Impact of Observation Duration in Action Observation Therapy: Manual Dexterity, Mirror Neuron System Activity, and Subjective Psychomotor Effort in Healthy Adults

<b>Background/Objectives:</b> Action observation therapy (AOT) has gained attention as a rehabilitation method for motor function recovery following nerve injury. Although the total observation time and daily session duration have been studied, the effective observation duration per trial remains unclear. This study examined the effect of different observation durations on manual dexterity, mirror neuron system activity, and subjective psychomotor effort in healthy adults. <b>Methods:</b> Twenty-four healthy right-handed adults participated in this crossover study under four conditions: observing ball rotations with the dominant hand for one, two, or three minutes, or geometric patterns (control) for two minutes. The outcomes included maximum rotations and errors by both hands during a ball rotation task and interpersonal motor resonance (IMR), indicating mirror neuron system activity. These measures were compared before and after intervention. Subjective ratings of concentration, physical fatigue, and mental fatigue were assessed post-intervention. <b>Results:</b> Rotation performance significantly increased for the intervention hand after a 2 min observation and showed a notable effect (<i>p</i> = 0.113, r = 0.48) for the non-intervention hand after a 3 min observation compared to the control. The IMR was significantly greater during the 2 min observation than in the control. Compared to the 1 min observation, the 2 min and 3 min observations resulted in higher mental fatigue, and the 3 min observation showed lower concentration levels. <b>Conclusions:</b> These findings indicate that the observation duration has varying effects on manual dexterity and mirror neuron system activity, with optimal effects observed at specific time intervals while also highlighting the relationship between observational learning and psychomotor effort.