Vestibular contribution to balance control during a sit-to-walk task

Abstract

Predictive simulation is a powerful tool that can be used to examine the impacts of aging on complex movement behaviors. These models rely on neuromuscular controllers that modulate sensory feedback, including vestibular feedback, in order to transition between different movement phases. Current models, however, define the phase transitions based on the kinematics of movement without consideration for the underlying neurophysiological feedback mechanisms driving actual behavior. Here, we studied sit-to-walk movements, a challenging task commonly faced by aging populations, and examined how vestibular feedback is modulated for the control of balance. We estimated the coupling between an electrical vestibular stimulus and ground reaction forces in healthy participants (N = 16) while they performed a sit-to-walk task. Because sit-to-walk transitions are thought to be comprised of simultaneous transitions of standing up and walking, we also compared the sit-to-walk (STW) task to sit-to-stand (STS) (N= 8) and gait-initiation (GI) tasks (N = 8). Four main phases of vestibular control were identified for STW: quiet sitting, flexion, transition, and gait. Similarly, four main phases were identified for STS, though they differed after the first two: quiet sitting, flexion, rising/stabilizing, and quiet standing. In contrast, five main phases were identified for GI: quiet standing, adjustment I, adjustment II, transition, and gait. Importantly, the timings of the identified phases differed from the timings of the events used to define kinematic phases, and the magnitude of the vestibular responses was modulated gradually between phases. We also found that the vestibular modulation observed in STW could be explained as a sharp shift from an STS task just after flexion, around seat-off, into a GI task starting at transition. These results demonstrate that defining the timing of neuromuscular controllers in predictive simulation based on neurophysiological events may be better suited to improving their accuracy.