Is It Really Worth the Effort? Examining the Effects of Mental Fatigue on Physical Activity Effort Discounting.

Physical activity (PA) guidelines are informed by epidemiological evidence but do not account for people's motivation for exerting physical effort. Previous research has shown that people are less motivated to engage in moderate- to vigorous-intensity PA when fatigued. In a two-study series, we investigated how intensity and duration affected people's willingness to engage in PA using an effort-discounting paradigm. A secondary purpose was to examine whether effort discounting was affected by mental fatigue. Both studies revealed a significant Intensity × Duration interaction demonstrating a reduced willingness to engage in PA of higher intensities across increasing duration levels. Study 1 demonstrated greater effort discounting for vigorous-intensity PA with increasing mental fatigue; however, this effect was not observed in Study 2. Findings provide novel insight toward people's motivation for engaging in PA based on the properties of the task, and some evidence suggesting feelings of fatigue may also influence motivation to exert physical effort.