We implemented a virtual reality system to quantify differences in the use of visual feedback to maintain balance during walking between healthy young (n = 12, mean age: 24 years) and healthy old (n = 11, 71 years) adults. Subjects walked on a treadmill while watching a speed-matched, virtual hallway with and without mediolateral visual perturbations. A motion capture system tracked center of mass (CoM) motion and foot kinematics. Spectral analysis, detrended fluctuation analysis, and local divergence exponents quantified old and young adults’ dynamic response to visual perturbations. Old and young adults walked normally with comparable CoM spectral characteristics, lateral step placement temporal persistence, and local divergence exponents. Perturbed visual flow induced significantly larger changes in mediolateral CoM motion in old vs. young adults. Moreover, visual perturbations disrupted the control of lateral step placement and compromised local dynamic stability more significantly in old than young adults. Advanced age induces a greater reliance on visual feedback to maintain balance during waking, an effect that may compensate for degradations in somatosensation. Our findings are relevant to the early diagnosis of sensory-induced balance impairments and also point to the potential use of virtual reality to evaluate sensory rehabilitation and balance training programs for old adults.

The gastrocnemius and soleus both contribute to the ankle plantarflexor moment during the midand terminal stance phases of gait. The gastrocnemius also generates a knee flexion moment that may lead to dynamic function that is unique from the soleus. This study used a muscle stimulation protocol to experimentally compare the contributions of individual plantarflexors to vertical support, forward propulsion and center of pressure (CoP) movement during normal gait. Twenty subjects walked on an instrumented treadmill at self-selected speeds with stimulating surface electrodes affixed over the medial gastrocnemius and soleus muscles. Short duration pulse trains (90 ms) were used to stimulate either the gastrocnemius or soleus at 20% or 30% of the gait cycle (GC) of random strides. Changes in ground reactions between stimulated and non-stimulated strides were evaluated to characterize the influence of each muscle on whole body movement during mid- (stimulation onset at 20% GC) and late (30% GC) stance. The gastrocnemius and soleus each induced an increase in vertical support and anterior progression of the CoP in mid-stance. However, late stance gastrocnemius activity induced forward acceleration, while both mid- and terminal stance soleus activity induced braking of forward velocity. The results suggested that the individual plantarflexors exhibit unique functions during normal gait, with the two muscles having opposite effects on forward propulsion. These empirical results are important both for enhancing the veracity of models used to predict muscle function in gait and also clinically as physicians seek to normalize gait in patients with plantarflexor dysfunction.

Gait variability measures have been linked to fall risk in older adults. However, challenging walking tasks may be required to elucidate increases in variability that arise from subtle age-related changes in cognitive processing and sensorimotor function. Hence, the study objective was to investigate the effects of visual perturbations, increased cognitive load, and narrowed step width on gait variability in healthy old and young adults. Eleven old (OA, 71.2 ± 4.2 years) and twelve young (YA, 23.6 ± 3.9 years) adults walked on a treadmill while watching a speed-matched virtual hallway. Subjects walked: 1) normally, 2) with mediolateral visual perturbations, 3) while performing a cognitive task (serial seven subtractions), and 4) with narrowed step width. We computed the mean and variability of step width (SW and SWV, respectively) and length (SL, SLV) over one three minute trial per condition. Walking normally, old and young adults exhibited similar SWV and SLV. Visual perturbations significantly increased gait variability in old adults (by more than 100% in both SWV and SLV), but not young adults. The cognitive task and walking with narrowed step width did not show any effect on SWV or SLV in either group. The dramatic increase in step width variability when old adults were subjected to mediolateral visual perturbations was likely due to increased reliance on visual feedback for assessing whole body position. Further work is needed to ascertain whether these findings may reflect sub-clinical balance deficits that could contribute to the increased fall risk seen with advancing age.

Distinguishing gastrocnemius and soleus muscle function is relevant for treating gait disorders in which abnormal plantarflexor activity may contribute to pathological movement patterns. Our objective was to use experimental and computational analysis to determine the influence of gastrocnemius and soleus activity on lower limb movement, and determine if anatomical variability of the gastrocnemius affected its function. Our hypothesis was that these muscles exhibit distinct functions, with the gastrocnemius inducing limb flexion and the soleus inducing limb extension. To test this hypothesis, the gastrocnemius or soleus of twenty healthy participants was electrically stimulated for brief periods (90 ms) during mid-or terminal stance of a random gait cycle. Muscle function was characterized by the induced change in sagittal pelvis, hip, knee, and ankle angles occurring during the 200 ms after stimulation onset. Results were corroborated with computational forward dynamic gait models, by perturbing gastrocnemius or soleus activity during similar portions of the gait cycle. Mid- and terminal stance gastrocnemius stimulation induced posterior pelvic tilt, hip flexion and knee flexion. Mid-stance gastrocnemius stimulation also induced ankle dorsiflexion. In contrast, mid-stance soleus stimulation induced anterior pelvic tilt, knee extension and plantarflexion, while late-stance soleus stimulation induced relatively little change in motion. Model predictions of induced hip, knee, and ankle motion were generally in the same direction as the experiments, though the gastrocnemius’ results were shown to be quite sensitive to its knee-to-ankle moment arm ratio.

Visuomotor entrainment, or the synchronization of motor responses to visual stimuli, is a naturally emergent phenomenon in human standing. Our purpose was to investigate the prevalence and resolution of visuomotor entrainment in walking and the frequency-dependent response of walking balance to perturbations. We used a virtual reality environment to manipulate optical flow in ten healthy young adults during treadmill walking. A motion capture system recorded trunk, sacrum, and heel marker trajectories during a series of 3-min conditions in which we perturbed a virtual hallway mediolaterally with systematic changes in the driving frequencies of perceived motion. We quantified visuomotor entrainment using spectral analyses and changes in balance control using trunk sway, gait variability, and detrended fluctuation analyses (DFA). ML kinematics were highly sensitive to visual perturbations, and instinctively synchronized (i.e., entrained) to a broad range of driving frequencies of perceived ML motion. However, the influence of visual perturbations on metrics of walking balance was frequency-dependent and governed by their proximity to stride frequency. Specifically, we found that a driving frequency nearest to subjects' average stride frequency uniquely compromised trunk sway, gait variability, and step-to-step correlations. We conclude that visuomotor entrainment is a robust and naturally emerging phenomenon during human walking, involving coordinated and frequency-dependent adjustments in trunk sway and foot placement to maintain balance at the whole-body level. These findings provide mechanistic insight into how the visuomotor control of walking balance is disrupted by visual perturbations and important reference values for the emergence of balance deficits due to age, injury, or disease.