Reduced postural sway during quiet standing by light touch is due to finger tactile feedback but not mechanical support

It is well known that a light and voluntary touch with a fingertip on a fixed surface improves postural stability during quiet standing. To determine whether the effect of the light touch is due to the tactile sensory input, as opposed to mechanical support, we investigated the light touch effect on postural stability during quiet standing with and without somatosensory input from the fingertip. Seven young subjects maintained quiet standing on a force platform with (LT) and without (NT) lightly touching a fixed surface, and with (TIS) and without (CON) the application of tourniquet ischemia, which removed the tactile sensation from the fingertip. The mean velocity of centre of pressure (CoP) was calculated to assess the postural sway in each condition. The mean velocity of CoP was significantly smaller in the LT condition compared to the NT condition only under the CON condition, whereas the light touch effect was not significant under the TIS condition. We found that the reduction of the horizontal ground reaction force due to the light touch was about 20%, which was approximately equivalent to the reduction of mean velocity of CoP in the LT condition compared to the NT condition. Since the fingertip contact force was relatively large compared to the horizontal ground reaction force, one could say that the light touch effect might be due to the mechanical support provided by the contact itself. However, we demonstrated experimentally that light touch effects were diminished due to loss of finger tactile feedback induced by the tourniquet ischemia, but not due to the mechanical support provided by the light touch. One possible reason is the lack of feedback information in controlling posture, and the other is the altered control of the arm induced by the loss of tactile feedback.     

Unperceivable noise to active light touch effects on fast postural sway

Human postural sway during quiet standing is reduced when a fingertip lightly touches a stable surface. The tactile feedback information from the fingertip has been considered responsible for this effect of light touch. Studies have shown that a noise-like minute stimulation to the sensory system can improve the system's weak signal detection. In the present study, we investigated whether a noise-like unperceivable vibration on the fingertip enhances its tactile sensation and facilitates the effect of light touch during quiet standing. Thirteen volunteers maintained quiet standing while lightly touching a touch surface with the index fingertip. Based on each subject's vibrotactile threshold (VT), a noise-like vibration was applied to the touch surface at amplitudes under (0.5VT) or at VT (1.0VT), in addition to the normal light touch condition (no vibration, 0VT). The results showed that the mean velocities of the foot center of pressure (CoP) in both the anteroposterior (AP) and mediolateral (ML) directions were significantly reduced at 0.5VT compared to 0VT and 1.0VT (P < 0.05), while there was no significant difference between 1.0VT and 0VT (P > 0.05). Frequency analysis of CoP revealed that the power of high-frequency fluctuation (1-10 Hz) was significantly reduced at 0.5VT (P < 0.05), whereas no significant change was observed in that of low-frequency sway (below 1 Hz) (P > 0.05). These results indicate that an unperceivable noise-like vibration can facilitate the effect of light touch on postural stability, by further reducing fast postural sway.     

Postural control during quiet standing following cervical muscular fatigue: effects of changes in sensory inputs

The purpose of the present experiment was to investigate the effects of cervical muscular fatigue on postural control during quiet standing under different conditions of reliability and/or availability of somatosensory inputs from the plantar soles and the ankles and visual information. To this aim, 14 young healthy adults were asked to sway as little as possible in three sensory conditions (No vision, No vision-Foam support and Vision) executed in two conditions of No fatigue and Fatigue of the scapula elevator muscles. Centre of foot pressure (CoP) displacements were recorded using a force platform. Results showed that (1) the cervical muscular fatigue yielded increased CoP displacements in the absence of vision, (2) this effect was more accentuated when somatosensation was degraded by standing on a foam surface and (3) the availability of vision allowed the individuals to suppress this destabilising effect. On the whole, these findings not only stress the importance of intact cervical neuromuscular function on postural control during quiet standing, but also suggest a reweigthing of sensory cues in balance control following cervical muscular fatigue by increasing the reliance on the somatosensory inputs from the plantar soles and the ankles and visual information.     

Controlling posture using a plantar pressure-based,tongue-placed tactile biofeedback system

The present paper introduces an original biofeedback system for improving human balance control, whose underlying principle consists in providing additional sensory information related to foot sole pressure distribution to the user through a tongue-placed tactile output device. To assess the effect of this biofeedback system on postural control during quiet standing, ten young healthy adults were asked to stand as immobile as possible with their eyes closed in two conditions of No-biofeedback and Biofeedback. Centre of foot pressure (CoP) displacements were recorded using a force platform. Results showed reduced CoP displacements in the Biofeedback relative to the No-biofeedback condition. The present findings evidenced the ability of the central nervous system to efficiently integrate an artificial plantar-based, tongue-placed tactile biofeedback for controlling control posture during quiet standing.     

Inter-individual variability in sensory weighting of a plantar pressure-based, tongue-placed tactile biofeedback for controlling posture

The purpose of the present experiment was to investigate whether the sensory weighting of a plantar pressure-based, tongue-placed tactile biofeedback for controlling posture could be subject to inter-individual variability. To achieve this goal, 60 young healthy adults were asked to stand as immobile as possible with their eyes closed in two conditions of No-biofeedback and Biofeedback. Centre of foot pressure (CoP) displacements were recorded using a force platform. Overall, results showed reduced CoP displacements in the Biofeedback relative to the No-biofeedback condition, evidencing the ability of the central nervous system to efficiently integrate an artificial plantar-based, tongue-placed tactile biofeedback for controlling posture during quiet standing. Results further showed a significant positive correlation between the CoP displacements measured in the No-biofeedback condition and the decrease in the CoP displacements induced by the use of the biofeedback. In other words, the degree of postural stabilization appeared to depend on each subject's balance control capabilities, the biofeedback yielding a greater stabilizing effect in subjects exhibiting the largest CoP displacements when standing in the No-biofeedback condition. On the whole, by evidencing a significant inter-individual variability in sensory weighting of an additional tactile information related to foot sole pressure distribution for controlling posture, the present findings underscore the need and the necessity to address the issue of inter-individual variability in the field of neuroscience.     

Effects of maintaining touch contact on predictive and reactive balance

Light touch contact between the body and an environmental referent reduces fluctuations of center of pressure (CoP) in quiet standing although the contact forces are insufficient to provide significant forces to stabilize standing balance. Maintenance of upright standing posture (with light touch contact) may include both predictive and reactive components. Recently Dickstein et al. (2003) demonstrated that reaction to temporally unpredictable displacement of the support surface was affected by light touch raising the question whether light touch effects also occur with predictable disturbance to balance. We examined the effects of shoulder light touch on SD of CoP rate (dCoP) during balance perturbations associated with forward sway induced by pulling on (voluntary), or being pulled by (reactive), a hand-held horizontal load. Prior to perturbation, SD dCoP was lower with light touch, corresponding to previous findings. Immediately after perturbation, SD dCoP(AP) was greater with light touch in the case of voluntary pull, whereas no difference was found for reflex pull. However, in the following time course, light touch contact again resulted in a significantly lower SD dCoP and faster stabilization of SD dCoP. We conclude that shoulder light touch contact affects immediate postural responses to voluntary pull but also stabilization after voluntary and reflex perturbation. We suggest that in voluntary perturbation CoP fluctuations are differentially modulated in anterioposterior and mediolateral directions to maintain light touch, which not only provides augmented sensory feedback about body self-motion, but may act as a "constraint" to the postural control system when preparing postural adjustments.     

Vibratory noise to the fingertip enhances balance improvement associated with light touch

Light touch of a fingertip on an external stable surface greatly improves the postural stability of standing subjects. The hypothesis of the present work was that a vibrating surface could increase the effectiveness of fingertip signaling to the central nervous system (e.g., by a stochastic resonance mechanism) and hence improve postural stability beyond that achieved by light touch. Subjects stood quietly over a force plate while touching with their right index fingertip a surface that could be either quiescent or randomly vibrated at two low-level noise intensities. The vibratory noise of the contact surface caused a significant decrease in postural sway, as assessed by center of pressure measures in both time and frequency domains. Complementary experiments were designed to test whether postural control improvements were associated with a stochastic resonance mechanism or whether attentional mechanisms could be contributing. A full curve relating body sway parameters and different levels of vibratory noise resulted in a U-like function, suggesting that the improvement in sway relied on a stochastic resonance mechanism. Additionally, no decrease in postural sway was observed when the vibrating contact surface was attached to the subject’s body, suggesting that no attentional mechanisms were involved. These results indicate that sensory cues obtained from the fingertip need not necessarily be associated with static contact surfaces to cause improvement in postural stability. A low-level noisy vibration applied to the contact surface could lead to a better performance of the postural control system.     

Head position-based electrotactile tongue biofeedback affects postural responses to Achilles tendon vibration in humans

The purpose of the present experiment was to investigate whether postural responses to ankle proprioceptive perturbation Achilles tendon vibration were affected by the availability of augmented sensory information about head orientation/motion with respect to gravitational vertical, i.e., normally provided by the vestibular system. To achieve this goal, ten standing subjects were exposed to Achilles tendon vibration in two No Biofeedback and Biofeedback conditions. The No Biofeedback condition served as a control condition. In the Biofeedback condition, subjects performed the postural task using a head position-based electrotactile tongue-placed biofeedback system. Center of foot pressure (CoP) displacements were recorded using a force platform. Results showed that (1) Achilles tendon vibration increased CoP displacements in the No Biofeedback condition and (2) this destabilizing effect was less accentuated in the Biofeedback condition. These results are consistent with and discussed in terms of sensory re-weighting mechanisms involved in postural control. In the condition of Achilles tendon vibration, which renders ankle proprioceptive information less reliable for controlling posture, the central nervous system was able to integrate alternatively available augmented sensory information suitable and usable in upright postural control to reduce the destabilizing effect of the ankle proprioceptive perturbation.     

Postural stabilization from fingertip contact: I. Variations in sway attenuation, perceived stability and contact forces with aging

In this study, we compared the ability of young (n=10, 19–32 years) and older subjects (n=35, 60–86 years) to use fingertip contact as a balance aid during quiet stance under various conditions to determine whether aging would influence contact strategies. Experimental trials (duration, 60 s) included two visual conditions (vision; no vision), three fingertip contact conditions (no touch; smooth touch; rough touch) and two support surface conditions (firm; foam). In trials with contact, participants were required to maintain a light contact with their right index fingertip on an instrumented touch-plate. Subjects were not constrained to exert minimal contact force, although they were aware that the touch-plate was not designed for physical support. From displacements of the centre of foot pressure (COP), mean sway amplitude (MSA) was computed in the anterior-posterior (COP_AP) and medio-lateral (COP_ML) directions. Subjective estimates of stability were also obtained by asking participants to rate perceived stability on a visual analog scale in each condition. Mean normal force (F _N ) and mean resultant tangential force (F _TAN) were computed from contact force data applied on the touch plate. In both age groups, touch conditions had a substantial effect on MSA in the AP direction under both support surface conditions, with reductions averaging between 40–55% when touch was allowed. Reductions in the ML direction, though less important (8–12% on average), were nevertheless highly significant, especially in the older subjects when standing on the foam. In the two groups, vision and texture had only marginal impact on MSA computed on both support surfaces. Contrasting with sway measurements, stability ratings were highly influenced by visual conditions in both age groups. Only in conditions of deficient support (foam surface) and absent vision did the perceived effect of touch exceed that of vision. Age had a major impact, however, on contact forces deployed during trials with touch. While individuals in the young group typically produced forces of <1 N (mean F _N , 0.32±0.15 N) to achieve postural stabilization, older subjects tended to use higher, though not too excessive, contact forces (mean F _N , 1.21±0.75 N) under the same conditions. From these findings, we conclude that the ability to use contact cues from the fingertip as a source of sensory information to improve postural stability is largely preserved in healthy older adults. The increase in contact force deployed by older individuals to achieve postural stabilization is interpreted as a compensatory strategy to help overcome age-related loss in tactile sensation, an issue that will be further addressed in a companion paper.     

Sensory supplementation system based on electrotactile tongue biofeedback of head position for balance control

The present study aimed at investigating the effects of an artificial head position-based tongue-placed electrotactile biofeedback on postural control during quiet standing under different somatosensory conditions from the support surface. Eight young healthy adults were asked to stand as immobile as possible with their eyes closed on two Firm and Foam support surface conditions executed in two conditions of No-biofeedback and Biofeedback. In the Foam condition, a 6-cm thick foam support surface was placed under the subjects' feet to alter the quality and/or quantity of somatosensory information at the plantar sole and the ankle. The underlying principle of the biofeedback consisted of providing supplementary information about the head orientation with respect to gravitational vertical through electrical stimulation of the tongue. Centre of foot pressure (CoP) displacements were recorded using a force platform. Larger CoP displacements were observed in the Foam than Firm conditions in the two conditions of No-biofeedback and Biofeedback. Interestingly, this destabilizing effect was less accentuated in the Biofeedback than No-biofeedback condition. In accordance with the sensory re-weighting hypothesis for balance control, the present findings evidence that the availability of the central nervous system to integrate an artificial head orientation information delivered through electrical stimulation of the tongue to limit the postural perturbation induced by alteration of somatosensory input from the support surface.     

Sensory supplementation system based on electrotactile tongue biofeedback of head position for balance control

The present study aimed at investigating the effects of an artificial head position-based tongue-placed electrotactile biofeedback on postural control during quiet standing under different somatosensory conditions from the support surface. Eight young healthy adults were asked to stand as immobile as possible with their eyes closed on two Firm and Foam support surface conditions executed in two conditions of No-biofeedback and Biofeedback. In the Foam condition, a 6-cm thick foam support surface was placed under the subjects’ feet to alter the quality and/or quantity of somatosensory information at the plantar sole and the ankle. The underlying principle of the biofeedback consisted of providing supplementary information about the head orientation with respect to gravitational vertical through electrical stimulation of the tongue. Centre of foot pressure (CoP) displacements were recorded using a force platform. Larger CoP displacements were observed in the Foam than Firm conditions in the two conditions of No-biofeedback and Biofeedback. Interestingly, this destabilizing effect was less accentuated in the Biofeedback than No-biofeedback condition. In accordance with the sensory re-weighting hypothesis for balance control, the present findings evidence that the availability of the central nervous system to integrate an artificial head orientation information delivered through electrical stimulation of the tongue to limit the postural perturbation induced by alteration of somatosensory input from the support surface.     

Multisensory information for human postural control: integrating touch and vision

Despite extensive research on the influence of visual, vestibular and somatosensory information on human postural control, it remains unclear how these sensory channels are fused for self-orientation. The focus of the present study was to test whether a linear additive model could account for the fusion of touch and vision for postural control. We simultaneously manipulated visual and somatosensory (touch) stimuli in five conditions of single- and multisensory stimulation. The visual stimulus was a display of random dots projected onto a screen in front of the standing subject. The somatosensory stimulus was a rigid plate which subjects contacted lightly (<1 N of force) with their right index fingertip. In each condition, one sensory stimulus oscillated (dynamic) in the medial-lateral direction while the other stimulus was either dynamic, static or absent. The results qualitatively supported five predictions of the linear additive model in that the patterns of gain and variability across conditions were consistent with model predictions. However, a strict quantitative comparison revealed significant deviations from model predictions, indicating that the sensory fusion process clearly has nonlinear aspects. We suggest that the sensory fusion process behaved in an approximately linear fashion because the experimental paradigm tested postural control very close to the equilibrium point of vertical upright.     

Experimental neck muscle pain impairs standing balance in humans

Impaired postural control has been reported in patients with chronic neck pain of both traumatic and non-traumatic etiologies, but whether painful stimulation of neck muscle per se can affect balance control during quiet standing in humans remains unclear. The purpose of the present experiment was thus to investigate the effect of experimental neck muscle pain on standing balance in young healthy adults. To achieve this goal, 16 male university students were asked to stand upright as still as possible on a force platform with their eyes closed in two conditions of No pain and Pain of the neck muscles elicited by experimental painful electrical stimulation. Postural control and postural performance were assessed by the displacements of the center of foot pressure (CoP) and of the center of mass (CoM), respectively. The results showed increased CoP and CoM displacements variance, range, mean velocity, and mean and median frequencies in the Pain relative to the No pain condition. The present findings emphasize the destabilizing effect of experimental neck muscle pain per se, and more largely stress the importance of intact neck neuromuscular function on standing balance.     

Age differences in visual sensory integration

Summary Numerous authors have reported that elderly persons are more affected than young adults when submitted to reduced or conflicting sensory inputs conditions. These results, however, do not permit to evaluate whether the elderly suffer from a reduced peripheral sensibility or from a deficit in the central integrative mechanisms responsible for configuring the postural set. The present experiment evaluated the ability of elderly to reconfigure the postural set when submitted to successive reduced and augmented visual sensory conditions. Results showed that young and elderly subjects' sway dispersion increased when they were exposed to a reduced visual sensory condition (i.e., vision/no-vision transition). However, when exposed to augmented sensory condition (i.e., no-vision/vision transitions) young adults were able to adapt rapidly and reduced their sway dispersion whereas the elderly exhibited an increased sway dispersion. This inability to adapt to an augmented sensory condition suggest that elderly persons, in addition to a reduced peripheral sensibility, have a deficit with central integrative mechanisms responsible for reconfiguring the postural set.     

Can a plantar pressure–based tongue-placed electrotactile biofeedback improve postural control under altered vestibular and neck proprioceptive conditions?

We investigated the effects of a plantar pressure-based tongue-placed electrotactile biofeedback on postural control during quiet standing under normal and altered vestibular and neck proprioceptive conditions. To achieve this goal, 14 young healthy adults were asked to stand upright as immobile as possible with their eyes closed in two Neutral and Extended head postures and two conditions of No-biofeedback and Biofeedback. The underlying principle of the biofeedback consisted of providing supplementary information related to foot sole pressure distribution through a wireless embedded tongue-placed tactile output device. Center of foot pressure (CoP) displacements were recorded using a plantar pressure data acquisition system. Results showed that (1) the Extended head posture yielded increased CoP displacements relative to the Neutral head posture in the No-biofeedback condition, with a greater effect along the anteroposterior than mediolateral axis, whereas (2) no significant difference between the two Neutral and Extended head postures was observed in the Biofeedback condition. The present findings suggested that the availability of the plantar pressure-based tongue-placed electrotactile biofeedback allowed the subjects to suppress the destabilizing effect induced by the disruption of vestibular and neck proprioceptive inputs associated with the head extended posture. These results are discussed according to the sensory re-weighting hypothesis, whereby the CNS would dynamically and selectively adjust the relative contributions of sensory inputs (i.e. the sensory weights) to maintain upright stance depending on the sensory contexts and the neuromuscular constraints acting on the subject.     

Light touch contribution to balance in normal bipedal stance

 It has previously been shown that light contact with the finger tip on a fixed surface reduces centre of pressure (CoP) fluctuations in the frontal plane when standing in an unstable posture with the feet in line (tandem Romberg stance). Positive cross-correlations between horizontal finger forces and CoP fluctuations with finger forces exhibiting a phase lead suggest the hand provides sensory input for postural stability. The present study investigates whether this is the case for normal posture. We report reduced CoP fluctuations in the sagittal plane when light touch is permitted during normal bipedal stance. Moreover, we find positive crosscorrelations between finger tip forces and CoP fluctuations which are of similar magnitude and phase lag to those observed in tandem Romberg stance. This shows the utility of hand touch input for regulation of normal upright posture as well as inherently unstable postures such as tandem Romberg. Received: 24 November 1997 / Accepted: 20 December 1998     

Fingertip contact influences human postural control

Touch and pressure stimulation of the body surface can strongly influence apparent body orientation, as well as the maintenance of upright posture during quiet stance. In the present study, we investigated the relationship between postural sway and contact forces at the fingertip while subjects touched a rigid metal bar. Subjects were tested in the tandem Romberg stance with eyes open or closed under three conditions of fingertip contact: no contact, touch contact (<0.98 N of force), and force contact (as much force as desired). Touch contact was as effective as force contact or sight of the surroundings in reducing postural sway when compared to the no contact, eyes closed condition. Body sway and fingertip forces were essentially in phase with force contact, suggesting that fingertip contact forces are physically counteracting body sway. Time delays between body sway and fingertip forces were much larger with light touch contact, suggesting that the fingertip is providing information that allows anticipatory innervation of musculature to reduce body sway. The results are related to observations on precision grip as well as the somatosensory, proprioceptive, and motor mechanisms involved in the reduction of body sway.     

Re-weighting of somatosensory inputs from the foot and the ankle for controlling posture during quiet standing following trunk extensor muscles fatigue

The present study focused on the effects of trunk extensor muscles fatigue on postural control during quiet standing under different somatosensory conditions from the foot and the ankle. With this aim, 20 young healthy adults were asked to stand as immobile as possible in two conditions of No fatigue and Fatigue of trunk extensor muscles. In Experiment 1 (n = 10), somatosensation from the foot and the ankle was degraded by standing on a foam surface. In Experiment 2 (n = 10), somatosensation from the foot and ankle was facilitated through the increased cutaneous feedback at the foot and ankle provided by strips of athletic tape applied across both ankle joints. The centre of foot pressure displacements (CoP) were recorded using a force platform. The results showed that (1) trunk extensor muscles fatigue increased CoP displacements under normal somatosensatory conditions (Experiment 1 and Experiment 2), (2) this destabilizing effect was exacerbated when somatosensation from the foot and the ankle was degraded (Experiment 1), and (3) this destabilizing effect was mitigated when somatosensation from the foot and the ankle was facilitated (Experiment 2). Altogether, the present findings evidenced re-weighting of sensory cues for controlling posture during quiet standing following trunk extensor muscles fatigue by increasing the reliance on the somatosensory inputs from the foot and the ankle. This could have implications in clinical and rehabilitative areas.     

Fingertip contact influences human postural control

Touch and pressure stimulation of the body surface can strongly influence apparent body orientation, as well as the maintenance of upright posture during quiet stance. In the present study, we investigated the relationship between postural sway and contact forces at the fingertip while subjects touched a rigid metal bar. Subjects were tested in the tandem Romberg stance with eyes open or closed under three conditions of fingertip contact: no contact, touch contact (< 0.98 N of force), and force contact (as much force as desired). Touch contact was as effective as force contact or sight of the surroundings in reducing postural sway when compared to the no contact, eyes closed condition. Body sway and fingertip forces were essentially in phase with force contact, suggesting that fingertip contact forces are physically counteracting body sway. Time delays between body sway and fingertip forces were much larger with light touch contact, suggesting that the fingertip is providing information that allows anticipatory innervation of musculature to reduce body sway. The results are related to observations on precision grip as well as the somatosensory, proprioceptive, and motor mechanisms involved in the reduction of body sway.     

Differential integration of kinaesthetic signals to postural control

The purpose of the present experiment was to identify whether non-visual sensory cues involved in the maintenance of balance control could be weighted differently from one subject to another in condition during which kinaesthetic signals, stemming from the ankle proprioceptors and plantar pressure somatosensory sensors, were altered. A large population of blindfolded healthy young university students (n = 140) were asked to sway as little as possible on: (1) a firm support (Firm condition) and (2) an unstable support used to impair the exploitation of the kinematic ankle proprioceptive and plantar pressure somatosensation (Foam condition). Centre of foot pressure (CoP) displacements were recorded using a force platform. Analyses of the surface area, range, and mean velocity of the CoP displacements showed significant negative correlations between the postural sway observed in the Firm condition and the increase in postural sway observed in the Foam condition. In other words, the alteration of ankle proprioception had a greater destabilising effect in subjects exhibiting the smallest CoP displacements when standing in a normal proprioception condition. The present findings suggest that the exploitation of the kinaesthetic relationships to postural control varied from one subject to another, hence evidencing the need to introduce differential approach to assess the general impact of preferential modes of spatial referencing in postural control.