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

Pupil size scaled with overall tapping speed during a sustained repetitive wrist-tapping task, but its time course was not pacing-dependent, indicating effort tracking without motor fatigability.

• Why it matters: It was unclear whether pupil dynamics during prolonged movement reflect only physical effort or also the buildup of motor fatigability.
• What they did: Participants performed paced wrist tapping at three tapping speeds while measuring force, electromyography, and pupillometry to separate effort from speed-dependent motor slowing.
• The result: Motor slowing and increased antagonistic coactivation emerged selectively at the fastest speed, yet pupil temporal change was not pacing-dependent, supporting pupil size as a marker of physical effort but not motor fatigability.

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