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.

Adaptation of gait termination on a slippery surface in Parkinson's disease.

Parkinson's disease (PD) causes instability and difficulty adapting to changing environmental and task demands. We examined the effects of PD on the adaptation of gait termination (GT) on a slippery surface under unexpected and cued circumstances. An unexpected slip perturbation during GT was followed by a slip perturbation during GT under two conditions: planned over multiple steps and cued one step prior to GT. Feed forward and feedback-based responses to the perturbation were compared to determine (1) how PD affects the ability to integrate adaptive feed forward and feedback-based GT strategies on a slippery surface, (2) if adaptations can be implemented when GT is required within one step, and (3) if behaviour changes with repeated exposure. Similar to the control group (n=10), the PD group (n=8) adapted and integrated feed forward and feedback-based components of GT under both stop conditions. Feed forward adaptations included a shorter, wider step, and appropriate stability margin modifications. Feedback-based adaptations included a longer, wider subsequent step. When cued to stop quickly, both groups maintained most of these adaptations: foot angle at contact increased in the first cued stop but adapted with practice. The group with PD differed in their ability to adapt GT with slower, wider steps and less stability.

Compensatory postural adaptations during continuous, variable amplitude perturbations reveal generalized rather than sequence-specific learning.

We examined changes in the motor organization of postural control in response to continuous, variable amplitude oscillations evoked by a translating platform and explored whether these changes reflected implicit sequence learning. The platform underwent random amplitude (maximum +/- 15 cm) and constant frequency (0.5 Hz) oscillations. Each trial was composed of three 15-s segments containing seemingly random oscillations. Unbeknownst to participants, the middle segment was repeated in each of 42 trials on the first day of testing and in an additional seven trials completed approximately 24 h later. Kinematic data were used to determine spatial and temporal components of total body centre of mass (COM) and joint segment coordination. Results showed that with repeated trials, participants reduced their magnitude of COM displacement, shifted from a COM phase lag to a phase lead relative to platform motion and increased correlations between ankle/platform motion and hip/platform motion as they shifted from an ankle strategy to a multi-segment control strategy involving the ankle and hip. Maintenance of these changes across days provided evidence for learning. Similar improvements for the random and repeated segments, indicated that participants did not exploit the sequence of perturbations to improve balance control. Rather, the central nervous system may have been tuning into more general features of platform motion. These findings provide important insight into the generalizabilty of improved compensatory balance control with training.