Have you seen those people who come out of an exercise class with a spring in their step and self-satisfied smile on their face? They really pushed themselves this time and now they’re riding that endorphin high. To them, the ache and burn feels good. But it’s not so for everyone. Others find exercise unpleasant and unrewarding – the aches just, well, ache. Psychologists call this difference the “affective response to exercise” and in a paper in Psychology of Sport and Exercise researchers in the Netherlands report new evidence that it is to a significant degree genetically inherited.
Nienke Schutte at VU University Amsterdam and his colleagues recruited 115 pairs of identical twins who share the same genes, 111 pairs of non-identical siblings who share roughly half their genes, 35 of their non-twin siblings, and another 6 non-twin sibling pairs (aged 12 to 25). The participants completed a 20 minute non-vigorous ride on an exercise bike and a non-vigorous 20 minute treadmill run. Their breathing was monitored to ensure the exercise didn’t become vigorous. There were also warm up and warm down periods and there was a second brief exercise ride to exhaustion.
While exercising, participants periodically rated how they were feeling on a scale from very bad to very good, how much they felt they were exerting themselves, and how much they were feeling various adjectival states, such as energetic, lively, jittery or tense. From these measures the researchers established the participants’ affective response to exercise. The researchers also interviewed the participants about how much and how often they participated in voluntary sport and exercise in their everyday lives.
By looking to see if affective response to exercise was more similar between identical twins than non-identical twins, the researchers were able to deduce how much it is genetically inherited (its “heritability” – in other words, how much the differences in the subjective experience of exercise between people is due to genetic influences).
The heritability for the affective response to exercise was significant. For instance, using the simple feeling scale, heritability was 15 per cent for the moderate cycle; for perceived exertion during the treadmill it was 35 per cent; and for the adjectival rating scale, heritability varied from 17 to 37 per cent for non-vigorous exercise, and 12 to 37 per cent for the ride to exhaustion, with the figure depending on the adjectives looked at.
Moreover, and as logic would dictate, affective response to exercise correlated positively with how much exercise the participants did in their everyday lives: those who found exercise more pleasant engaged in more exercise. And the analysis suggested genetic overlap between these measures: in other words, many of the same genes that influence the affective experience of exercise seem to be the same genes that influence how active people are in their lives.
One caveat: causal direction hasn’t been established here. It’s possible that doing more exercise can change how we experience exercise. In which case perhaps it is the known heritability of inclination to do exercise that is driving the apparent heritability of the affective response to exercise. Likely the influences are mutual. Another possibility is genetic pleiotropy in which the same genes have separate effects on different outcomes.
This study has uncovered a point of principle – that whether we experience exercise as pleasant or unpleasant is to a significant degree influenced by our genes. Establishing what those specific genes are, and what their other functions might be, is for the future. One candidate gene that the researchers mention is the gene that codes for brain-derived neurotrophic factor, a peptide that has been shown to moderate the influence of exercise on mood.
Perhaps most interesting is the practical implications this line of research could have for interventions to help people take up more exercise. If some of us are genetically inclined to find exercise less fun and rewarding we might especially benefit from personalised exercise programmes that aim to reduce its arduousness and make it as enjoyable as possible.