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Neural mechanisms of human standing are expected to be elucidated for preventing fallings. Postural response evoked by sudden external perturbation originates from various areas in the central nervous system. Recent studies have revealed that the corticospinal pathway is one of the key nodes for an appropriate postural response. The corticospinal pathway that mediates the early part of the electromyographic response is modulated with prediction before a perturbation occurs. Temporal prediction explicitly exhibiting an onset timing contributes to enhancing corticospinal excitability. However, how the cortical activities in the sensorimotor area with temporal prediction are processed before the corticospinal pathway enhancement remains unclear. In this study, using electroencephalography, we investigated how temporal prediction affects both neural oscillations and synchronization between sensorimotor and distal areas. Our results revealed that desynchronization of cortical oscillation at α‐ and β‐bands was observed in the sensorimotor and parietooccipital areas (Cz, CPz, Pz and POz), and those are nested in the phase at θ‐band frequency. Furthermore, a reduction in the interareal phase synchrony in the α‐band was induced after the timing cue for the perturbation onset. The phase synchrony at the low frequency can relay the temporal prediction among the distant areas and initiate the modulation of the local cortical activities. Such modulations contribute to the preparation for sensory processing and motor execution that are necessary for optimal responses. [ABSTRACT FROM AUTHOR]

Background: It has been shown that humans adapt their postural sway to oscillatory, visually simulated self-motion. However, little is still known about the way individual body segments contribute to this adjustment of body sway and how this contribution varies with different environmental conditions.Research Question: How do the centre of pressure (COP) and individual body segments phase-lock to a sinusoidal visual drive depending on the frequency of stimulation?Methods: In this study, we introduce phase coupling as a method for assessing full body motion in response to visual stimuli presented in virtual reality (VR). 12 participants (mean age: 31 ± 9 years) stood inside a virtual tunnel which oscillated sinusoidally in the anterior-posterior direction at a frequency of 0.2 Hz, 0.8 Hz or 1.2 Hz. Primary outcome measures were the trajectories of their COP as well as of 25 body segments obtained by a motion tracking system.Results: Subjects significantly coupled the phase of their COP and body segments to the visual drive. Our analysis yielded significant phase coupling of the COP to the stimulus for all tested frequencies. The phase coupling of body segments revealed a shift in postural response as a function of frequency. At the low frequency of 0.2 Hz, we found strong and significant phase coupling homogeneously distributed across the body. At the higher frequencies of 0.8 Hz and 1.2 Hz, however, overall phase coupling became weaker and was centred around the lower torso and hip segments.Significance: Information on how the visual percept of self-motion affects balance control is crucial for understanding visuomotor processing in health and disease. Our setup and methods constitute a reliable tool for assessing perturbed balance control, which can be utilized in future clinical trials. [ABSTRACT FROM AUTHOR]

Vision plays a central role in maintaining balance. When humans perceive their body as moving, they trigger counter movements. This results in body sway, which has typically been investigated by measuring the body's center of pressure (COP). Here, we aimed to induce visually evoked postural responses (VEPR) by simulating self-motion in virtual reality (VR) using a sinusoidally oscillating "moving room" paradigm. Ten healthy subjects participated in the experiment. Stimulation consisted of a 3D-cloud of random dots, presented through a VR headset, which oscillated sinusoidally in the anterior–posterior direction at different frequencies. We used a force platform to measure subjects' COP over time and quantified the resulting trajectory by wavelet analyses including inter-trial phase coherence (ITPC). Subjects exhibited significant coupling of their COP to the respective stimulus. Even when spectral analysis of postural sway showed only small responses in the expected frequency bands (power), ITPC revealed an almost constant strength of coupling to the stimulus within but also across subjects and presented frequencies. Remarkably, ITPC even revealed a strong phase coupling to stimulation at 1.5 Hz, which exceeds the frequency range that has generally been attributed to the coupling of human postural sway to an oscillatory visual scenery. These findings suggest phase-locking to be an essential feature of visuomotor control. [ABSTRACT FROM AUTHOR]

Longer latency of postural response in multiple sclerosis (MS) may be linked to imbalance and increased likelihood of falls. It may be caused by the compromised microstructural integrity in the spinal cord, as evidenced by slowed somatosensory conduction in the spinal cord. Thus, the purpose of this study is to investigate the correlation between latency of postural responses and microstructural integrity of the cervical spinal cord, the region particularly related to the disease severity in MS, using diffusion tensor imaging (DTI) metrics. Seventeen persons with MS with mild-to-moderate disease severity were enrolled in this study. Postural response latencies of each patient were measured using electromyography of the tibialis anterior muscle (TA) and gastrocnemius muscle (GN) in response to surface perturbations. Cervical spinal cord DTI images were obtained from each patient. DTI mean, radial, axial diffusivity, and fractional anisotropy (FA) were measured between segments C4 and C6. Correlations of DTI metrics with postural response latencies, expanded disability status scale (EDSS) scores, and 25-foot walk (T25FW) were assessed using the Spearman's rank correlation coefficient at α = 0.05. Lower FA was significantly correlated with longer latencies measured on right TA in response to forward postural perturbations (r = −0.51, p =.04). DTI metrics showed no significant correlations with EDSS scores (r = −0.06–0.09, p =.73–0.95) or T25FW (r = −0.1–0.14, p =.6–0.94). DTI metrics showed no significant differences between subjects with and without spinal cord lesions (p =.2–0.7). Our results showed a significant correlation between lower FA in the cervical spinal cord and longer latencies measured on right TA in response to forward postural perturbations in persons with MS, suggesting that impaired cervical spinal cord microstructure assessed by DTI may be associated with the delayed postural responses. [ABSTRACT FROM AUTHOR]

The prediction of upcoming perturbation modulates postural responses in the ankle muscles. The effects of this prediction on postural responses vary according to predictable factors. When the amplitude of perturbation can be predicted, the long-latency response is set at an appropriate size for the required response, whereas when the direction of perturbation can be predicted, there is no effect. The neural mechanisms underlying these phenomena are poorly understood. Here, we examined how the corticospinal excitability of the ankle muscles [i.e., the tibialis anterior (TA), the soleus (SOL), and the medial gastrocnemius (MG), with a focus on the TA], would be modulated in five experimental conditions: (1) No-perturbation; (2) Low (anterior translation with small amplitude); (3) High (anterior translation with large amplitude); (4) Posterior (posterior translation with large amplitude); and (5) Random (Low, High, and Posterior in randomized order). We measured the motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) at 50 ms before surface-translation in each condition. The electromyographic (EMG) responses evoked by surface-translations were also measured. The results showed that the TA-MEP amplitude was greater in the High condition (where the largest TA-EMG response was evoked among the five conditions) compared to that in the No-perturbation, Low, and Posterior conditions (High vs. No-perturbation, p < 0.001; High vs. Low, p = 0.001; High vs. Posterior, p = 0.001). In addition, the MEP amplitude in the Random condition was significantly greater than that in the No-perturbation and Low conditions (Random vs. No-perturbation, p = 0.002; Random vs. Low, p = 0.002). The EMG response in the TA evoked by perturbation was significantly smaller when a perturbation can be predicted (predictable vs. unpredictable, p < 0.001). In the SOL and MG muscles, no prominent modulations of the MEP amplitude or EMG response were observed, suggesting that the effects of prediction on corticospinal excitability differ between the dorsiflexor and plantar flexor muscles. These findings suggest that the corticospinal excitability in the TA is scaled in parallel with the prediction of the direction and magnitude of an upcoming perturbation in advance.

The present study examined whether an internal or external attentional focus would affect participants' feet-in-place balance response to postural stance perturbations. A movable platform automatically slid forward or backward while healthy participants stood on it and (a) performed no cognitive activity (control), (b) focused on the pelvis or upper body sway (internal focus), (c) memorized a number displayed immediately before the platform slid (external focus), or (d) kept the equilibrium of an unstable cylinder over the arm (external focus). The forward displacement of the pelvis induced by the platform sliding forward was smaller when participants focused on their pelvic sway, although such effect was absent when they focused on their upper body sway, indicating that the internal focus was effective for the postural response when attention was paid to the pelvic sway. Regarding an external attention focus, the forward displacement of the pelvis induced by the platform sliding forward was smaller when participants focused on the equilibrium of an unstable object over the arm, but this effect was absent when they focused on the number, indicating that an external focus was only effective when the unstable object focused upon was relevant to the equilibrium of one's own body. No attentional intervention was effective during backward sliding of the support surface, indicating that central set for responding to postural perturbation depends on the direction of the postural perturbation. [ABSTRACT FROM AUTHOR]

Background: A plurality of fatal falls to lower levels involve ladders. After a slip/misstep on a ladder, climbers use their upper and lower limbs to reestablish contact with the ladder.Research Question: This study investigates the impact of upper body strength, hand placement and foot placement on fall severity after a ladder climbing perturbation.Methods: Participants performed upper body strength tests (breakaway and grip strength) and climbed a vertical, fixed ladder while a misstep perturbation was applied under the foot. After the perturbation, three hand placement and two foot placement responses were generally observed. Common hand placement responses included the hand moving two rungs, one rung, or did not move to a different rung. Foot placement responses included at least one foot or no feet reestablished contact with the ladder rung(s). Fall severity was quantified by the peak harness force observed after the perturbation.Results: Increased strength, reestablishing at least one foot on the ladder, and ascending (compared with descending) the ladder was associated with a reduction in fall severity. An interaction effect indicated that the impact of hand placement was altered by climbing direction. Moving the hand one rung during ascent and moving the hand two rungs during descent was associated with an increased fall severity. Cases where the hand decoupled from the ladder was associated with higher fall severity. Upper body strength assessed using a portable grip dynamometer was sufficient to predict fall severity.Discussion: This study confirms the multifactor role of upper body strength, hand placement and foot placement in preventing falls from ladders. Furthermore, a portable dynamometer shows potential to screen for high-risk individuals. Results of this investigation may guide targeted interventions to prevent falls from ladders. [ABSTRACT FROM AUTHOR]

The prediction of upcoming perturbation modulates postural responses in the ankle muscles. The effects of this prediction on postural responses vary according to predictable factors. When the amplitude of perturbation can be predicted, the long-latency response is set at an appropriate size for the required response, whereas when the direction of perturbation can be predicted, there is no effect. The neural mechanisms underlying these phenomena are poorly understood. Here, we examined how the corticospinal excitability of the ankle muscles [i.e., the tibialis anterior (TA), the soleus (SOL), and the medial gastrocnemius (MG), with a focus on the TA], would be modulated in five experimental conditions: (1) No-perturbation; (2) Low (anterior translation with small amplitude); (3) High (anterior translation with large amplitude); (4) Posterior (posterior translation with large amplitude); and (5) Random (Low, High, and Posterior in randomized order). We measured the motor-evoked potentials (MEPs) induced by transcranial magnetic stimulation (TMS) at 50 ms before surface-translation in each condition. The electromyographic (EMG) responses evoked by surface-translations were also measured. The results showed that the TA-MEP amplitude was greater in the High condition (where the largest TA-EMG response was evoked among the five conditions) compared to that in the No-perturbation, Low, and Posterior conditions (High vs. No-perturbation, p < 0.001; High vs. Low, p = 0.001; High vs. Posterior, p = 0.001). In addition, the MEP amplitude in the Random condition was significantly greater than that in the No-perturbation and Low conditions (Random vs. No-perturbation, p = 0.002; Random vs. Low, p = 0.002). The EMG response in the TA evoked by perturbation was significantly smaller when a perturbation can be predicted (predictable vs. unpredictable, p < 0.001). In the SOL and MG muscles, no prominent modulations of the MEP amplitude or EMG response were observed, suggesting that the effects of prediction on corticospinal excitability differ between the dorsiflexor and plantar flexor muscles. These findings suggest that the corticospinal excitability in the TA is scaled in parallel with the prediction of the direction and magnitude of an upcoming perturbation in advance. [ABSTRACT FROM AUTHOR]

Low back pain (LBP) has a point prevalence of nearly 10% and ranks highest in global disease burden for years lived with disability; Parkinson’s disease (PD) ranks in the top 100 most disabling health conditions for years lost and years lived with disability (1). Recent evidence suggests that people with chronic, recurrent LBP exhibit many postural impairments reminiscent of a neurological postural disorder such as PD. We compare and contrast postural impairments associated with LBP and PD in order to inform treatment strategies for both conditions. The literature suggests that both LBP and PD associate with impaired proprioceptive function, sensory orientation during standing balance, anticipatory postural adjustments, automatic postural responses, and striatal-cortical function. Although postural impairments are similar in nature for LBP and PD, the postural impairments with LBP appear more specific to the trunk than for PD. Likewise, although both health conditions associate with altered striatal-cortical function, the nature of the altered neural structure or function differ for PD and LBP. Due to the high prevalence of LBP associated with PD, focused treatment of LBP in people with PD may render benefit to their postural impairments and disabilities. In addition, LBP would likely benefit from being considered more than just a musculoskeletal injury; as such, clinicians should consider including approaches that address impairments of postural motor control.

Postural control undergoes rapid changes during child development. However, the influence of balance training (BT) on the compensation of perturbations has not yet been investigated in children. For this purpose, young (6.7 ± 0.6 years) and old children (12.0 ± 0.4 years) were exposed to externally induced anticipated (direction known) and non-anticipated (direction unknown) perturbations on a free swinging platform before and after either child-oriented BT (INT; young: n = 12, old: n = 18) or regular physical education (CON; young: n = 9, old: n = 9). At baseline, old children exhibited less platform sway after perturbations than young children (p = .004; η2p = 0.17). However, no differences were found between anticipated and non-anticipated perturbations. After training, INT reduced the platform sway path while CON remained stable (-11.1% vs. +2.7%; p < .001; η2p = 0.26). Furthermore, the young INT group adapted statistically similarly in anticipated and non-anticipated situations (-7.9% vs. -12.6%; p = .556; r = 0.33), whereas the old INT group tended to improve more in anticipated perturbations (-15.1% vs. -8.2%; p = .052; r = 0.51). Thus, the maturity of the postural system seems to influence the extent of training adaptations in anticipated perturbations. Furthermore, this study provides evidence that BT can improve postural responses to external perturbations in children and may represent a useful intervention to prevent falls. [ABSTRACT FROM AUTHOR]

Low back pain (LBP) has a point prevalence of nearly 10% and ranks highest in global disease burden for years lived with disability; Parkinson's disease (PD) ranks in the top 100 most disabling health conditions for years lost and years lived with disability (1). Recent evidence suggests that people with chronic, recurrent LBP exhibit many postural impairments reminiscent of a neurological postural disorder such as PD. We compare and contrast postural impairments associated with LBP and PD in order to inform treatment strategies for both conditions. The literature suggests that both LBP and PD associate with impaired proprioceptive function, sensory orientation during standing balance, anticipatory postural adjustments, automatic postural responses, and striatal-cortical function. Although postural impairments are similar in nature for LBP and PD, the postural impairments with LBP appear more specific to the trunk than for PD. Likewise, although both health conditions associate with altered striatal-cortical function, the nature of the altered neural structure or function differ for PD and LBP. Due to the high prevalence of LBP associated with PD, focused treatment of LBP in people with PD may render benefit to their postural impairments and disabilities. In addition, LBP would likely benefit from being considered more than just a musculoskeletal injury; as such, clinicians should consider including approaches that address impairments of postural motor control. [ABSTRACT FROM AUTHOR]

We used a musculoskeletal model to investigate the possible biomechanical and neural bases of using consistent muscle synergy patterns to produce functional motor outputs across different biomechanical conditions, which we define as generalizability. Experimental studies in cats demonstrate that the same muscle synergies are used during reactive postural responses at widely varying configurations, producing similarly-oriented endpoint force vectors with respect to the limb axis. However, whether generalizability across postures arises due to similar biomechanical properties or to neural selection of a particular muscle activation pattern has not been explicitly tested. Here, we used a detailed cat hindlimb model to explore the set of feasible muscle activation patterns that produce experimental synergy force vectors at a target posture, and tested their generalizability by applying them to different test postures. We used three methods to select candidate muscle activation patterns: (1) randomly-selected feasible muscle activation patterns, (2) optimal muscle activation patterns minimizing muscle effort at a given posture, and (3) generalizable muscle activation patterns that explicitly minimized deviations from experimentally-identified synergy force vectors across all postures. Generalizability was measured by the deviation between the simulated force direction of the candidate muscle activation pattern and the experimental synergy force vectors at the test postures. Force angle deviations were the greatest for the randomly selected feasible muscle activation patterns (e.g. >100°), intermediate for effort-wise optimal muscle activation patterns (e.g. ~20°), and smallest for generalizable muscle activation patterns (e.g.

The aim of this study was twofold: first to examine whether the choice of balance training device has any influence on the overall therapeutic outcome and secondly whether it affects postural strategy in patients with low-back pain. Six patients used a Gamma trainer with virtual reality games and five patients used a wobble board. Before and after the treatment the postural responses were tested. Five out of 11 patients improved their postural responses in terms of latency and stability. The contribution of the balance training to the improvement of postural responses was not statistically significant (ANOVA, p > 0.05), but differences in the functional reaching test were statistically significant (p = 0.0215) for each group (p = 0.0419), while differences between the groups were not found significant (p = 0.1257). In spite of the small number of participating subjects, we suggest that balance training improves postural responses latencies and functional reaching in people with low back pain regardless of the choice of the balance training device. [ABSTRACT FROM AUTHOR]

We used a musculoskeletal model to investigate the possible biomechanical and neural bases of using consistent muscle synergy patterns to produce functional motor outputs across different biomechanical conditions, which we define as generalizability. Experimental studies in cats demonstrate that the same muscle synergies are used during reactive postural responses at widely varying configurations, producing similarly-oriented endpoint force vectors with respect to the limb axis. However, whether generalizability across postures arises due to similar biomechanical properties or to neural selection of a particular muscle activation pattern has not been explicitly tested. Here, we used a detailed cat hindlimb model to explore the set of feasible muscle activation patterns that produce experimental synergy force vectors at a target posture, and tested their generalizability by applying them to different test postures. We used three methods to select candidate muscle activation patterns: (1) randomly-selected feasible muscle activation patterns, (2) optimal muscle activation patterns minimizing muscle effort at a given posture, and (3) generalizable muscle activation patterns that explicitly minimized deviations from experimentally-identified synergy force vectors across all postures. Generalizability was measured by the deviation between the simulated force direction of the candidate muscle activation pattern and the experimental synergy force vectors at the test postures. Force angle deviations were the greatest for the randomly selected feasible muscle activation patterns (e.g., >100°), intermediate for effort-wise optimal muscle activation patterns (e.g., ~20°), and smallest for generalizable muscle activation patterns (e.g., <5°). Generalizable muscle activation patterns were suboptimal in terms of effort, often exceeding 50% of the maximum possible effort (cf. ~5% in minimum-effort muscle activation patterns). The feasible muscle activation ranges of individual muscles associated with producing a specific synergy force vector was reduced by ~45% when generalizability requirements were imposed. Muscles recruited in the generalizable muscle activation patterns had less sensitive torque-producing characteristics to changes in postures. We conclude that generalization of function across postures does not arise from limb biomechanics or a single optimality criterion. Muscle synergies may reflect acquired motor solutions globally tuned for generalizability across biomechanical contexts, facilitating rapid motor adaptation. [ABSTRACT FROM AUTHOR]

Ladder falling accidents are a significant, growing and severe occupational hazard. The factors that contribute to falls from ladders and specifically those that influence the motor response from ladder falls are not well understood. The aims of this research were to determine the effects of hand placement (rung versus rail) on muscle activation onset and peak activity timing in response to slipping on a ladder and to sequence the timing of events following slip initiation. Fifteen unexpected slips from 11 experienced ladder climbers were induced with a freely spinning rung under the foot, while subjects were randomly assigned to a rung versus rail hand grasping strategy. EMG onset time and peak activity time from five bilateral muscles (semitendinosis, vastus lateralis, triceps, biceps and anterior deltoid) were analyzed. Results indicated that significantly slower muscle activation onset and peak response times occurred during rail hand placement, suggesting that grasping ladder rungs may be preferable for improving the speed of the motor response. The triceps muscle activated and reached peak activity earlier in the slip indicating that subjects may initially extend their arms prior to generating hand forces. The study also revealed that slips tended to occur around the time that a foot and hand were in motion and there were just two points of contact (one hand and the slipping foot). [ABSTRACT FROM AUTHOR]

The ease with which we avoid falling down belies a highly sophisticated and distributed neural network for controlling reactions to maintain upright balance. Although historically these reactions were considered within the sub cortical domain, mounting evidence reveals a distributed network for postural control including a potentially important role for the cerebral cortex. Support for this cortical role comes from direct measurement associated with moments of induced instability as well as indirect links between cognitive task performance and balance recovery. The cerebral cortex appears to be directly involved in the control of rapid balance reactions but also setting the central nervous system in advance to optimize balance recovery reactions even when a future threat to stability is unexpected. In this review the growing body of evidence that now firmly supports a cortical role in the postural responses to externally induced perturbations is presented. Moreover, an updated framework is advanced to help understand how cortical contributions may influence our resistance to falls and on what timescale. The implications for future studies into the neural control of balance are discussed. [ABSTRACT FROM AUTHOR]

The purpose of this study was to determine the role of somatosensory input to the sensory reference system in quiet standing. We applied vibration (0.5 mm amplitude, 1–60 Hz) to the triceps surae and the forefoot sole to stimulate the muscle spindles and the mechanoreceptors, respectively, and evaluated postural responses. Thirteen young healthy adults who showed backward-lean and forward-lean responses to vibration at high and low frequencies, respectively, participated in the full experiment. The lowest vibration frequencies inducing backward-lean responses (B-LF) were 15–55 Hz for the triceps surae and 16–60 Hz for the forefoot sole. The highest frequencies inducing forward-lean responses (F-HF) were 3–18 Hz for the triceps surae and 1–20 Hz for the forefoot sole. When vibration was simultaneously applied to the triceps surae and forefoot sole at F-HF, no response was induced in 70% of trials. A forward-lean response was induced in the remaining 30% of trials. Simultaneous vibration of the triceps surae and forefoot sole at B-LF induced backward-lean responses in all trials. All postural responses occurred 0.5–4.3 s after vibration onset. Postural responses to high-frequency vibration conceivably occur as a compensatory movement to the illusionary perception that standing position is deviating forward from quiet standing, which must be a reference position. Postural responses to low-frequency vibration possibly occur to equalize the positional information that is received from the triceps surae and the forefoot sole. Both postural responses are likely to involve the sensory reference system, which is located in the supraspinal nervous system. [ABSTRACT FROM AUTHOR]

Balance and postural response strategies change when subjects are exposed to horizontal translations of the floor or virtual reality or both. This may impact the balance training strategy and balance capabilities assessment in the future telerehabilitation. In the study 15 neurologically intact volunteers participated. Balance standing frame with virtual reality tasks and our novel haptic floor able to generate horizontal translations were used. The postural responses were measured with center of gravity and muscle electromyography of plantar-dorsiflexors, quadriceps, hamstrings, hip and spine muscles in three scenarios. The results demonstrated that center of gravity and electromyographic activity were comparable; with low latency at translation only, longer latency at combination with virtual reality and long latency when only virtual reality was applied. Soleus and semimembranousis demonstrated lower latency at back-right horizontal translations when virtual reality was present. The outcomes suggests that the postural strategy changes from ankle to ankle-hip strategy with availability of additional sensory systems which may be an important issue for objective balance evaluation in the clinical environment and remote telerehabilitation. [ABSTRACT FROM AUTHOR]

Visual stimulation alone is sufficient to produce visually induced postural reactivity (VIPR). While some studies have shown that VIPR increases with the velocity of a moving visual stimulus, others have shown that it decreases with the temporal frequency of an oscillating visual stimulus. These results seem contradictory given that these two variables co-vary in the same direction. The purpose of this study is to determine whether the VIPR can be different depending on the frequency range being considered. Twelve subjects were placed standing up in a virtual reality environment that simulated a black and white checkerboard at floor level. This checkerboard oscillated at seven frequencies (0.03-2.0 Hz) and three amplitudes (2, 4, and 8°), corresponding to nine velocities (0.125-32°/s). The virtual floor oscillated from left to right (mediolateral) or from front to back (anteroposterior). We calculated the subjects' mean velocity (Ω) based on data from electromagnetic sensors positioned on the head and lower back. Our experiment shows that for temporal frequencies below 0.12 Hz, VIPR is visually dependent and increases with stimulus velocity. When stimulus velocity becomes too high, the body becomes incapable of following, and the VIPR saturates between 0.12 and 0.25 Hz. In this frequency range, maximal postural oscillation seems to depend on biomechanical constraints imposed by the positioning of the feet. For frequencies above 0.5 Hz, the body can no longer maintain the same oscillation state. This saturation may be linked to proprioceptive feedback mechanisms in the postural system. [ABSTRACT FROM AUTHOR]

Appropriate muscular response following an external perturbation is essential in preventing falls. Transtibial prosthesis users lack a foot-ankle complex and associated sensorimotor structures on the side with the prosthesis. The effect of this lack on rapid responses of the lower limb to external surface perturbations is unknown. The aim of the present study was to compare electromyogram (EMG) response latencies of otherwise healthy, unilateral, transtibial prosthesis users (n = 23, mean +/– standard deviation [SD] age = 48 +/– 14 yr) and a matched control group (n = 23, mean +/– SD age = 48 +/– 13 yr) following sudden support-surface rotations in the pitch plane (toes-up and toes-down). Perturbations were elicited in various weight-bearing and limb-perturbed conditions. The results indicated that transtibial prosthesis users have delayed responses of multiple muscles of the lower limb following perturbation, both in the intact and residual limbs. Weight-bearing had no influence on the response latency in the residual limb, but did on the intact limb. Which limb received the perturbation was found to influence the muscular response, with the intact limb showing a significantly delayed response when the perturbation was received only on the side with a prosthesis. These delayed responses may represent an increased risk of falling for individuals who use transtibial prostheses. [ABSTRACT FROM AUTHOR]

The aim of this study was to determine the postural response accompanying Achilles tendon vibration stimulation during various phases of the sit-to-stand movement. Twelve healthy young adults performed the sit-to-stand movement in response to an auditory signal 2 s after a first one. Vibration stimulation with a 100 Hz frequency was applied to both Achilles tendons during the following phases: (1) 10 s of sitting before standing up; (2) 10 s plus a period until the standing position was achieved; and (3) 5 s after standing. The postural response after standing was analyzed with the center of foot pressure in the anteroposterior direction. Forward-leaning responses were identified in 78.3% and 63.3% of trials under conditions 1 and 2, respectively. Backward-leaning responses were identified in 93.3% of the trials under condition 3. Response latency (± standard deviation) was significantly longer under conditions 1 and 2 than under condition 3 (1: 872 ± 576, 2: 1026 ± 542, and 3: 555 ± 322 ms; ps < 0.05). Sensory information at the standing point might be anticipated based on sensory information received while sitting. Consequently, postural response as a compensatory movement would occur via the sensory reference system within the supraspinal nervous system. [ABSTRACT FROM AUTHOR]

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For optimal balance, the postural system needs to quickly detect and respond to perturbations. The aim of this study was to investigate the immediate and long-term impact of walking at different speeds on standing balance and postural stability. Center of pressure (COP) motion was measured from 14 young individuals at discrete time intervals after they walked on a treadmill at three speeds (preferred walking speed (PWS), 120 %-PWS, 140 %-PWS). Results revealed that walking at a faster speed had the greatest impact on postural stability. This was reflected by increases in the amount (path length, range, 95 % ellipse), variability (standard deviation, SD), and structure (approximate entropy, ApEn) of COP motion and were most evident when compared to pre-walking assessments. In subsequent trials following pre-walking assessments there was a leveling-off for specific COP variables (range, variability, and ApEn) and a decline in path length. This plateau effect was observed even though measures of physical exertion (HR, RPE) continued to increase over the entire walking trial. Together, these results indicate that, despite the constant task demands induced by fast walking, the postural system was able to rapidly compensate and adjust appropriately. [ABSTRACT FROM AUTHOR]

Abstract: Postural responses are usually investigated as reflexes. Several trials are averaged, and trial-to-trial variations are interpreted as noise. Several studies providing single-trial data plots revealed oscillations that may be cancelled out in averaged time series. Variations between single trials may also be interpreted as a consequence of changed dynamic properties of the neural circuitries. Therefore, we propose a Matsuoka oscillator model to describe single-trial postural responses to external perturbations. The applicability of the model was demonstrated by a comparison between simulations and experimental electromyographic (EMG) data. Vertical force perturbations of durations 0.4s and 0.2s were applied via a handle to 10 subjects. Handle force was used as model input, and EMG data from the external oblique muscles was compared with simulation output. Model coefficients were optimized by a least-squares algorithm. The optimization produced a good similarity between simulation and experimental data with determination coefficients of and greater. Furthermore, as a model validation, the model coefficients were used to predict other perturbation trials with similarities between predictions and respective EMG data of about , which was in the range of trial-to-trial EMG variability. The observed oscillations are assumed to originate from the central nervous system with changes in the neural circuitries between trials. Hence, the oscillations in single trial responses which are usually regarded as noise might be generated by the dynamics of a neural oscillator. [Copyright &y& Elsevier]

Summary: A method for assessing balance, which was sensitive to changes in the postural control system is presented. This paper describes the implementation of a force-sensing platform, with force sensing resistors as the sensing element. The platform is capable of measuring destabilized postural perturbations in dynamic and static postural conditions. Besides providing real-time qualitative assessment, the platform quantifies the postural control of the subjects. This is done by evaluating the weighted center of applied pressure distribution over time. The objective of this research was to establish the feasibility of using the force-sensing platform to test and gauge the postural control of individuals. Tests were conducted in Eye Open and Eye Close states on Flat Ground (static condition) and the balance trainer (dynamic condition). It was observed that the designed platform was able to gauge the sway experienced by the body when subject’s states and conditions changed. [Copyright &y& Elsevier]

Abstract: Sudden addition or removal of visual information can be particularly critical to balance control. The promptness of adaptation of stance control mechanisms is quantified by the latency at which body oscillation and postural muscle activity vary after a shift in visual condition. In the present study, volunteers stood on a force platform with feet parallel or in tandem. Shifts in visual condition were produced by electronic spectacles. Ground reaction force (center of foot pressure, CoP) and EMG of leg postural muscles were acquired, and latency of CoP and EMG changes estimated by t-tests on the averaged traces. Time-to-reach steady-state was estimated by means of an exponential model. On allowing or occluding vision, decrements and increments in CoP position and oscillation occurred within about 2s. These were preceded by changes in muscle activity, regardless of visual-shift direction, foot position or front or rear leg in tandem. These time intervals were longer than simple reaction-time responses. The time course of recovery to steady-state was about 3s, shorter for oscillation than position. The capacity of modifying balance control at very short intervals both during quiet standing and under more critical balance conditions speaks in favor of a necessary coupling between vision, postural reference, and postural muscle activity, and of the swiftness of this sensory reweighing process. [Copyright &y& Elsevier]

The purposes of this study were: (1) to examine the reactive control of the resultant joint moments at the lower limbs in response to a novel and unannounced slip; (2) to establish individualized forward-dynamics models; and (3) to explore personal potential by determining the operation limits of these moments at each lower limb joint, beyond which the resulting motion at this or other joints will exceed its/their normal range(s). Ten young subjects' kinematics and kinetics, collected during regular walking and during their first exposure to a novel and unannounced slip, were randomly selected from an existing database. An inverse-dynamics approach was applied to derive their ( original) resultant joint moments, which were then used as input to establish forward-dynamics models, each including an individualized 16-element foot model to simulate ground reaction force. A simulated annealing (SA) algorithm was applied to modify the original moments, so that the subsequent output ( baseline) moments can closely reproduce these subjects' recorded motion. A systematic alteration of the baseline moments was employed to determine the operation limits. The results revealed that the subjects reactively increased the hip extensor and knee flexor moments and reduced their ankle plantar flexor moments of their single-stance limb following slip onset. The 'baseline' correction of the original moments can reach as much as 21% of the original moments. The analysis of the operation limits revealed that these individuals may be able to further increase their knee flexors more so than increase the hip extensors or reduce ankle plantar flexors before causing abnormal joint movement. Such systematic approach opens the possibility to properly assess an individual's rehabilitation potential, and to identify whether this person's strength is the limiting factor for stability training. [ABSTRACT FROM AUTHOR]