Changes in aerobic fitness are not needed to have reductions in depression
- Adherence to the exercise programme in inpatients and outpatients seemed to match those of the general population.
Unlike the previous review this one does not have the shortcoming of poor subject selection. It did however, only look at quasi-experimental designs which does exclude a large proportion of the literature (Craft et al 1998). North et al (1990) examined the relationship between exercise and depression in a total of 80 studies and found an overall effect size of -0.53. This indicated that depression decreased more in the exercise groups than the comparison groups. Craft and Landers (1998) found similar effects to these in their meta analysis on clinical depression and depression resulting from mental illness. Results from 30 studies, many of which were on unpublished dissertations, showed an effect size of -.72, therefore depressed individuals who exercised were -.72 of a standard deviation less depressed than those who did not exercise. An interesting finding that merged from this study was that the researcher did not find any difference in effect size depending on the type of exercise that was conducted (aerobic or non aerobic). Type of exercise has been the topic of much discussion within the field and the majority of research seems to show that effects of exercise does not differ depending on the activity completed. In the review study conducted by (Byrne & Byrne, 1993) the authors concluded that aerobic activity was no better than aerobic activity in alleviating depression. However, this review has been criticised by Craft and Landers (1998) as they did not find any improvement in fitness from the aerobic exercise. In Craft and Landers own study (1998) they predicted that aerobic and non aerobic exercise programmes would not be any different in alleviating the symptoms of depression. They found that running produced the largest effect although this was not significantly different from the reduction in depression from non-aerobic activity.
A further study carried out by Martinsen, Hoffart, & Solberg, (1989) compared aerobic exercise including jogging and cycling to non-aerobic exercise including a circuit class on a multigym. The study found both methods of exercise to give significant reductions on scores of depression, although the aerobic exercise gave a greater increase in aerobic capacity
As well as the type of exercise to be completed what has also been considered is the severity of the depression in relation to the effectiveness of exercise. Craft and Landers (1998) predicted in their review study that effect would increase with severity. The researcher found that the moderately to severely depressed group benefited the most from the exercise. A more recent study (Dimeo, Bauer, Varahram, Proest, and Halter, 2001) looked at patients with severe depression, classified by DSM-IV (See appendix 1). The mean duration of the depressive symptoms was 21 weeks and the exercise that was completed consisted of walking on a treadmill following an interval training pattern for the duration of 30 minutes a day for 10 days. Depression was rated using the Hamilton Rating Scale for Depression (Hedlung and Vieweg 1979) and results showed a significant reduction in depression after the intervention.
Psychological mechanisms
There has also been a large amount of research conducted on the mechanisms of exercise and depression. Much of this research focuses on self-evaluations; data suggests that negative self-evaluations can actually lead to depression onset. (Bandura, Pastorelli, Barbaranelli, & Caprara, 1999; Maciejewski, Prigerson, & Mazure, 2000) Research does seem to points to the fact that global self-evaluations (for example self-esteem) (Blumenthal et al., 1999) and specific self-evaluations (such as exercise self-efficacy) (McAuley, Blissmer, Katula, Duncan, & Mihalko, 2000) do indeed change with exercise. Looking specifically at self efficacy research Craft (2003) considers mastery experiences to be one of the most influential factors in changing efficacy beliefs. The author stated that exercise may provide a way to enhance efficacy beliefs as it can provide mastery experience. By learning how to monitor themselves through exercise, set goals and develop relationships with instructors and other exercisers can lead to enhanced self efficacy. Research to date on self efficacy, exercise and depression has been vague (Brown, Welsh, Labbe, Vitulli, & Kulkarni, 1992) and had not considered clinically depressed individuals. Therefore the role that exercise has on improving self efficacy and reducing depression requires further study. Another explanation of the alleviation of symptoms is that of distraction. The idea behind this is that physical activity serves as a distraction from worries, anxiety and depressed mood (Gleser & Mendelberg, 1990) One idea relating to the cause of depression is that it is related to Response Style Theory (Nolen-Hoeksema, 1991) This theory states that there are two different ways that an individual responds to feelings of depression. Distraction is one of these two which involves engaging in an activity in attempt to avoid focusing on the depressed mood. Rumination is the other way and is distinguished by a tendency to passively focus on ones negative feelings of depression and is thought to lead to a maintained depressed mood (Lyubomirsky & Nolen-Hoeksema, 1995)Research in the area had round that a distraction type style has a more positive outcome for depressives. Craft (2003) completed a study to test the hypothesis that “depressed individuals who engage in a chronic exercise intervention will report a reduced tendency to ruminate on their depression and an increased use of distraction across a 9-week intervention” (p5) Further, it was hypothesised that response style would be associated with symptoms of depression. Exercise was associated with a reduction in the use of rumination across the study whilst distraction style increased at week 3. However, the results did not support the hypothesis that response style would be associated with depression among exercise group participants.
Critique and Future Directions
Much of the research examining the relationship between exercise and depression has some methodological flaws that can hinder the area. For example, many of the studies have not used control groups or random assignment of subjects (Blue, 1979) Adding to this, as depression can be such a serious illness many of the subjects were completing other methods of treatment such as pharmacology, this makes it difficult to distinguish which of the treatments is having the effect. Studies have also been found to have quite small control groups and whilst this can be difficult to avoid due to the nature of the topic, they do restrict generalisability of the findings (Craft 2003). It is also difficult to make large generalisations in the area of clinical depression since no study can exactly replicate what goes on in clinical practice.
As started earlier exercise on prescription schemes are becoming ever more popular (Biddle et al 1991), however, maintenance of these programmes for any clinical problem is between 20 and 40% (Fox, Biddle, Edmunds, Bowler, & Killoran, 1997). Assumptions can be made, therefore that the compliance for depression alone would probably be similar or worse. (Lawlor & Hopker, 2001). Having said this, a recent survey conducted by the National Association for Mental Health (Mind) (2001) found that 65% of people with mental health problems said exercise helped relieve the symptoms of depression. However, what was also an interesting finding was that 58% of people with mental health problems did not know that GP’s can sometimes prescribe exercise. Related to this finding is the fact that 68% of gym members thought that their mentality would suffer if they stopped exercising. It therefore seems plausible to suggest that research into the barriers stopping depressed people from exercising should be examined, including looking more closely at the health care professionals. The main barrier seems to be a lack of knowledge and understanding about exercise as a treatment as once people take the initial step they do feel the benefits.
Many of the studies discussed accounted for extraneous variables such as gender, age and disability. However, there are other factors that were not accounted for which would make interesting further study. For example, social skills and socio economic status may also be reasons for inactivity and depression (Biddle et al 2002).
Future research should also consider dichotomous outcomes as well as continuous ones as they are said to be more understandable in clinical settings. (Lawlor & Hopker, 2001). Lawlor et al also state that they do not think it is possible to define the effectiveness of exercise in treating depression from the available literature although it may alleviate some of the symptoms
Until more is known about the effectiveness of exercise on depression standard treatment should continue especially in the more severe cases. More longitudinal designs are needed (for 12 months or more) (Craft and Landers 1998) within the area. In a study by (Martinsen, Medhus, & Sandvik, 1985) subjects who had fitness gains of over 30% had greater reductions in depression than those in the 15-30% bracket. This supports the notion of maintained levels of activity for increased benefits to health. Further to this, the majority of studies do seem to be for the duration of 12 months or less, however, the American College of Sport Medicine (1978) states that in order for there to be any gains to an individual they need to continue exercising for between 15 and 20 months. Therefore interventions should be conducted for this length of time and the results compared to shorter term interventions.
Total Word count: 3142 (excluding references)
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