Pupil size reflects effort but not motor fatigability during a sustained motor task

Pupil size has been shown to reflect physical effort during brief, discrete motor tasks. It remains unknown whether pupil size similarly scales with effort during sustained repetitive movement, or whether it also reflects the accumulation of motor fatigability, a reduction in motor performance during prolonged movements. In our previous work, sustained maximal tapping induced motor slowing, a subtype of motor fatigability, characterized by decreasing movement speed and associated with increased antagonistic muscle coactivation. Next to these behavioural effects, pupil initially dilated and then gradually decreased over time. Whether this temporal change in pupil size is shaped by physical effort alone or also by motor fatigability remains unclear. Here, we combined a repetitive wrist tapping paradigm with force measurement, electromyography, and pupillometry, and paced participants at three different tapping speeds to dissociate physical effort from motor slowing, which has been shown to be speed-dependent. Fatigability-related motor slowing emerged selectively at the fastest speed. Moreover, we showed that antagonistic muscle coactivation increased more steeply at faster speeds, even in the absence of motor slowing, suggesting it may reflect an early change in motor control. While we found clear evidence that pupil size scaled with overall tapping speed, we did not find evidence that the temporal change of the pupil was pacing-dependent. These findings extend the pupil effort relationship to sustained, dynamic motor tasks and suggest that pupil size is a marker of physical effort but not motor fatigability during sustained repetitive movement.