Trajectory elevation in reach-to-grasp movements under increased arm weight and repeated trials

This study examined how three-dimensional upper-limb movement trajectories are modulated by changes in gravitational load. We hypothesized that individuals adopt a trajectory elevation strategy in reach-to-grasp movements performed on a desk to compensate for increased movement variability arising from motor noise. To increase motor demands and create conditions under which trial-related fatigue effects could emerge, we manipulated arm weight and used an extended repeated-trial protocol. Participants completed 400 reach-to-grasp trials under three weight conditions (0 g, 200 g, and 400 g), with weights applied to the forearm during the middle 300 trials in the 200 g and 400 g conditions. From the first weighted block-when substantial muscle fatigue was unlikely to have developed-maximum trajectory height was greater in the weighted conditions than in the 0 g condition. Across trials, trajectory height gradually decreased in the 0 g and 200 g conditions, whereas a slight increase was observed in the 400 g condition. In the 400 g condition, trajectory elevation across trials was positively correlated with trajectory variability in later trials. These findings are consistent with the hypothesis that trajectory elevation may serve as a strategy to mitigate detrimental effects on task performance caused by fluctuations in motor output, whether arising from added weight or trial-related fatigue effects. Additionally, exploratory analyses examined the effects of within-day repeated practice on aperture control during reach-to-grasp movements.