Control of Posture During Lifting: Effects of Weight Knowledge and Load Position Asymmetry

Lifting objects challenges postural stability and requires the generation of anticipatory (APA) and compensatory postural adjustments. This study investigated the combined effects of perturbation predictability and asymmetric load placement on postural control during a bimanual lifting task. Sixteen healthy adults completed a randomized crossover study involving repeated lifts of a box containing a load equal to 5% of their body weight that was positioned asymmetrically under three conditions: known, unpredictable, and predictable. Surface electromyography from trunk and lower limb muscles and center-of-pressure (COP) displacements were recorded. Electromyography integrals and COP displacements were analyzed separately for APA and compensatory postural adjustment phases using linear mixed-effects models. Explicit knowledge of load position resulted in larger and more direction-specific APAs, characterized by larger anticipatory trunk muscle activation and a proactive anterior shift of the COP (all p < .05). Predictability acquired through repeated exposure produced smaller APAs (p < .05). Under unpredictable conditions, participants exhibited reduced anticipatory COP shifts and larger compensatory responses for right-sided loads (all p < .05), whereas left-sided loads were characterized by a posterior COP shift before lift initiation. Postural adjustments during lifting are modulated by both explicit task information and experience-based predictability, and load position asymmetry alters anticipatory and compensatory control strategies. Enhancing information about load position may therefore improve postural stability and reduce reliance on compensatory mechanisms during manual material handling.