Reaching accuracy declines with postural demand during whole-body leaning.

Introduction: Whole-body movements in everyday tasks challenge the neuromuscular system. Prior studies have shown that appropriate postural control supports both task success and postural balance during quiet standing or under modest postural demands. However, it remains unclear how the central nervous system controls whole-body posture under high-demand conditions, such as maximal leaning postures. Here, we investigated how varying postural demands influence task performance in whole-body tasks. Methods: We used an upper-body reaching task in which participants leaned their upper body mediolaterally to reach a target without falling. Our task required various upper-body positions and accelerations by varying target positions and time constraints to reach a target. Therefore, our task induced participants to different magnitudes of destabilizing torque, which is defined as postural demand. Task performance in response to different target positions and time constraints was evaluated based on reaching accuracy. Results: Target position, time constraint, and movement distance significantly affected reaching accuracy. Specifically, participants could accurately reach targets requiring upright or moderately leaning postures. However, when targets required greater leaning postures, participants failed to reach them. Furthermore, the detrimental effects of shorter time constraints and longer movement distances on reaching accuracy became more pronounced when target positions required greater leaning postures. Discussion: These findings demonstrate that reaching accuracy was preserved under low to modest postural demands, but deteriorated once postural demands became high.