Advanced age brings a greater reliance on visual feedback to maintain balance during walking

The effects of cognitive load and optical flow on antagonist leg muscle coactivation during walking for young and older adults

The purpose of this study was to compare how healthy aging interacts with environments that challenge cognitive load and optical flow to affect antagonist leg muscle coactivation during walking. We measured leg muscle activity in sixteen older adults (70.4 ± 4.2 years) and twelve young adults (23.6 ± 3.9 years) walking on a treadmill at their preferred speed while watching a speed-matched virtual hallway. Cognitive load was challenged using a dual-task to interfere with available attentional resources. Optical flow was challenged using perturbations designed to create a perception of lateral imbalance. We found antagonist coactivation increased with aging, independent of condition. We also found that, compared to unperturbed walking, only in the presence of optical flow perturbations did the older adults increase their antagonist coactivation. Antagonist coactivation in the young adults was not affected by either condition. Our findings provide evidence that antagonist leg muscle coactivation in healthy older adults is more sensitive to walking environments that challenge optical flow than environments that challenge cognitive load. As increased antagonist coactivation may indicate compromised balance, these findings may be relevant in the design of living environments to reduce falls risk.

Age and falls history effects on antagonist leg muscle coactivation during walking with balance perturbations

BACKGROUND

Inspired by a reliance on visual feedback for movement control in older age, optical flow perturbations provide a unique opportunity to study the neuromuscular mechanisms involved in walking balance control, including aging and falls history effects on the response to environmental balance challenges. Specifically, antagonist leg muscle coactivation, which increases with age during walking, is considered a neuromuscular defense against age-associated deficits in balance control. The purpose of this study was to investigate the effects of age and falls history on antagonist leg muscle coactivation during walking with and without optical flow perturbations of different amplitudes.

METHODS

Eleven young adults [mean (standard deviation) age: 24.8 (4.8) years], eleven older non-fallers [75.3 (5.4) years] and eleven older fallers [age: 78 (7.6) years] participated in this study. Participants completed 2-minute walking trials while watching a speed-matched virtual hallway that, in some conditions, included mediolateral optical flow perturbations designed to elicit the visual perception of imbalance.

FINDINGS

We first found that lower leg antagonist muscle coactivation during normal walking increased with age, independent of falls history. We also found that older but not young adults increased antagonist leg muscle coactivation in the presence of optical flow perturbations, with more pervasive effects in older adults with a history of falls.

INTERPRETATION

Our findings allude to a greater susceptibility to optical flow perturbations in older fallers during walking, which points to a higher potential for risk of instability in more complex and dynamic everyday environments. These findings may also have broader impacts related to the design of innovative training paradigms and neuromuscular targets for falls prevention.

Neural Control of Balance During Walking

Neural control of standing balance has been extensively studied. However, most falls occur during walking rather than standing, and findings from standing balance research do not necessarily carry over to walking. This is primarily due to the constraints of the gait cycle: Body configuration changes dramatically over the gait cycle, necessitating different responses as this configuration changes. Notably, certain responses can only be initiated at specific points in the gait cycle, leading to onset times ranging from 350 to 600 ms, much longer than what is observed during standing (50-200 ms). Here, we investigated the neural control of upright balance during walking. Specifically, how the brain transforms sensory information related to upright balance into corrective motor responses. We used visual disturbances of 20 healthy young subjects walking in a virtual reality cave to induce the perception of a fall to the side and analyzed the muscular responses, changes in ground reaction forces and body kinematics. Our results showed changes in swing leg foot placement and stance leg ankle roll that accelerate the body in the direction opposite of the visually induced fall stimulus, consistent with previous results. Surprisingly, ankle musculature activity changed rapidly in response to the stimulus, suggesting the presence of a direct reflexive pathway from the visual system to the spinal cord, similar to the vestibulospinal pathway. We also observed systematic modulation of the ankle push-off, indicating the discovery of a previously unobserved balance mechanism. Such modulation has implications not only for balance but plays a role in modulation of step width and length as well as cadence. These results indicated a temporally-coordinated series of balance responses over the gait cycle that insures flexible control of upright balance during walking.

The motor repertoire of older adult fallers may constrain their response to balance perturbations

Older adults are at a high risk of falls, and most falls occur during locomotor activities like walking. This study aimed to improve our understanding of changes in neuromuscular control associated with increased risk of falls in older adults in the presence of dynamic balance challenges during walking. Motor module (also known as muscle synergy) analyses identified changes in the neuromuscular recruitment of leg muscles during walking with and without perturbations designed to elicit the visual perception of lateral instability. During normal walking we found that a history of falls (but not age) was associated with reduced motor module complexity and that age (but not a history of falls) was associated with increased step-to-step variability of module recruitment timing. Furthermore, motor module complexity was unaltered in the presence of optical flow perturbations. The specific effects of a history of falls on leg muscle recruitment included an absence and/or inability to independently recruit motor modules normally recruited to perform biomechanical functions important for walking balance control. These results suggest that fallers do not recruit the appropriate motor modules necessary for well-coordinated walking balance control even in the presence of perturbations. The identified changes in the modular control of walking balance in older fallers may either represent a neural deficit that leads to poor balance control or a prior history of falls that results in a compensatory motor adaptation. In either case, our study provides initial evidence that a reduced motor repertoire in older adult fallers may be a constraint on their ability to appropriately respond to balance challenges during walking. NEW & NOTEWORTHY This is the first study to demonstrate a reduced motor repertoire during walking in older adults with a history of falls but without any overt neurological deficits. Furthermore, using virtual reality during walking to elicit the visual perception of lateral instability, we provide initial evidence that a reduced motor repertoire in older adult fallers may be a constraint on their ability to appropriately respond to balance challenges during walking.

Differentiation in Theta and Beta Electrocortical Activity between Visual and Physical Perturbations to Walking and Standing Balance

Human balance is a complex process in healthy adults, requiring precisely timed coordination among sensory information, cognitive processing, and motor control. It has been difficult to quantify brain dynamics during human balance control due to limitations in brain-imaging modalities. The goal of this study was to determine whether by using high-density electroencephalography (EEG) and independent component analysis, we can identify common cortical responses to visual and physical balance perturbations during walking and standing. We studied the responses of 30 healthy young adults to sensorimotor perturbations that challenged their balance. Subjects performed four 10 min trials of beam walking and tandem stance while either being mediolaterally pulled at the waist or viewing brief 20° field-of-view rotations in virtual reality. We recorded high-density EEG, motion capture, lower leg electromyography (EMG), and neck EMG. We hypothesized that both physical pull and visual rotation perturbations would elicit time-frequency fluctuations in theta (4-8 Hz) and beta (13-30 Hz) bands, with increased occipito-parietal activity during visual rotations compared with pull perturbations. Our results confirmed this hypothesis. For both perturbations, we found early theta synchronization and late alpha-beta (8-30 Hz) desynchronization following perturbation onset. This pattern was strongest in occipito-parietal areas during visual perturbations and strongest in sensorimotor areas during pull perturbations. These results suggest a similar time-frequency electrocortical pattern when humans respond to sensorimotor conflict, but with substantive differences in the brain areas involved for visual versus physical perturbations. Our findings may have important implications for assessing and training balance in individuals with and without motor disabilities.

Age-Related Imbalance Is Associated With Slower Walking Speed: An Analysis From the National Health and Nutrition Examination Survey

BACKGROUND AND PURPOSE

Walking speed is an important dimension of gait function and is known to decline with age. Gait function is a process of dynamic balance and motor control that relies on multiple sensory inputs (eg, visual, proprioceptive, and vestibular) and motor outputs. These sensory and motor physiologic systems also play a role in static postural control, which has been shown to decline with age. In this study, we evaluated whether imbalance that occurs as part of healthy aging is associated with slower walking speed in a nationally representative sample of older adults.

METHODS

We performed a cross-sectional analysis of the previously collected 1999 to 2002 National Health and Nutrition Examination Survey (NHANES) data to evaluate whether age-related imbalance is associated with slower walking speed in older adults aged 50 to 85 years (n = 2116). Balance was assessed on a pass/fail basis during a challenging postural task-condition 4 of the modified Romberg Test-and walking speed was determined using a 20-ft (6.10 m) timed walk. Multivariable linear regression was used to evaluate the association between imbalance and walking speed, adjusting for demographic and health-related covariates. A structural equation model was developed to estimate the extent to which imbalance mediates the association between age and slower walking speed.

RESULTS

In the unadjusted regression model, inability to perform the NHANES balance task was significantly associated with 0.10 m/s slower walking speed (95% confidence interval: -0.13 to -0.07; P < .01). In the multivariable regression analysis, inability to perform the balance task was significantly associated with 0.06 m/s slower walking speed (95% confidence interval: -0.09 to -0.03; P < .01), an effect size equivalent to 12 years of age. The structural equation model estimated that age-related imbalance mediates 12.2% of the association between age and slower walking speed in older adults.

CONCLUSIONS

In a nationally representative sample, age-related balance limitation was associated with slower walking speed. Balance impairment may lead to walking speed declines. In addition, reduced static postural control and dynamic walking speed that occur with aging may share common etiologic origins, including the decline in visual, proprioceptive, and vestibular sensory and motor functions.

Visually fixating or tracking another person decreases balance control in young and older females walking in a real-world scenario

Balance control during overground walking was assessed in 10 young (23.6 ± 3.4) and 10 older (71.0 ± 5.5 years) healthy females during free gaze, and when fixating or tracking another person in an everyday use waiting room. Balance control was characterised by medial/lateral sacrum acceleration dispersion, and gaze fixations were simultaneously assessed with eye tracking equipment. The results showed decreased balance control when fixating a stationary (p = 0.003, g_av = 0.19) and tracking a walking (p = 0.027, g_av = 0.16) person compared to free gaze. The older adults exhibited reduced baseline stability throughout, but the decrease caused by the visual tasks was not more profound than the younger adults. The decreased balance control when fixating on or tracking the observed person was likely due to more challenging conditions for interpreting retinal flow, which facilitated less reliable estimates of self-motion through vision. The older adults either processed retinal flow during the tasks as effectively as the young adults, or they adopted a more rigid posture to facilitate visual stability, which masked any ageing effect of the visual tasks. The decrease in balance control, the first to be shown in this context, may warrant further investigation in those with ocular or vestibular dysfunction.

Performance of an attention-demanding task during treadmill walking shifts the noise qualities of step-to-step variation in step width

BACKGROUND

The fractal scaling evident in the step-to-step fluctuations of stepping-related time series reflects, to some degree, neuromotor noise.

RESEARCH QUESTION

The primary purpose of this study was to determine the extent to which the fractal scaling of step width, step width and step width variability are affected by performance of an attention-demanding task. We hypothesized that the attention-demanding task would shift the structure of the step width time series toward white, uncorrelated noise.

METHODS

Subjects performed two 10-min treadmill walking trials, a control trial of undisturbed walking and a trial during which they performed a mental arithmetic/texting task. Motion capture data was converted to step width time series, the fractal scaling of which were determined from their power spectra.

RESULTS

Fractal scaling decreased by 22% during the texting condition (p < 0.001) supporting the hypothesized shift toward white uncorrelated noise. Step width and step width variability increased 19% and five percent, respectively (p < 0.001). However, a stepwise discriminant analysis to which all three variables were input revealed that the control and dual task conditions were discriminated only by step width fractal scaling.

SIGNIFICANCE

The change of the fractal scaling of step width is consistent with increased cognitive demand and suggests a transition in the characteristics of the signal noise. This may reflect an important advance toward the understanding of the manner in which neuromotor noise contributes to some types of falls. However, further investigation of the repeatability of the results, the sensitivity of the results to progressive increases in cognitive load imposed by attention-demanding tasks, and the extent to which the results can be generalized to the gait of older adults seems warranted.

Age-induced changes in the lower limb muscle activities during uphill walking at steep grades

BACKGROUND

Uphill walking is particularly challenging for elderly persons. However, there is a lack of age-differentiated studies investigating the underlying differences in muscle activation when walking on sloped surfaces. These studies are needed, e.g., for planning of evacuations of large modern cruise ships with long walking distances on often inclined surfaces.

METHODS

An age-differentiated, gender-balanced study with 26 young (20-30 years) and 26 elderly people (60-77 years) was therefore conducted, investigating uphill walking at 7° and at 14° contrasted to level walking on a treadmill. EMG signals of musculus gluteus maximus (GMAX), m. biceps femoris (BF), m. rectus femoris (RF), m. vastus medialis (VM), m. gastrocnemius medialis (GAS) and m. soleus (SOL) were analysed with regard to mean and maximum muscle activities and timing during the gait cycle.

RESULTS

The results showed that walking uphill at 14° was highly strainful for elderly people. In line with previous research, young people mostly "pushed" themselves uphill with the GAS and SOL. In contrast, elderly people not only used the known compensatory ability of hip muscles to propel the trailing leg, but also showed a high level of BF activation prolonged until the mid-stance phase at the steepest uphill gradient of 14°.

SIGNIFICANCE

The strikingly long activation of the BF until the mid-stance phase in elderly people at steep gradients is an unexpected, new finding. It suggests that, instead of pushing, elderly people "pull" themselves uphill. In cases of evacuations, the lower physical fitness levels of elderly passengers and their increased need for assistance on inclined surfaces have to be planned for in advance. Considering the findings in (home) training programmes might help elderly people to strengthen lower limb muscles and to enhance the efficiency of muscle activation patterns enabling them to manage steep inclinations more easily.

Does local dynamic stability during unperturbed walking predict the response to balance perturbations? An examination across age and falls history

BACKGROUND

Older adults are at an exceptionally high risk of falls, and most falls occur during locomotor activities such as walking. Reduced local dynamic stability in old age is often interpreted to suggest a lessened capacity to respond to more significant balance challenges encountered during walking and future falls risk. However, it remains unclear whether local dynamic stability during normal, unperturbed walking predicts the response to larger external balance disturbances.

RESEARCH QUESTION

We tested the hypothesis that larger values of local dynamic instability during unperturbed walking would positively correlate with larger changes thereof due to optical flow balance perturbations.

METHODS

We used trunk kinematics collected in subjects across a spectrum of walking balance integrity - young adults, older non-fallers, and older fallers - during walking with and without mediolateral optical flow perturbations of four different amplitudes.

RESULTS

We first found evidence that optical flow perturbations of sufficient amplitude appear capable of revealing independent effects of aging and falls history that are not otherwise apparent during normal, unperturbed walking. We also reject our primary hypothesis; a significant negative correlation only in young adults indicated that individuals with more local dynamic instability during normal, unperturbed walking exhibited smaller responses to optical flow perturbations. In contrast, most prominently in older fallers, the response to optical flow perturbations appeared independent of their baseline level of dynamic instability.

SIGNIFICANCE

We propose that predicting the response to balance perturbations in older fallers, at least that measured using local dynamic stability, likely requires measuring that response directly.

Aging effects on leg joint variability during walking with balance perturbations

BACKGROUND

Older adults are more susceptible to balance perturbations during walking than young adults. However, we lack an individual joint-level understanding of how aging affects the neuromechanical strategies used to accommodate balance perturbations.

RESEARCH QUESTION

We investigated gait phase-dependence in and aging effects on leg joint kinematic variability during walking with balance perturbations. We hypothesized that leg joint variability would: 1) vary across the gait cycle and 2) increase with balance perturbations. We also hypothesized that perturbation effects on leg joint kinematic variability would be larger and more pervasive in older versus young adults.

METHODS

We collected leg joint kinematics in young and older adults walking with and without mediolateral optical flow perturbations of different amplitudes.

RESULTS

We first found that leg joint variability during walking is gait phase-dependent, with step-to-step adjustments occurring predominantly during push-off and early swing. Second, young adults accommodated perturbations almost exclusively by increasing coronal plane hip joint variability, likely to adjust step width. Third, perturbations elicited larger and more pervasive increases in all joint kinematic outcome measures in older adults. Finally, we also provide insight into which joints contribute more to foot placement variability in walking, adding that variability in sagittal plane knee and coronal plane hip joint angles contributed most to that in step length and step width, respectively.

SIGNIFICANCE

Taken together, our findings may be highly relevant to identifying specific joint-level therapeutic targets to mitigate balance impairment in our aging population.

Altered postural control variability in older-aged individuals with a history of lateral ankle sprain

The current study aimed to examine postural control performance during a single-leg balance task in elderly individuals with and without a previous history of lateral ankle sprain (LAS). Eighteen adults with a previous history of LAS (mean age = 66 years old) and 12 healthy controls (mean age = 65 years old) were included in the study. Participants performed three trials of a single-leg balance task during an eyes-opened condition for 20-s. Center of pressure (COP) trajectories in the anteroposterior (AP) and mediolateral (ML) directions were collected with a force plate. The following postural control measures were calculated in the AP and ML directions: 1) Sample Entropy (SampEn); 2) Approximate Entropy (ApEn); 3) mean of Time-to-Boundary minima (mean TTB); and 4) COP velocity (COPV). Older-age participants with a history LAS exhibited lower ApEn-AP, SampEn-AP, and SampEn-ML values compared to healthy controls (p < 0.05). The information gained from this investigation indicates more rigid postural control patterns, less adaptability, and more difficulty maintaining COP during a single-leg balance task in adults with a previous history of LAS. Our data suggest that there is a need to consider history of musculoskeletal injury when evaluating factors for postural control and fall risk in the elderly. Future investigations are needed to assess the effect of LAS on age-related declines in postural control and discern associations between potential risk factors of fall-related injuries and LAS in an elderly population.

Does dynamic stability govern propulsive force generation in human walking?

Before succumbing to slower speeds, older adults may walk with a diminished push-off to prioritize stability over mobility. However, direct evidence for trade-offs between push-off intensity and balance control in human walking, independent of changes in speed, has remained elusive. As a critical first step, we conducted two experiments to investigate: (i) the independent effects of walking speed and propulsive force (F_P) generation on dynamic stability in young adults, and (ii) the extent to which young adults prioritize dynamic stability in selecting their preferred combination of walking speed and F_P generation. Subjects walked on a force-measuring treadmill across a range of speeds as well as at constant speeds while modulating their F_P according to a visual biofeedback paradigm based on real-time force measurements. In contrast to improvements when walking slower, walking with a diminished push-off worsened dynamic stability by up to 32%. Rather, we find that young adults adopt an F_P at their preferred walking speed that maximizes dynamic stability. One implication of these findings is that the onset of a diminished push-off in old age may independently contribute to poorer balance control and precipitate slower walking speeds.

Smooth pursuits decrease balance control during locomotion in young and older healthy females

Dynamic balance control-characterised as movement of the trunk and lower limbs-was assessed during fixation of a fixed target, smooth pursuits and saccadic eye movements in ten young (22.9 ± 1.5 years) and ten older (72.1 ± 8.2 years) healthy females walking overground. Participants were presented with visual stimuli to initiate eye movements, and posture and gaze were assessed with motion analysis and eye tracking equipment. The results showed an increase in medial/lateral (ML) trunk movement (C7: p = 0.012; sacrum: p = 0.009) and step-width variability (p = 0.052) during smooth pursuits compared to a fixed target, with no changes for saccades compared to a fixed target. The elders demonstrated greater ML trunk movement (sacrum: p = 0.037) and step-width variability (p = 0.037) than the younger adults throughout, although this did not interact with the eye movements. The findings showed that smooth pursuits decreased balance control in young and older adults similarly, which was likely a consequence of more complicated retinal flow. Since healthy elders are typically already at a postural disadvantage, further decreases in balance caused by smooth pursuits are undesirable.

The Neuromuscular Origins of Kinematic Variability during Perturbed Walking

We investigated the neuromuscular contributions to kinematic variability and thus step to step adjustments in posture and foot placement across a range of walking speeds in response to optical flow perturbations of different amplitudes using a custom virtual environment. We found that perturbations significantly increased step width, decreased step length, and elicited larger trunk sway compared to normal walking. However, perturbation-induced effects on the corresponding variabilities of these measurements were much more profound. Consistent with our hypotheses, we found that: (1) perturbations increased EMG activity of the gluteus medius and postural control muscles during leg swing, and increased antagonist leg muscle coactivation during limb loading in early stance, and (2) changes in the magnitude of step to step adjustments in postural sway and lateral foot placement positively correlated with those of postural control and gluteus medius muscle activities, respectively, in response to perturbations. However, (3) interactions between walking speed and susceptibility to perturbations, when present, were more complex than anticipated. Our study provides important mechanistic neuromuscular insight into walking balance control and important reference values for the emergence of balance impairment.

Neuroimaging of Human Balance Control: A Systematic Review

This review examined 83 articles using neuroimaging modalities to investigate the neural correlates underlying static and dynamic human balance control, with aims to support future mobile neuroimaging research in the balance control domain. Furthermore, this review analyzed the mobility of the neuroimaging hardware and research paradigms as well as the analytical methodology to identify and remove movement artifact in the acquired brain signal. We found that the majority of static balance control tasks utilized mechanical perturbations to invoke feet-in-place responses (27 out of 38 studies), while cognitive dual-task conditions were commonly used to challenge balance in dynamic balance control tasks (20 out of 32 studies). While frequency analysis and event related potential characteristics supported enhanced brain activation during static balance control, that in dynamic balance control studies was supported by spatial and frequency analysis. Twenty-three of the 50 studies utilizing EEG utilized independent component analysis to remove movement artifacts from the acquired brain signals. Lastly, only eight studies used truly mobile neuroimaging hardware systems. This review provides evidence to support an increase in brain activation in balance control tasks, regardless of mechanical, cognitive, or sensory challenges. Furthermore, the current body of literature demonstrates the use of advanced signal processing methodologies to analyze brain activity during movement. However, the static nature of neuroimaging hardware and conventional balance control paradigms prevent full mobility and limit our knowledge of neural mechanisms underlying balance control.

Do kinematic metrics of walking balance adapt to perturbed optical flow?

Visual (i.e., optical flow) perturbations can be used to study balance control and balance deficits. However, it remains unclear whether walking balance control adapts to such perturbations over time. Our purpose was to investigate the propensity for visuomotor adaptation in walking balance control using prolonged exposure to optical flow perturbations. Ten subjects (age: 25.4±3.8years) walked on a treadmill while watching a speed-matched virtual hallway with and without continuous mediolateral optical flow perturbations of three different amplitudes. Each of three perturbation trials consisted of 8min of prolonged exposure followed by 1min of unperturbed walking. Using 3D motion capture, we analyzed changes in foot placement kinematics and mediolateral sacrum motion. At their onset, perturbations elicited wider and shorter steps, alluding to a more cautious, general anticipatory balance control strategy. As perturbations continued, foot placement tended toward values seen during unperturbed walking while step width variability and mediolateral sacrum motion concurrently increased. Our findings suggest that subjects progressively shifted from a general anticipatory balance control strategy to a reactive, task-specific strategy using step-to-step adjustments. Prolonged exposure to optical flow perturbations may have clinical utility to reinforce reactive, task-specific balance control through training.

Visuomotor Entrainment and the Frequency-Dependent Response of Walking Balance to Perturbations

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 balance deficits 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.

Perspectives on Aging Vestibular Function

Much is known about age-related anatomical changes in the vestibular system. Knowledge regarding how vestibular anatomical changes impact behavior for older adults continues to grow, in line with advancements in diagnostic testing. However, despite advancements in clinical diagnostics, much remains unknown about the functional impact that an aging vestibular system has on daily life activities such as standing and walking. Modern diagnostic tests are very good at characterizing neural activity of the isolated vestibular system, but the tests themselves are artificial and do not reflect the multisensory aspects of natural human behavior. Also, the majority of clinical diagnostic tests are passively applied because active behavior can enhance performance. In this perspective paper, we review anatomical and behavioral changes associated with an aging vestibular system and highlight several areas where a more functionally relevant perspective can be taken. For postural control, a multisensory perturbation approach could be used to bring balance rehabilitation into the arena of precision medicine. For walking and complex gaze stability, this may result in less physiologically specific impairments, but the trade-off would be a greater understanding of how the aging vestibular system truly impacts the daily life of older adults.

Gait variability in healthy old adults is more affected by a visual perturbation than by a cognitive or narrow step placement demand

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 3-min 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% for 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.