Attentional demands for static and dynamic equilibrium

Upright standing and walking tasks require the integration of several sources of sensory information. In a normal and highly predictable environment, locomotor synergies involving several muscles may take place at lower spinal levels with neural circuitry tuned by local loops of assistance or self-organizing processes generated in coordinative networks. When ongoing regulation of gait is necessary (obstacles, changes in direction) supraspinal involvement is necessary to perform movements adapted to the environment. Using a classical information processing framework and a dual-task methodology, it is possible to evaluate the attentional demands for performing static and dynamic equilibrium tasks. The present experiment evaluates whether the attentional requirements for a control sitting condition and for standing and walking conditions vary with the intrinsic balance demands of the tasks. The results show that standing and walking conditions required more attention than sitting in a chair. The attentional cost for walking was also significantly greater than for standing. For the walking task, reaction times when subjects were in singlesupport phase (small base of support) were significantly longer than those in double-support phase, suggesting that the attentional demands increased with an increase in the balance requirements of the task. Balance control requires a continuous regulation and integration of sensory inputs; increasing balance demands loads the higher level cognitive system.     

Selective use of visual information signaling objects' center of mass for anticipatory control of manipulative fingertip forces

The present study examines whether visual information indicating the center of mass (CM) of an object can be used for the appropriate scaling of fingertip forces at each digit during precision grip. In separate experiments subjects lifted an object with various types of visual cues concerning the CM location several times and then rotated and lifted it again to determine whether the visual cues signaling the new location of the CM could be used to appropriately scale the fingertip forces. Specifically, subjects had either no visual cues, visual instructional cues (i.e., an indicator) or visual geometric cues where the longer axis of the object indicated the CM. When no visual cues were provided, subjects were unable to appropriately scale the load forces at each digit following rotation despite their knowledge of the new weight distribution. When visual cues regarding the CM location were provided, the nature of the visual cues determined their effectiveness in retrieval of internal representations underlying the anticipatory scaling of fingertip forces. Specifically, when subjects were provided with visual instructional information, they were unable to appropriately scale the forces. More appropriate scaling of the load forces occurred when the visual cues were ecologically meaningful, i.e., when the shape of the object indicated the CM location. We suggest that visual instructional cues do not have access to the implicit processes underlying dynamic force control, whereas visual geometric cues can be used for the retrieval of the internal representation related to CM for appropriate partitioning of the forces in each digit. Electronic Publication     

Parkinsonian deficits in context-dependent regulation of standing postural control

This study explored whether patients with Parkinson's disease alter the regulation of upright standing according to constraints imposed by the environmental context. The provision of context-dependent adaptations was inferred from the presence of adjustments to standing postural control that would serve to reduce fall risk when balance was challenged by a threatening environmental context. Participants were asked to stand as still as possible in two environmental context conditions that differed in the level of imposed postural threat: LOW threat and HIGH threat. Eight levodopa dependent patients with Parkinson's disease (PD) and eight age-matched control subjects (CTRL) provided the subject sample. PD patients were tested following a 12-h withdrawal of anti-Parkinsonian medications and approximately 1h post-medication. The CTRL group showed altered postural control in the HIGH threat condition, in a manner that was indicative of appropriate context-dependent regulation of standing. PD patients, in the non-medicated or medicated states, did not modify stance regulation when the environmental context heightened postural threat. Our results extend the current understanding of Parkinsonian deficits in the context-dependent regulation of postural control to include upright standing.     

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.     

The impact of light fingertip touch on haptic cortical processing during a standing balance task

Availability of fingertip touch onto a stable surface reduces body sway for subjects standing with eyes closed. This is largely associated with sensory feedback from the fingertip when mechanical load is limited. Here, it is possible that the central nervous system facilitates cortical sensory processing to augment feedback to control upright stance. To test this, we compared cortical sensory excitability between tasks with and without light finger touch while standing. Subjects stood in tandem on a force plate with eyes closed while lightly touching a stable surface with the index finger. This was, in two different studies, compared to: (1) no haptic contact or (2) light touch on an object not referenced to balance. Throughout testing, the median nerve was stimulated and electroencephalography was used to measure somatosensory evoked potentials (SEPs). As expected, availability of stable light touch reduced medial–lateral COP sway. Peak amplitudes for SEP components revealed reduced P100 (48%), but increased P50 (31%), N140 (80%), and P200 (20%) during stable touch versus no touch. The modulation of P50 and N140 was no longer present when comparing stable to control (touch), which suggested that attending to touch on either surface, regardless of stability reference, accounted for these changes. Conversely, P200 was increased (19%) when touching the stable surface. Our data show SEP modulation during a standing balance task related to hand contact. Facilitation of P200 in particular may indicate task-specific regulation of the cortical representation of fingertip afferent input when it is relevant to providing stable cues for static balance control.     

Epigenetic development of postural responses for sitting during infancy

This study examined whether postural responses emerge in children in a predetermined way before independent sitting is achieved, and in what respect postural responses in infants differ from those in adults. Children just able to sit independently and children not yet able to sit were exposed to surface perturbations (translation and rotation) while body movement and electromyographic (EMG) responses were recorded. Perturbations causing a backward sway of the body (i.e., forward translation and legs-up rotation), elicited consistent patterns of muscle activity in ventral hip, trunk, and neck muscles in the independently sitting children. A high tonic EMG background activity in trunk and neck extensor muscles was inhibited at the onset of the ventral muscle activity. Kinematic analysis revealed that backward rotation of the pelvis was the first detectable body movement, while head movements (linear and angular displacement) were irregular and occurred later than the pelvis movement. Perturbations in the opposite direction, causing a forward sway, evoked variable responses in dorsal trunk and neck muscles, suggesting that the excitability level for postural responses was set according to the stability limits of the body. Children not yet able to sit without support were tested when the support around the waist, given by the experimenter's hands, was released prior to the onset of the platform perturbation. Postural responses were elicited in ventral muscles following a backward sway in all children and in about 60% of all trials. Often, only some of the ventral muscles were activated. No distinct responses were evoked during perturbations imposing a forward sway. These results suggest that (1) backward rotation of the pelvis triggers the postural adjustments in the independently sitting children; (2) a basic form of the postural adjustment develops in a predetermined manner before children practice independent sitting; and (3) the basic structure of ventral muscle activation pattern resembles that of adults, while the activation of the dorsal muscles (inhibition) differs in several aspects. These findings are in agreement with a recent model of central pattern generators for postural responses consisting of two operative levels. At the first level, which is triggered by backward rotation of the pelvis, the basic activation pattern is generated. At the second level, the pattern is shaped and fine-tuned by multisensory interactions from all activated sensory systems. The basic pattern present in the youngest infants may be produced mainly by neural networks at the first level, while the shaping function develops during practice, the shaping function being subjected to a learning process in which appropriate responses are formed in conjunction with the establishment of an internal neural representation for sitting.     

Coherence analysis of muscle activity during quiet stance

Studies of muscle activation during perturbed standing have demonstrated that the typical patterns of coordination (“ankle strategy” and “hip strategy”) are controlled through multiple muscles activated in a distal-to-proximal or proximal-to-distal temporal pattern. In contrast, quiet stance is thought to be maintained primarily through the ankle musculature. Recently, spectral analysis of inter-segment body motion revealed the coexistence of both ankle and hip patterns of coordination during quiet stance, with the predominating pattern dependent on the frequency of body sway. Here we use frequency domain techniques to determine if these patterns are associated with the same muscular patterns as observed during perturbed stance. Six of the seven muscles measured showed a linear relationship to the sway of at least one body segment, all being leg muscles. Muscle–segment phases were consistent with that required to resist gravity at low frequencies, with increasing phase lag as frequency increased. Visual information had effects only at frequencies below 0.5 Hz, where the shift from in-phase to anti-phase trunk–leg co-phase was observed. These results indicate that co-existence of the ankle and hip pattern during quiet stance involves only leg musculature. Anti-phase movement of the trunk relative to the legs at higher frequencies arises from indirect biomechanical control from posterior leg muscles.     

Reciprocal angular acceleration of the ankle and hip joints during quiet standing in humans

Human quiet standing is often modeled as a single inverted pendulum rotating around the ankle joint, under the assumption that movement around the hip joint is quite small. However, several recent studies have shown that movement around the hip joint can play a significant role in the efficient maintenance of the center of body mass (COM) above the support area. The aim of this study was to investigate how coordination between the hip and ankle joints is controlled during human quiet standing. Subjects stood quietly for 30 s with their eyes either opened (EO) or closed (EC), and we measured subtle angular displacements around the ankle (thetaa) and hip (thetah) joints using three highly sensitive CCD laser displacement sensors. Reliable data were obtained for both angular displacement and angular velocity (the first derivative of the angular displacement). Further, measurement error was not predominant, even among the angular acceleration data, which were obtained by taking the second derivative of the angular displacement. The angular displacement, velocity, and acceleration of the hip were found to be significantly greater (P<0.001) than those of the ankle, confirming that hip-joint motion cannot be ignored, even during quiet standing. We also found that a consistent reciprocal relationship exists between the angular accelerations of the hip and ankle joints, namely positive or negative angular acceleration of ankle joint is compensated for by oppositely directed angular acceleration of the hip joint. Principal component analysis revealed that this relationship can be expressed as: thetah=gammathetaa with gamma=-3.15+/-1.24 and gamma=-3.12+/-1.46 (mean +/-SD) for EO and EC, respectively, where theta is the angular acceleration. There was no significant difference in the values of y for EO and EC, and these values were in agreement with the theoretical value calculated assuming the acceleration of COM was zero. On the other hand, such a consistent relationship was never observed for angular displacement itself. These results suggest that the angular motions around the hip and ankle joints are not to keep the COM at a constant position, but rather to minimize acceleration of the COM.     

Motion parallax is used to control postural sway during walking

Three experiments tested the hypothesis that postural sway during locomotion is visually regulated by motion parallax as well as optical expansion. Oscillating displays of three-dimensional scenes were presented to participants walking on a treadmill, while postural sway was recorded. Displays simulated: (a) a cloud, in which parallax and expansion are congruent, (b) a hallway, (c) the side walls of the hallway, (d) a ground surface, (e) a wall, (f) the wall with a central hole, (g) a hall farther from the observer, and (h) a wall farther from the observer. In contrast to previous results with a hallway, responses with the cloud were isotropic and directionally specific. The other displays demonstrated that motion parallax was more effective than simple horizontal flow in eliciting lateral sway. These results are consistent with the hypothesis that adaptive control of sway during walking is based on congruent expansion and parallax in natural environments.     

How performing a mental arithmetic task modify the regulation of centre of foot pressure displacements during bipedal quiet standing

We investigated the effect of performing a mental arithmetic task with two levels of difficulty on the regulation of centre of foot pressure (COP) displacements during bipedal quiet standing in young healthy individuals. There was also a control condition in which no concurrent task was required. A space-time-domain analysis showed decreased COP displacements, along the antero–posterior axis, when participants concurrently performed the most difficult mental arithmetic task. Frequency-domain and stabilogram-diffusion analyses further suggested these decreased COP displacements to be associated with an increased stiffness and a reduction of the exploratory behaviours in the short term, respectively.     

Non-linear development of postural control and strategy use in young children: a longitudinal study

This longitudinal analysis confirmed a non-monotonic pattern of postural control development in children from age 5 to 8 years suggested by previous cross-sectional studies. Postural control was considered in terms of control strategy and its variability operationalized by mean and standard deviation of center of pressure (COP) velocity; and of effectiveness and its variability operationalized by mean and standard deviation of COP anteroposterior (a-p) excursion. Periods of significant variability were used to indicate behavioral transitions. Seventeen healthy children (nine males, eight females) aged 5-6 years (61.5-75 months) were tested at 3- to 4-month intervals up to age 8 years (83-97 months) in eyes-open quiet stance on a force platform for 30 s in each of ten trials. Data were reorganized into six developmental categories based on adjacent test dates prior to (-1) and after (+1, +2, +3, +4) a subject's trial with the lowest COP velocity (0). Developmental category is proposed to represent level of sensorimotor integrative skill. Within-subject ANCOVAs revealed a significant effect (P<0.0001) for developmental category with covariance due to height, weight and actual age removed. Post hoc tests showed a significant effect (P<0.0001) on measures of strategy. However, differences in COP velocity (type of strategy used) and differences in its variability (denoting a transition between strategies) were not always coincident. Performance outcome (COP a-p excursion) changed near linearly across categories. It was concluded that a non-monotonic change in control strategy as indicated by COP velocity describes the development of quiet stance equilibrium. A transition occurs from a primarily open-loop to incorporation of open- and closed-loop components of control. Honing of strategy used precedes and follows transitions. Constriction of velocity and excursion may typify the early stages of bimodal strategy. Developmental categories describe affiliation with the strategy employed and may represent differentiable levels of sensorimotor integrative skill. They may be more useful in assessing progression of equilibrium control than consecutive age in years.     

Early development of postural adjustments in standing with and without support

This study investigates the early development of postural adjustments during external perturbations in two different standing positions: standing with support and standing without support. The aim of the study was to assess a group of 13 infants four times during the period in life when independent standing is achieved; at 8, 10, 12 and 14 months. However, longitudinal data could be achieved only in four infants. Muscle activations of the neck, hip and ankle were recorded using surface electromyography. Based on earlier studies and controversies, three main issues were addressed: (1) Is direction specificity present before independent standing is established? (2) How do postural adjustments change with increasing age (8–14 months)? (3) Are postural adjustments task-specific in the young child? The results showed that our small sample of infants aged 8 and 10 months, who were not yet able to stand independently, exhibited direction-specific postural adjustments both during standing with and without support, though not consistently during all trials and at all body levels. Therefore, we argue that direction specificity might constitute a prerequisite for the development of independent standing. We also found that the development of postural adjustments in standing with support resembles that of sitting, i.e. great variation in the postural adjustments at early age, and fine-tuning to the situation with increasing age and experience. This, we find that this is in agreement with the proposal that postural control develops through a selection process of the most suitable postural adjustments for the situation from a repertoire of direction-specific postural adjustments. The development of postural adjustments during standing without support is discussed. Additionally, differences in response rates were noted between the two standing positions, indicating that even before independent standing is established, sophisticated sensorimotor integration enables task-specific postural adjustments.     

How a plantar pressure-based,tongue-placed tactile biofeedback modifies postural control mechanisms during quiet standing

The purpose of the present study was to determine the effects of a plantar pressure-based, tongue-placed tactile biofeedback on postural control mechanisms during quiet standing. To this aim, 16 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, recorded using a force platform, were used to compute the horizontal displacements of the vertical projection of the centre of gravity (CoG _v ) and those of the difference between the CoP and the vertical projection of the CoG (CoP-CoG _v ). Analysis of the CoP-CoG _v displacements showed larger root mean square (RMS) and mean power frequencies (MPF) in the Biofeedback than in the No-biofeedback condition. Stabilogram-diffusion analysis further showed a concomitant increased spatial and reduced temporal transition point co-ordinates at which the corrective processes were initiated and an increased persistent behaviour of the CoP-CoG _v displacements over the short-term region. Analysis of the CoG _v displacements showed decreased RMS and increased MPF in the Biofeedback relative to the No-biofeedback condition. Stabilogram-diffusion analysis further indicated that these effects mainly stem from reduced spatio-temporal transition point co-ordinates at which the corrective process involving CoG _v displacements is initiated and an increased anti-persistent behaviour of the CoG _v displacements over the long-term region. Altogether, the present findings suggest that the main way the plantar pressure-based, tongue-placed tactile biofeedback improves postural control during quiet standing is via both a reduction of the correction thresholds and an increased efficiency of the corrective mechanism involving the CoG _v displacements.     

Control of the triceps surae during the postural sway of quiet standing

AIM: The present study investigated how the triceps surae are controlled at the spinal level during the naturally occurring postural sway of quiet standing. METHODS: Subjects stood on a force platform as electrical stimuli were applied to the posterior tibial nerve when the center of pressure (COP) was either 1.6 standard deviations anterior (COP(ant)) or posterior (COP(post)) to the mean baseline COP signal. Peak-to-peak amplitudes of the H-reflex and M-wave from the soleus (SOL) and medial gastrocnemius (MG) muscles were recorded to assess the efficacy of the Ia pathway. RESULTS: A significant increase in the H(max) : M(max) ratio for both the SOL (12 +/- 6%) and MG (23 +/- 6%) was observed during the COP(ant) as compared to the COP(post) condition. The source of the modulation between COP conditions cannot be determined from this study. However, the observed changes in the synaptic efficacy of the Ia pathway are unlikely to be simply a result of an altered level of background electromyographic activity in the triceps surae. This was indicated by the lack of differences observed in the H(max) : M(max) ratio when subjects stood without postural sway (via the use of a tilt table) at two levels of background activity. CONCLUSIONS: It is suggested that the phase-dependent modulation of the triceps surae H-reflexes during the postural sway of quiet standing functions to maintain upright stance and may explain the results from previous studies, which, until now, had not taken the influence of postural sway on the H-reflex into consideration.     

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.     

Effect of the hip motion on the body kinematics in the sagittal plane during human quiet standing

Human quiet stance is often modeled as a single-link inverted pendulum pivoting only around the ankle joints in the sagittal plane. However, several recent studies have shown that movement around the hip joint cannot be negligible, and the body behaves like a double-link inverted pendulum. The purpose of this study was to examine how the hip motion affects the body kinematics in the sagittal plane during quiet standing. Ten healthy subjects were requested to keep a quiet stance for 30 s on a force platform. The angular displacements of the ankle and hip joints were measured using two highly sensitive CCD laser sensors. By taking the second derivative of the angular displacements, the angular accelerations of both joints were obtained. As for the angular displacements, there was no clear correlation between the ankle and hip joints. On the other hand, the angular accelerations of both joints were found to be modulated in a consistent anti-phase pattern. Then we estimated the anterior–posterior (A–P) acceleration of the center of mass (CoM) as a linear summation of the angular acceleration data. Simultaneously, we derived the actual CoM acceleration by dividing A–P share force by body mass. When we estimated CoM acceleration using only the angular acceleration of the ankle joint under the assumption that movement of the CoM is merely a scaled reflection of the motion of the ankle, it was largely overestimated as compared to the actual CoM acceleration. Whereas, when we take the angular acceleration of the hip joint into the calculation, it showed good coincidence with the actual CoM acceleration. These results indicate that the movement around the hip joint has a substantial effect on the body kinematics in the sagittal plane even during quiet standing.     

Modulation of cortical activity as a result of voluntary postural sway direction: An EEG study

There is increasing evidence demonstrating the role of the cerebral cortex in human postural control. Modulation of EEG both in voltage and frequency domains has been observed preceding and following self-paced postural movements and those induced by external perturbations. The current study set out to provide additional evidence regarding the role of cerebral cortex in human postural control by specifically examining modulation of EEG as a function of postural sway direction. Twelve neurologically normal subjects were instructed to produce self-paced voluntary postural sways in the anterior–posterior (AP) and medial–lateral (ML) directions. The center of pressure dynamics and EEG both in voltage and frequency domains were extracted by averaging and Morlet wavelet techniques, respectively. The amplitude of movement-related cortical potentials (MRCP) was significantly higher preceding ML sways. Also, time–frequency wavelet coefficients (TF) indicated differential modulation of EEG within alpha, beta and gamma bands as a function of voluntary postural sway direction. Thus, ML sway appear to be more difficult and energy demanding tasks than the AP sway as reflected in differential modulation of EEG. These results are discussed within the conceptual framework of differential patterns of brain activation as a result of postural task complexity.     

Effects of 20-day bed rest with and without strength training on postural sway during quiet standing

Aim: To examine the effect of unweighting as a possible contributory factor to a reduced calf muscle volume on postural sway during quiet standing, changes in postural sway following bed rest with or without strength training were investigated. Methods: Twelve young men participated in a 20‐day bed‐rest study. Subjects were divided into a non‐training group (BR‐Con) and a strength training group (BR‐Tr). For the BR‐Tr group, training was comprised of dynamic calf‐raise and leg‐press exercises to maintain the muscle volume of the plantar flexors. Before and after bed rest, subjects maintained quiet standing in a barefoot position on a force platform with their eyes open or closed. During the quiet stance, foot centre‐of‐pressure (CoP) and the mean velocity of CoP was calculated. Muscle volume of the plantar flexors was computed using axial magnetic resonance images of the leg. Results: After the bed‐rest period, the muscle volume decreased in the BR‐Con group but not in the BR‐Tr group. The mean velocity of CoP as an assessment of postural sway, however, increased in both groups. These results indicate that the strength training during bed rest cannot counteract the increase in postural sway. Conclusion: We concluded that postural sway increases following 20 days of bed rest despite maintenance of the muscle volume of plantar flexors as the main working muscles for the human postural standing.