Human motor responses to different assistance onset timings during powered elbow flexion

Exoskeletons often initiate assistance at unexpected moments, disrupting natural movement and reducing user acceptance. The timing of mechanical assistance is critical for enabling users to perform fast and accurate movements without conscious effort. This study examined how assistance onset timing affects movement accuracy, muscle activation, and subjective evaluation during simple tasks. Twenty healthy young participants performed visually guided elbow flexions (30°-120°) within 1.15 s, lifting a dumbbell equal to 25% of their maximum voluntary contraction with 40% mechanical assistance under seven onset conditions (±300 ms, ±200 ms, ±100 ms, and 0 ms relative to the start cue). The findings indicate that both early and delayed assistance provide unique advantages but are subject to a fundamental trade-off: early assistance improves movement accuracy, whereas delayed assistance better preserves the sense of agency. The most effective cooperation occurred within a ±100 ms window, balancing accuracy and agency-factors essential for improving device acceptance.