Anticipatory postural adjustment for compensating forces and moments associated with isometric force production in an upper limb.

Abstract

Anticipatory postural adjustments (APAs) play a critical role in maintaining postural stability during voluntary movements, compensating proactively for expected perturbations. While previous studies have extensively documented APAs for linear perturbations associated with upper-limb movements, less is known about the anticipatory adjustments involved in counteracting rotational perturbations generated by unilateral force production. The present study aimed to elucidate how APAs are achieved through modulation of ground reaction forces (GRFs) under each foot, in preparation for horizontal-plane rotational perturbations induced by isometric unilateral handle-pulling tasks. Fifteen healthy participants performed rapid, isometric handle-pulling tasks in different horizontal directions (left, forward, and right) using either hand while standing on two force plates. Our results demonstrated that functional and direction-specific adjustments of bilateral GRFs were observed 50 ms before the onset of hand force. Specifically, anticipatory modulations in anterior-posterior GRFs under the left and right feet created condition-specific horizontal moments, effectively compensating for predicted rotational perturbations. Furthermore, statistical parametric mapping (SPM1D) analysis revealed that condition-specific GRF moments preceded hand-generated moments, reinforcing the feedforward nature of these adjustments. These findings highlight the importance of coordinated bilateral GRF control as a key mechanism for anticipatory postural stabilization.