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.

By monitoring head movement and orientation in space, the vestibular system can evoke appropriate muscle responses in order to maintain standing balance. The present study investigates whether vestibular-evoked muscle responses are dependent on sensory cues of gravity by examining these responses across varying load and gravity conditions. Standing subjects were exposed to a stochastic electrical vestibular stimulus (EVS, ±5 mA, 0-25 Hz) that induced a vestibular error signal, while vertical loading forces or vestibular signals of gravity were independently modified. A backboard structure limited subjects’ whole-body rotation to the sagittal plane which corresponded with the EVS-evoked sway responses in anteroposterior direction, as the subject’s head was rotated in yaw. Vestibular-evoked muscle responses were greatest when sensory cues of gravity matched the expected terrestrial force of gravity, and decreased when these cues were modified. The reduction was largest when both load- and vestibular-related cues of gravity were different from normal. Our results indicate that the vestibular drive for standing balance control is attenuated when sensory cues of gravity are not congruent to normal (i.e. terrestrial) expectations of standing balance and that the degree of attenuation is dependent upon the cumulative incongruency that arises from multiple sensory cues.