Postural responses and spatial orientation to neck proprioceptive and vestibular inputs during locomotion in young and older adults

Measuring Vestibular Contributions to Age-Related Balance Impairment: A Review

Aging is associated with progressive declines in both the vestibular and human balance systems. While vestibular lesions certainly contribute to imbalance, the specific contributions of age-related vestibular declines to age-related balance impairment is poorly understood. This gap in knowledge results from the absence of a standardized method for measuring age-related changes to the vestibular balance pathways. The purpose of this manuscript is to provide an overview of the existing body of literature as it pertains to the methods currently used to infer vestibular contributions to age-related imbalance.

Podokinetic After-Rotation Is Transiently Enhanced or Reversed by Unilateral Axial Muscle Proprioceptive Stimulation

Unilateral axial muscle vibration, eliciting a proprioceptive volley, is known to incite steering behavior. Whole-body rotation while stepping in place also occurs as an after-effect of stepping on a circular treadmill (podokinetic after-rotation, PKAR). Here, we tested the hypothesis that PKAR is modulated by axial muscle vibration. If both phenomena operate through a common pathway, enhancement or cancellation of body rotation would occur depending on the stimulated side when vibration is administered concurrently with PKAR. Seventeen subjects participated in the study. In one session, subjects stepped in place eyes open on the center of a platform that rotated counterclockwise 60°/s for 10 min. When the platform stopped, subjects continued stepping in place blindfolded. In other session, a vibratory stimulus (100 Hz, 2 min) was administered to right or left paravertebral muscles at lumbar level at two intervals during the PKAR. We computed angular body velocity and foot step angles from markers fixed to shoulders and feet. During PKAR, all subjects rotated clockwise. Decreased angular velocity was induced by right vibration. Conversely, when vibration was administered to the left, clockwise rotation velocity increased. The combined effect on body rotation depended on the time at which vibration was administered during PKAR. Under all conditions, foot step angle was coherent with shoulder angular velocity. PKAR results from continuous asymmetric input from the muscles producing leg rotation, while axial muscle vibration elicits a proprioceptive asymmetric input. Both conditioning procedures appear to produce their effects through a common mechanism. We suggest that both stimulations would affect our straight ahead by combining their effects in an algebraic mode.

Postural Control in Bilateral Vestibular Failure: Its Relation to Visual, Proprioceptive, Vestibular, and Cognitive Input

Patients with bilateral vestibular failure (BVF) suffer from postural and gait unsteadiness with an increased risk of falls. The aim of this study was to elucidate the differential role of otolith, semicircular canal (SSC), visual, proprioceptive, and cognitive influences on the postural stability of BVF patients. Center-of-pressure displacements were recorded by posturography under six conditions: target visibility; tonic head positions in the pitch plane; horizontal head shaking; sensory deprivation; dual task; and tandem stance. Between-group analysis revealed larger postural sway in BVF patients on eye closure; but with the eyes open, BVF did not differ from healthy controls (HCs). Head tilts and horizontal head shaking increased sway but did not differ between groups. In the dual task condition, BVF patients maintained posture indistinguishable from controls. On foam and tandem stance, postural sway was larger in BVF, even with the eyes open. The best predictor for the severity of bilateral vestibulopathy was standing on foam with eyes closed. Postural control of our BVF was indistinguishable from HCs once visual and proprioceptive feedback is provided. This distinguishes them from patients with vestibulo-cerebellar disorders or functional dizziness. It confirms previous reports and explains that postural unsteadiness of BVF patients can be missed easily if not examined by conditions of visual and/or proprioceptive deprivation. In fact, the best predictor for vestibular hypofunction (VOR gain) was examining patients standing on foam with the eyes closed. Postural sway in that condition increased with the severity of vestibular impairment but not with disease duration. In the absence of visual control, impaired otolith input destabilizes BVF with head retroflexion. Stimulating deficient SSC does not distinguish patients from controls possibly reflecting a shift of intersensory weighing toward proprioceptive-guided postural control. Accordingly, proprioceptive deprivation heavily destabilizes BVF, even when visual control is provided.

Mastoid Vibration Affects Dynamic Postural Control During Gait

Our objective was to investigate how manipulating sensory input through mastoid vibration (MV) could affect dynamic postural control during walking, with and without simultaneous manipulation of the visual and the somatosensory systems. We used three levels of MV (none, unilateral, and bilateral) via vibrating elements placed on the mastoid processes. We combined this with the six conditions of the Locomotor Sensory Organization Test (LSOT) paradigm to challenge the visual and somatosensory systems. We hypothesized that MV would affect both amount and temporal structure measures of sway variability during walking and that, in combination with manipulations of the visual and the somatosensory inputs, MV would augment the effects previously observed. The results confirmed that MV produced a significant increase in the amount of sway variability in both anterior-posterior and medial-lateral directions. Significant changes in the temporal structure of sway variability were only observed in the anterior-posterior direction. Bilateral MV produced larger effects than unilateral stimulation. We concluded that sensory input while walking could be affected using MV. Combining MV with manipulations of visual and somatosensory input could allow us to better understand the contributions of the sensory systems during locomotion.

Contribution of Head Position, Standing Surface, and Vision to Postural Control in Community-Dwelling Older Adults

Postural control requires the integration of sensorimotor information to maintain balance and to properly position and orient the body in response to external stimuli. Age-related declines in peripheral and central sensory and motor function contribute to postural instability and falls. This study investigated the contribution of head position, standing surface, and vision on postural sway in 26 community-dwelling older adults. Participants were asked to maintain a stable posture under conditions that varied standing surface, head position, and the availability of visual information. Significant main and interaction effects were found for all three factors. Findings from this study suggest that postural sway responses require the integration of available sources of sensory information. These results have important implications for fall risks in older adults and suggest that when standing with the head extended and eyes closed, older adults may place themselves at risk for postural disequilibrium and loss of balance.

Techniques and Methods for Testing the Postural Function in Healthy and Pathological Subjects

The different techniques and methods employed as well as the different quantitative and qualitative variables measured in order to objectify postural control are often chosen without taking into account the population studied, the objective of the postural test, and the environmental conditions. For these reasons, the aim of this review was to present and justify the different testing techniques and methods with their different quantitative and qualitative variables to make it possible to precisely evaluate each sensory, central, and motor component of the postural function according to the experiment protocol under consideration. The main practical and technological methods and techniques used in evaluating postural control were explained and justified according to the experimental protocol defined. The main postural conditions (postural stance, visual condition, balance condition, and test duration) were also analyzed. Moreover, the mechanistic exploration of the postural function often requires implementing disturbing postural conditions by using motor disturbance (mechanical disturbance), sensory stimulation (sensory manipulation), and/or cognitive disturbance (cognitive task associated with maintaining postural balance) protocols. Each type of disturbance was tackled in order to facilitate understanding of subtle postural control mechanisms and the means to explore them.

Assessment of gait direction changes during straight-ahead walking in healthy elderly and Huntington disease patients using a shank worn MIMU

The aim of this study was to propose and comparatively evaluate four methods for assessing stride-by-stride changes of direction of progression, during straight walking using measurements of a magnetic and inertial unit placed above the malleolus. The four methods were evaluated by comparing their estimate of the gait changes of direction of progression with that obtained from an instrumented gait mat used as a gold standard. The methods were applied to the data obtained from the gait of both healthy subjects and patients with Huntington Disease, the latter characterized by a jerky swing phase. The results showed that the errors associated to the best estimates of the gait direction changes were about 10% of its range of variability for the healthy subjects and increased to about 30% for the patients, both walking at comfortable speed when the range of variability is the largest. Additional testing on gait at various radius of curvature should be carried out to fully validate the MIMU-based estimates.

Neck proprioception shapes body orientation and perception of motion

This review article deals with some effects of neck muscle proprioception on human balance, gait trajectory, subjective straight-ahead (SSA), and self-motion perception. These effects are easily observed during neck muscle vibration, a strong stimulus for the spindle primary afferent fibers. We first remind the early findings on human balance, gait trajectory, SSA, induced by limb, and neck muscle vibration. Then, more recent findings on self-motion perception of vestibular origin are described. The use of a vestibular asymmetric yaw-rotation stimulus for emphasizing the proprioceptive modulation of motion perception from the neck is mentioned. In addition, an attempt has been made to conjointly discuss the effects of unilateral neck proprioception on motion perception, SSA, and walking trajectory. Neck vibration also induces persistent aftereffects on the SSA and on self-motion perception of vestibular origin. These perceptive effects depend on intensity, duration, side of the conditioning vibratory stimulation, and on muscle status. These effects can be maintained for hours when prolonged high-frequency vibration is superimposed on muscle contraction. Overall, this brief outline emphasizes the contribution of neck muscle inflow to the construction and fine-tuning of perception of body orientation and motion. Furthermore, it indicates that tonic neck-proprioceptive input may induce persistent influences on the subject's mental representation of space. These plastic changes might adapt motion sensitiveness to lasting or permanent head positional or motor changes.

Stimulating the cerebellum affects visuomotor adaptation but not intermanual transfer of learning

When systematic movement errors occur, the brain responds with a systematic change in motor behavior. This type of adaptive motor learning can transfer intermanually; adaptation of movements of the right hand in response to training with a perturbed visual signal (visuomotor adaptation) may carry over to the left hand. While visuomotor adaptation has been studied extensively, it is unclear whether the cerebellum, a structure involved in adaptation, is important for intermanual transfer as well. We addressed this question with three experiments in which subjects reached with their right hands as a 30° visuomotor rotation was introduced. Subjects received anodal or sham transcranial direct current stimulation on the trained (experiment 1) or untrained (experiment 2) hemisphere of the cerebellum, or, for comparison, motor cortex (M1). After the training period, subjects reached with their left hand, without visual feedback, to assess intermanual transfer of learning aftereffects. Stimulation of the right cerebellum caused faster adaptation, but none of the stimulation sites affected transfer. To ascertain whether cerebellar stimulation would increase transfer if subjects learned faster as well as a larger amount, in experiment 3 anodal and sham cerebellar groups experienced a shortened training block such that the anodal group learned more than sham. Despite the difference in adaptation magnitude, transfer was similar across these groups, although smaller than in experiment 1. Our results suggest that intermanual transfer of visuomotor learning does not depend on cerebellar activity and that the number of movements performed at plateau is an important predictor of transfer.

Assessment of gait deviation on the Babinski-Weill test in healthy Brazilians

OBJECTIVE

The aim of this study was to validate a simple and reproducible method for assessing gait deviation on the Babinski-Weill test in a representative sample of healthy Brazilians.

METHODS

Gait deviations were measured in 75 individuals (median=30 years, 41 women) for forward, backwards, and Babinski-Weill steps. The test entailed blindfolded individuals walking 10 paces at a frequency of 1 Hz with deviations subsequently measured by a protractor.

RESULTS

Mean gait deviation forward was 0.53° with standard deviation (SD)=4.22 and backwards was 2.14° with SD=4.29. No significant difference in deviation was detected between genders (t test p=0.40 forward and p=0.77 backwards) or for age (ANOVA, p=0.33 forward and p=0.63 backwards). On the Babinski-Weill test, mean gait deviation was 5.26°; SD=16.32 in women and -3.11°; SD=12.41 in men, with no significant difference between genders (t test, p=0.056).

DISCUSSION

Defining normative gait patterns helps distinguish pathological states.

Peripheral visual cues affect minimum-foot-clearance during overground locomotion

The importance of peripheral visual cues in the control of minimum-foot-clearance during overground locomotion on a clear path was investigated. Eleven subjects walked at their natural speed whilst wearing goggles providing four different visual conditions: upper occlusion, lower occlusion, circumferential-peripheral occlusion and full vision. Results showed that under circumferential-peripheral occlusion, subjects were more cautious and increased minimum-foot-clearance and decreased walking speed and step length. The minimum-foot-clearance increase can be interpreted as a motor control strategy aiming to safely clear the ground when online visual exproprioceptive cues from the body are not available. The lack of minimum-foot-clearance increase in lower occlusion suggests that the view of a clear pathway from beyond two steps combined with visual exproprioception and optic flow in the upper field were adequate to guide gait. A suggested accompanying safety strategy of reducing the amount of variability of minimum-foot-clearance under circumferential-peripheral occlusion conditions was not found, likely due to the lack of online visual exproprioceptive cues provided by the peripheral visual field for fine-tuning foot trajectory.

Head and neck position sense

Traumatic minor cervical strains are common place in high-impact sports (e.g. tackling) and premature degenerative changes have been documented in sports people exposed to recurrent impact trauma (e.g. scrummaging in rugby) or repetitive forces (e.g. Formula 1 racing drivers, jockeys). While proprioceptive exercises have been an integral part of rehabilitation of injuries in the lower limb, they have not featured as prominently in the treatment of cervical injuries. However, head and neck position sense (HNPS) testing and re-training may have relevance in the management of minor sports-related neck injuries, and play a role in reducing the incidence of ongoing pain and problems with function. For efficacious programmes to be developed and tested, fundamental principles associated with proprioception in the cervical spine should be considered. Hence, this article highlights the importance of anatomical structures in the cervical spine responsible for position sense, and how their interaction with the CNS affects our ability to plan and execute effective purposeful movements. This article includes a review of studies examining position sense in subjects with and without pathology and describes the effects of rehabilitation programmes that have sought to improve position sense. In respect to the receptors providing proprioceptive information for the CNS, the high densities and complex arrays of spindles found in cervical muscles suggest that these receptors play a key role. There is some evidence suggesting that ensemble encoding of discharge patterns from muscle spindles is relayed to the CNS and that a pattern recognition system is used to establish joint position and movement. Sensory information from neck proprioceptive receptors is processed in tandem with information from the vestibular system. There are extensive anatomical connections between neck proprioceptive inputs and vestibular inputs. If positional information from the vestibular system is inaccurate or fails to be appropriately integrated in the CNS, errors in head position may occur, resulting in an inaccurate reference for HNPS, and conversely if neck proprioceptive information is inaccurate, then control of head position may be affected. The cerebellum and cortex also play a role in control of head position, providing feed-forward and modulatory influences depending on the task requirements. Position-matching tasks have been the most popular means of testing position sense in the cervical spine. These allow the appreciation of absolute, constant and variable errors in positioning and have been shown to be reliable. The results of such tests indicate that errors are relatively low (2-5 degrees). It is apparent that error is not consistently affected by age, a finding similar to studies undertaken in peripheral joints. Furthermore, the range of motion in which subjects are tested does not consistently affect accuracy in a predictable manner. However, it is evident that impairments in position sense are observed in individuals who have experienced whiplash-type injuries and individuals with chronic head and neck pain of non-traumatic origin (e.g. cervical spondylosis). While researchers advocate comprehensive retraining protocols, which include eye and neck motion targeting tasks and coordination exercises, as well as co-contraction exercises to reduce such impairments, some studies show that more general exercises and manipulation may be of benefit. Overall, there is limited information concerning the efficacy of treatment programmes.

Visual-vestibular influences on locomotor adjustments for stepping over an obstacle

Combined visual and vestibular influences on locomotor control, particularly in changing environments, are little understood. We studied such influences on body orientation and foot trajectory control during level walking and obstacle avoidance. Six young adults walked on the level and over an obstacle while vision was present or occluded as well as while vestibular information was intact or perturbed using galvanic vestibular stimulation (GVS). The occlusion of vision caused a slowing of gait during obstacle avoidance as well as increased clearance of the leading limb over the obstruction. GVS caused lateral deviations in head and trunk roll angles as well as in foot and trunk displacements, but these lateral deviations were the same during both level walking and obstacle avoidance. In addition, GVS had no effect at all on sagittal plane factors such as speed, foot proximity to the obstacle and vertical clearance over the obstacle. Overall, there is a complex visual control of bilateral obstacle avoidance, but the lack of differences in GVS effects between level and obstructed walking shows that vestibular information is not upregulated for obstacle avoidance. In addition, the robust indifference of anterior foot placement and body displacement to significant lateral deviations from GVS suggests an orthogonally based sensori-locomotor control.

Visual–vestibular interaction during goal directed locomotion: effects of aging and blurring vision

Normal vision overrides perturbed vestibular information for the optimization of performance during goal directed locomotion, suggesting down-regulation of vestibular gain. However, it is not known if the responses to vestibular perturbation are accentuated when vision is impaired. Furthermore, both visual and vestibular systems deteriorate with age. It is not clear, however, how age-related decline in these sensory systems influences visual-vestibular interaction. Therefore, the dual purpose of the present study was to investigate the effects of aging and blurring vision, that simulated the consequences of cataracts, on visual-vestibular interaction. Young and healthy elderly walked to a target located straight ahead with either normal or blurring vision. On randomly selected trials vestibular system perturbation was achieved by applying transmastoidal galvanic vestibular stimulation (GVS). Two different galvanic stimulation intensities were used to provide insight into scaling effect of vestibular perturbation on locomotor performance and how age and vision influences this scaling effect. Maximum path deviation, frontal trunk tilt and postural coordination in the mediolateral direction were evaluated. The magnitude of the path deviation and the trunk tilt response were scaled to the magnitude of the vestibular perturbation in older adults independent of the visual condition. Older participants demonstrated increased coupling of the head and trunk segments irrespective of visual and vestibular perturbations. The results suggest that when visual information was available, the vestibular input reweighting was less effective in older individuals, as shown by the scaled responses to the GVS intensities and the inability to converge efficiently towards the target.

Eccentric eye and head positions in darkness induce deviation from the intended path

Head and gaze are aligned with the actual path during locomotion. Before a turn is made, gaze changes in the direction of the planned trajectory. We investigated whether eccentric horizontal head and/or eye position without vision causes deviations from the intended straight path. Twenty blindfolded healthy volunteers were asked to walk toward a previously seen target 10 m straight ahead. Various combinations of head and eye positions were tested (eye-in-head gaze straight ahead or 35 degrees left or right with head straight ahead or 70 degrees left or right). Head rotation to the left caused a gait deviation to the right (3.7 degrees ) and head rotation to the right caused a deviation to the left (2.7 degrees ; F(2,40) = 34.966; P < 0.00001). Eye position also showed a tendency to cause gait deviations opposite in direction to gaze, which was, however, not significant. Deviations from the intended straight path were largest with head rotation and eyes straight ahead (gaze 70 degrees off target) or eyes opposite to head rotation (gaze 35 degrees off target). Notably, when lateral eye deviation added to head rotation (gaze 105 degrees off target), i.e., gaze is directed backward, mean deviations decreased (2.3 degrees to the right and 1.2 degrees to the left). Thus, we show that (1) eccentric head positions induce direction-specific gait deviations that are independent of concurrent environmental visual information, and (2) that gait deviations are contraversive to eye-head gaze rather than ipsiversive as reported by others for visually controlled locomotion. The direction of deviation may reflect the compensation of an expected or perceived deviation in the direction of gaze.