Everyone knows that exercise is good for your physical health – it reduces your risk of developing type 2 diabetes, coronary heart disease, osteoporosis, cancer and dementia to name just a few conditions (NHS, 2018). However, exercise’s mental health benefits are less-celebrated, yet I believe, just as essential as what movement can do for us physically. Exercise is a free tool that we all have access to, in some form or another, that can reduce feelings of stress, anxiety and depression, and can help us feel empowered, joyful and truly alive.
There’s an ever-growing literature on the relationship between exercise and mental health. In one study, women with anxiety ran for 30 minutes at 65-85% of their maximum heart rate, whilst another group of women rested for the same amount of time. When tested immediately afterwards, the women in the running condition reported significant reductions in worry, anxiety and fatigue, and increases in energy, unlike the women who rested, whose feelings did not change (Herring, Hallgren and Campbell, 2017). Another study found that inpatients at a psychiatric hospital who took part in 40-60 minutes of exercise including Nordic walking, gym and ball sports, had improved moods and less rumination after just a single exercise session (Brand et al., 2018). Moreover, women with anxiety who took part in a nine week exercise programme showed reductions in anxiety at the end of the study, suggesting regular physical activity can help to reduce overall anxiety levels (Lucibello, Parker and Heisz, 2019). A review of six randomised controlled trials with 262 participants concluded that aerobic exercise is an effective treatment for anxiety and stress-related disorders (Stubbs et al., 2017). Similarly, a review of 33 studies comprising 1877 participants found that resistance training was significantly associated with a reduction in depressive symptoms, regardless of participants’ health status, training volume or strength improvements, and concluded that resistance training is an effective alternative and/or complementary therapy for depression (Gordon et al., 2018).
In this article I will discuss some of the possible biological, psychological and social explanations for this impressive relationship between exercise and mental health.
Perhaps the most commonly-proposed mechanism for why exercise may improve mental health is the ‘endorphin hypothesis’. This theory is popular because it is associated with the idea of a ‘runner’s high’, i.e. the feeling of elation reported by runners after they have been for a run. The endorphin hypothesis claims that when we participate in prolonged aerobic exercise, chemicals known as beta- are released in the brain, which numb pain and produce feelings of euphoria (Hoffman, 1997). However, it is difficult to measure brain endorphin levels in humans, so most of the evidence for this theory comes from animal studies and thus cannot be generalised from.
A more likely explanation for the ‘runner’s high’ may involve endocannabinoids, which are the same chemicals mimicked by cannabis. Endocannabinoids are produced by the brain during exercise and can reduce pain, promote calmness and improve mood. Although early research into this theory was carried out with mice, there is also human research to support it. Raichlen et al. (2013) assigned participants to use treadmills at three different speeds – one group walked, one group jogged and one group sprinted. Blood levels of endocannabinoids were tested immediately after exercising, and the group that had jogged showed three times more endocannabinoids than the other groups. This also correlated with the participants’ self-reported high in the jogging group. The same findings have been replicated with other moderate-intensity exercise including hiking and cycling (Brellenthin et al., 2017). It is thought that as long as the activity is moderately challenging and is carried out for at least twenty minutes the same results apply, suggesting that it is not just a ‘runner’s high’ but in fact a more generalised ‘moderate intensity exercise high’ that occurs.
Another biological explanation for the observed improved mental states following exercise is the ‘monoamine hypothesis’, which claims that neurotransmitters including dopamine, serotonin and norepinephrine are released during exercise (Chaouloff, 1997). Since levels of these neurotransmitters are typically depleted in individuals with depression and anxiety, it is thought that exercise can help to improve mood by increasing their concentration in the brain. This is similar to the way in which antidepressant drugs work, e.g. SSRIs inhibit the reuptake of serotonin in the brain, meaning more serotonin is available. However, this theory has been critiqued for being oversimplified (Dunn and Dishman, 1991), and research has so far only been carried out on animals, so we can’t assume that human brains respond in exactly the same way (Lin and Kuo, 2013).
Interesting new research suggests that lactate, which is a by-product of exercise that causes muscle soreness, could also play a part in the exercise and mental health relationship. Lactate affects not only the muscles but the brain too, and has been found to increase resilience to stress and reduce depressive symptoms in mice (Karnib et al., 2019). However, this research is in its infancy and much like other biological theories has only studied mice so far, so we can’t generalise this idea to humans.
Another proposed biological explanation is the ‘thermogenic hypothesis’ (De Vries, 1981). This theory claims that raising the temperature of the brain stem through exercise aids relaxation and reduces muscle tension, which thus reduces anxiety. However, this theory was based on the perceived benefits of saunas and hot showers, and there is a lack of research supporting the relationship between exercise, temperature and psychological state (Daley, 2002).
There may also be a genetic link to the effect that exercise has on one’s mental health. Certain strands of DNA have been associated with the mental health benefits of exercise, including reductions in anxiety and depression. This suggests that individuals with certain genetic profiles may be more sensitive to the mood-enhancing effects of exercise than others, and thus would perhaps benefit more from using exercise as a mental health intervention (Taylor et al., 2017; Haslacher et al., 2015).
Unlike the biological theories discussed above, which focus on physical changes in the body and brain, psychological theories revolve around what happens in our minds when we exercise.
One psychological theory proposed by for the relationship between exercise and mental health is the ‘distraction hypothesis’ (Bahrke and Morgan, 1978). This theory claims that it is the distraction from daily stressors, as opposed to the exercise per se, makes people feel better when they exercise. In a study by Bahrke and Morgan (1978) 75 men were randomly assigned to one of three experimental conditions for 20 minutes: aerobic exercise (jogging on a treadmill at 70% of maximum heart rate), meditation (using a relaxation method they were taught) or a control group (quiet rest in a chair). When assessed immediately afterwards, as well as 10 minutes-post intervention, men’s anxiety reduced equally and significantly in all three conditions. Other research has also found that rest, relaxation and even psychotherapy can be as effective as exercise at reducing anxiety and stress (Berger et al., 1988; Crocker and Grozelle, 1991; Boutcher, 1993). This evidence suggests that it is not the act of exercising itself but rather the ‘time out’ from daily routine that is responsible for the mental health benefits associated with exercise.
Another psychological theory is the ‘mastery hypothesis’. This argues that the successful completion of an effortful task results in feelings of accomplishment, which produces improved psychological states (Greist et al., 1979). This is similar to Bandura’s (1977) concept of self-efficacy, which is an individual’s belief in their ability to complete a task. A study by Bozoian et al. (1991) found that participants who had greater exercise self-efficacy, i.e. more belief that they could exercise, felt more revitalised and positive after exercising for 20 minutes than those with lower self-efficacy. This suggests that if we believe we can perform certain exercises we may be more likely to experience positive mental states afterwards.
A final possible explanation for the link between exercise and mental health is the social interaction hypothesis (Ransford, 1982) . This theory claims that the relationships and social support we encounter when we exercise are the reason for the mental health effects. This is easy to imagine in the context of team sports, when team mates cheer each other on and experience the ups and downs of their training and matches collectively. Evidence also shows that when spouses exercise together they both report more closeness later that day, including feeling loved and supported (Jeremy and Yorgason et al., 2018), reflecting the idea that exercising with others can improve our mental health.
However, a criticism of the social interaction theory is that some forms of exercise are carried out alone, e.g. running, so this does not explain the alleged ‘runner’s high’ and other reported mental health benefits following more individual forms of exercise. It has been suggested that certain groups of people may be more likely to benefit from the social aspect of exercise than others, such as the elderly or those with depression ( Biddle and Mutrie, 2001). Thus social interaction may not be a necessary requirement of exercise for it to benefit mental health, but it may exaggerate the positive mental health effects for certain individuals who may lack social interaction in other areas of their lives.
When I reflect on my own experiences, I can certainly relate to the mastery hypothesis when it comes to lifting weights. There is no feeling like hitting a PB on my deadlift or squat after months of grinding away at training, and the confidence boost and resulting happiness this rewards me with is huge. I also much prefer to exercise with my boyfriend than alone, as it helps with my motivation and I find having somebody to chat to makes time seem to go faster (especially when we are running!), so I can really relate to the importance of social interaction in exercise.
It’s unclear exactly which of the explanations discussed above is responsible for the relationship between exercise and mental health, but it is likely that a combination of biological, psychological and social factors come into play.
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