Visuomotor error augmentation affects mediolateral head and trunk stabilization during walking.

Prior work demonstrates that humans spontaneously synchronize their head and trunk kinematics to a broad range of driving frequencies of perceived mediolateral motion prescribed using optical flow. Using a closed-loop visuomotor error augmentation task in an immersive virtual environment, we sought to understand whether unifying visual with vestibular and somatosensory feedback is a control goal during human walking, at least in the context of head and trunk stabilization. We hypothesized that humans would minimize visual errors during walking - i.e., those between the visual perception of movement and actual movement of the trunk. We found that subjects did not minimize errors between the visual perception of movement and actual movement of the head and trunk. Rather, subjects increased mediolateral trunk range of motion in response to error-augmented optical flow with positive feedback gains. Our results are more consistent with our alternative hypothesis - that visual feedback can override other sensory modalities and independently compel adjustments in head and trunk position. Also, aftereffects following exposure to error-augmented optical flow included longer, narrower steps and reduced mediolateral postural sway, particularly in response to larger amplitude positive feedback gains. Our results allude to a recalibration of head and trunk stabilization toward more tightly regulated postural control following exposure to error-augmented visual feedback. Lasting reductions in mediolateral postural sway may have implications for using error-augmented optical flow to enhance the integrity of walking balance control through training, for example in older adults.

Time-dependent tuning of balance control and aftereffects following optical flow perturbation training in older adults.

BACKGROUND: Walking balance in older adults is disproportionately susceptible to lateral instability provoked by optical flow perturbations. The prolonged exposure to these perturbations could promote reactive balance control and increased balance confidence in older adults, but this scientific premise has yet to be investigated. This proof of concept study was designed to investigate the propensity for time-dependent tuning of walking balance control and the presence of aftereffects in older adults following a single session of optical flow perturbation training. METHODS: Thirteen older adults participated in a randomized, crossover design performed on different days that included 10 min of treadmill walking with (experimental session) and without (control session) optical flow perturbations. We used electromyographic recordings of leg muscle activity and 3D motion capture to quantify foot placement kinematics, lateral margin of stability, and antagonist coactivation during normal walking (baseline), early (min 1) and late (min 10) responses to perturbations, and aftereffects immediately following perturbation cessation (post). RESULTS: At their onset, perturbations elicited 17% wider and 7% shorter steps, higher step width and length variability (+171% and +132%, respectively), larger and more variable margins of stability (MoS), and roughly twice the antagonist leg muscle coactivation (p-values<0.05). Despite continued perturbations, most outcomes returned to values observed during normal, unperturbed walking by the end of prolonged exposure. After 10 min of perturbation training and their subsequent cessation, older adults walked with longer and more narrow steps, modest increases in foot placement variability, and roughly half the MoS variability and antagonist lower leg muscle coactivation as they did before training. CONCLUSIONS: Findings suggest that older adults: (i) respond to the onset of perturbations using generalized anticipatory balance control, (ii) deprioritize that strategy following prolonged exposure to perturbations, and (iii) upon removal of perturbations, exhibit short-term aftereffects that indicate a lessening of anticipatory control, an increase in reactive control, and/or increased balance confidence. We consider this an early, proof-of-concept study into the clinical utility of prolonged exposure to optical flow perturbations as a training tool for corrective motor adjustments relevant to walking balance integrity toward reinforcing task-specific, reactive control and/or improving balance confidence in older adults. TRIAL REGISTRATION: clinicaltrials.gov ( NCT03341728 ). Registered 14 November 2017.