Effects of mental fatigue and incentives on exercise decision-making
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People’s decisions regarding effort-based tasks such as engaging in physical activity depend on the subjective value of the activity: weighing the costs against the benefits (Chong et al., 2016). Exerting cognitive effort while performing one task negatively biases people's decisions to exert effort on subsequent cognitive tasks, suggesting a shift in their subjective valuation of the task due to mental fatigue (Kool & Botvinick, 2014). Similarly, exerting physical effort negatively biases decisions to further exert effort on a future physical task (Iodice et al., 2017a, Iodice et al., 2017b). The purpose of the present study was to investigate the effect of mental fatigue on people's decisions to engage in an acute bout of exercise and whether the effect of mental fatigue on decision-making was mediated by a benefit-cost analysis. Among those who decided to exercise, the study also aimed to investigate the relationship between mental fatigue and exercise behaviours during a self-selected, self-paced, bout of exercise. Recreationally active participants (N = 55, Mage = 19.04 ± 1.04 years) completed either a 10-minute, high cognitive demand (Stroop) task or low cognitive demand (documentary viewing) task to manipulate levels of mental fatigue. Participants then made a choice between engaging in a 20-minute self-paced moderate-to-vigorous intensity exercise task or a 20-minute non-exercise task. Prior to choosing, participants rated their mental fatigue and their perceived benefits and costs of the exercise task. The cognitive task had a strong effect on mental fatigue (p < .001, Cohen's d = 1.40). The mediation analysis showed no direct effect of mental fatigue on choice; however, there was a significant indirect effect indicating the benefit-cost score mediated the effect of mental fatigue on choice (95% C.I. = -.02 to -.0004). Higher levels of mental fatigue were associated with a lower benefit-cost score (r = -.33, p = .01) which, in turn, was associated with a decreased likelihood of choosing the exercise task (r = .31, p = .02). For those who chose to engage in the exercise task (N = 28), higher levels of mental fatigue were associated with higher ratings of perceived exertion (r = .38, p = .05). Findings provide insight into the effects of mental fatigue on people's exercise behaviours, illustrating a rational decision-making process that is dependent upon the subjective evaluation of the costs and benefits of engaging in physical activity or sedentary alternatives.
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