Time to disengage: holding an object influences the execution of rapid compensatory reach-to-grasp reactions for recovery from whole-body instability.

Rapid reach-to-grasp (RTG) reactions are important for balance recovery. Despite the benefit of having hands free to regain balance, people do not always release a handheld object. We investigated whether reluctance to release is related to central nervous system (CNS) processing delays that occur when the initial reaction is to drop the object rather than RTG. Young adults sat in a custom-designed chair that tilted backwards. Participants regained balance by reaching to a handle with hands free or while holding onto (1) a chair-fixed object or (2) a SMALL or LARGE free-moving object (unbreakable plastic tubes). EMG was collected from the upper limb to determine onset of reaction. Kinematic data from a digitized wrist marker were used to determine movement time. 9 of 10 participants released the object in every trial. Extensor digitorum onset occurred significantly later than anterior deltoid onset in all conditions. LARGE object release induced further delays in extensor onset while both SMALL and LARGE object release increased response and movement time. Object disengagement led to delays in perturbation-evoked, RTG reactions, particularly in the focal muscle (extensor digitorum) and when the objects' properties posed greater risk for a failed RTG response. We propose that time required for cognitive disengagement accounts for the observed delays. This study offers a potential explanation for the tendency to avoid disengaging from a handheld object during balance recovery. Results also provide insight into the challenges imposed upon the CNS during temporally urgent movements.