The effects of muscle vibration on anticipatory postural adjustments

Postural adjustments to self-triggered perturbations under conditions of changes in body orientation

We studied anticipatory and compensatory postural adjustments (APAs and CPAs) associated with self-triggered postural perturbations in conditions with changes in the initial body orientation. In particular, we were testing hypotheses on adjustments in the reciprocal and coactivation commands, role of proximal vs. distal muscles, and correlations between changes in indices of APAs and CPAs. Healthy young participants stood on a board with full support or reduced support area and held a standard load in the extended arms. They released the load in a self-paced manned with a standard small-amplitude arm movement. Electromyograms of 12 muscles were recorded and used to compute reciprocal and coactivation indices between three muscle pairs on both sides of the body. The subject's body was oriented toward one of three targets: straight ahead, 60° to the left, and 60° to the right. Body orientation has stronger effects on proximal muscle pairs compared to distal muscles. It led to more consistent changes in the reciprocal command compared to the coactivation command. Indices of APAs and CPAs showed positive correlations across conditions. We conclude that the earlier suggested hierarchical relations between the reciprocal and coactivation command could be task-specific. Predominance of negative or positive correlations between APA and CPA indices could also be task-specific.

The Effect of Additional Leg Supports in Control of Posture in Sitting

The objective of the study was to investigate the effect of leg supports on the anticipatory and compensatory postural adjustments of sitting subjects exposed to external perturbations in the anterior-posterior direction. Ten young participants received perturbations applied to the upper body while sitting on a stool with an anterior or posterior leg support and when using a footrest. Electromyographic activities of the trunk and leg muscles and center of pressure displacements were recorded and analyzed during the anticipatory and compensatory phases of postural control. Anticipatory activities were observed in the tibialis anterior, biceps femoris, and erector spinae muscles in the anterior leg support condition. Early onset of muscle activity was observed in the tibialis anterior, biceps femoris, rectus femoris, and erector spinae muscles in the posterior leg support condition compared to the feet support condition. Moreover, to maintain balance participants utilized co-contraction of muscles as the main mechanism of balance control in sitting regardless of the availability of the anterior or posterior leg support. There was no effect of a leg support on center of pressure displacements. The outcome of the study provides a background for future investigations of the effect of leg supports on control of balance in sitting when perturbed.

Anticipatory postural adjustments in older versus young adults: a systematic review and meta-analysis

BACKGROUND

Anticipatory postural adjustments (APAs) are a feedforward mechanism triggered in advance to a predictable perturbation, to help the individual counteract mechanical effects that the disturbance may cause. Whether or not this strategy is compromised in the elderly is not a consensus in the literature.

METHODS

In this systematic review with meta-analysis, we investigated aging effects on postural control, based on anticipatory postural adjustments (APAs). We selected 11 eligible articles of the following databases: Lilacs, SciELO, PubMed, Cochrane Central, Embase, and CINAHL, involving 324 research participants, assessing their methodological quality and extracting electromyographic, posturographic, and kinematic measurements. We included studies that investigated the occurrence of APAs in healthy younger and older adults, published before 10th August 2022, in English. Studies involving participant with conditions that may affect balance or that did not report measures of onset or amplitude of electromyography (EMG), COP, or kinematics were excluded. To analyze the aggregated results from these studies, we performed the analysis based on the outcome measures (EMG, COP, or kinematic measures) used in individual studies. We calculated differences between younger and older adult groups as the mean differences between the groups and the estimated effect. Egger's test was conducted to evaluate whether this meta-analysis had publication bias.

RESULTS

Through this review, older adults showed no significant difference in the velocity to perform a movement compared to the younger adults (MD 0.95, 95% CI -0.86, 2.76, I² = 82%), but both muscle onset and center of pressure (COP) onset were significantly more delayed in older than in younger adults: erector spinae (MD -31.44, 95% CI -61.79, -1.09, I² = 95%); rectus abdominis (RA) (MD -31.51, 95% CI -70.58, -3.57, I² = 85%); tibialis anterior (TA) (MD -44.70, 95% CI -94.30, 4.91, I² = 63%); soleus (SOL) (MD -37.74, 95% CI -65.43, -10.05, I² = 91%); gastrocnemius (GAS) (MD -120.59, 95% CI -206.70, -34.49, I² = 94%); quadriceps (Q) (MD -17.42, 95% CI -34.73, -0.12, I² = 0%); biceps femoris (BF) (MD -117.47, 95% CI -192.55, -42.70, I² = 97%); COP onset (MD -45.28, 95% CI -89.57, -0.98, I² = 93%), and COP apa (COPapa) (MD 2.35, 95% CI -0.09, 4.79, I² = 64%). These changes did not seem to be linked to the speed of movement but possibly to age-related physiological changes that indicated decreased motor control during APAs in older adults.

CONCLUSIONS

Older adults use different postural strategies that aim to increase the safety margin and stabilize the body to perform the movement, according to the requirements imposed, and this should be considered in rehabilitation protocols.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD420119143198.

Older adults utilize less efficient postural adaptations when they are uncertain about the magnitude of a perturbation

People frequently experience perturbations while standing or walking in crowded areas or when interacting with external objects. Balance maintenance in response to a perturbation is affected by the predictability of the magnitude of a body disturbance. The aim of this quasi-experimental study was to investigate the role of aging in maintenance of standing balance in response to perturbations of varying magnitudes. Twelve older adults and twelve young adults received a series of frontal perturbations of small or large magnitudes induced to their upper body by a pendulum impact while standing. The perturbation sequence included 10 trials of small, 15 trials of large, and 10 more trials of small magnitudes. The participants were exposed to either repetitive perturbations of known (predictable) magnitude or perturbations of unknown (unpredictable) magnitude as they were not told which of the perturbation magnitude (small, large) to expect. Electromyographic activity of six leg and trunk muscles and displacements of the center of pressure were recorded and analyzed during anticipatory (APAs) and compensatory (CPAs) phases of postural control. When exposed to both, repetitive perturbations of known magnitude and perturbations of unpredictable magnitude, older adults, compared to young adults, demonstrated delayed and smaller anticipatory and compensatory postural adaptations. Older adults also required more trials to modify postural adjustments, as compared to young adults. The findings imply that the ability to predict magnitudes of frontal perturbations is declined in older adults.

Anticipatory and Compensatory Postural Adjustments in Response to Dynamic Platform Perturbation during a Forward Step

We investigated the generation of anticipatory (APAs) and compensatory postural adjustments (CPAs) in preparation for a step during support surface perturbation. Changes in anticipatory muscle activation in the trunk segment were predominantly in the co-contraction indices from -600 t₀ -400 ms to foot-off. Reciprocal indices of the shank muscles were pronounced in the APA intervals. During the CPA intervals, larger reciprocal muscle activities were detected compared to the APA intervals. The results showed subjects co-varied the reciprocal and co-contraction activations in postural muscles to counteract the perturbation and generate mechanical effects sufficient for stepping during the APA and CPA intervals. This study enhances our understanding of the interaction between the APAs and CPAs in balance maintenance.

Role of angular position of the seat in control of posture in response to external perturbation

Ability of the human body to regain balance after being externally perturbed is important in the maintenance of vertical posture. The goal of this study was to investigate trunk and leg muscle response to external perturbation while sitting on a stool with varying seat inclinations. Ten healthy subjects were required to receive a perturbation applied to the upper body while sitting on an adjustable stool with 0°, 10° forward or 10° backward inclination of the seat and without footrest and leg support. Electromyographic activities of the trunk and leg muscles and center of pressure displacements were recorded and analyzed during the anticipatory (APA) and compensatory (CPA) phases of postural control. APAs and CPAs were generated in response to an external perturbation. Delays in the onset of anticipatory muscle activity were seen when seated on the inclined seat compared to sitting on the horizontal seat (p < 0.05). To maintain balance after a perturbation, participants activated the trunk and thigh muscles, the activity of which was modulated to a greater degree than that of leg muscles. Moreover, they utilized co-contraction of muscles as the main mechanism of balance control in sitting. Furthermore, there was no effect of a seat inclination on COP displacements. The outcome provides a background for future investigations of the effect of seat inclination on control of balance in sitting.

The role of predictability of the magnitude of a perturbation in control of vertical posture when catching an object

The predictability of perturbation magnitude plays an important role in control of standing posture. The aim of the study was to examine anticipatory (APAs) and compensatory (CPAs) postural adjustments in response to catching objects of uncertain mass. Twenty adults caught the same object with either light or heavy weight placed in it. Electromyographic activity of eight trunk and leg muscles, displacements of the center of pressure, and angular displacement of the shoulder joint were recorded and analyzed during the APAs and CPAs intervals. When the subjects experienced repeated catching of the object with the same weight, they estimated the object mass beforehand and generated APAs more precisely. When the object mass changed unpredictably, they generated APAs based on the most recent catch and needed four to six trials to optimize APAs and CPAs. The muscle co-contraction was a primary pattern for catching the object of uncertain mass. The results of the study suggest that catching the object of uncertain mass is a challenging task that involves co-contraction of postural muscles to maintain balance.

Changes of biomechanics induced by Equistasi® in Parkinson's disease: coupling between balance and lower limb joints kinematics

Axial disorders, including postural deformities, postural instability, and gait disturbances, are among the most disabling symptoms of Parkinson's disease (PD). Equistasi®, a wearable proprioceptive stabilizer device, has been proposed as neurological rehabilitative device for this set of symptoms. To investigate the effects of the device on gait and balance, 24 participants affected by PD were enrolled in this crossover double-dummy, randomized, controlled study. Subjects were assessed four times before and after 8 weeks treatment with either active or placebo device; one-month wash-out was taken between treatments, in a 20-week timeframe. Gait analysis and instrumented Romberg test were performed with the aid of a sterofotogrammetric system and two force plates. Joint kinematics, spatiotemporal parameters of gait and center of pressure parameters were extracted. Paired T-test (p < 0.05) was adopted after evidence of normality to compare the variables across different acquisition sessions; Wilcoxon was adopted for non-normal distributions. Before and after the treatment with the active device, statistically significant improvements were observed in trunk flexion extension and in the ankle dorsi-plantarflexion. Regarding balance assessment, significant improvements were reported at the frequencies corresponding to vestibular system. These findings may open new possibilities on PD's rehabilitative interventions. Research question, tailored design of the study, experimental acquisition overview, main findings, and conclusions.

Postural control in preparation to a step during support surface perturbation

We investigated changes in the anticipatory activity of the leg and trunk muscles in preparation for a step during support surface perturbation. Eight healthy subjects performed stepping tasks under three conditions: normal, forward perturbation, and backward perturbation. R and C indices were calculated for the reciprocal and co-activation patterns of muscle pairs within the time intervals typical of anticipatory postural adjustments. When the support surface perturbation occurred, anticipatory muscle activations were predominantly in the C indices in the leg muscles. Significant differences in the maximum displacement of the centre of pressure were seen between conditions (FS vs NS; BS vs FS). The results suggest that activation of the leg muscles rather than the trunk muscles was modified to ensure equilibrium for taking a step in response to support surface perturbation.

Characteristics of medial-lateral postural control while exposed to the external perturbation in step initiation

Controllability of posture in the medial-lateral direction is critical for balance maintenance, particularly in step initiation. The objective of the current study was to examine  the effects of external perturbation and landing orientation on medial-lateral control stability in step initiation. Eleven young healthy participants stood on the force platform and waited for the instruction of taking a step while experiencing a pendulum perturbation applied at the lateral side of the right shoulder. Eight experimental conditions were conducted by two levels of step side (right or left), two levels of perturbation (with or without), and two levels of landing orientation (forward or diagonal). The center of pressure (COP), pelvic movements, and muscle activities were recorded and analyzed as the onset of COP and pelvic movement, the COP displacement, and cocontraction and reciprocal muscle activation pattern. The temporal events of COP and pelvic movement were not significantly different in all experimental conditions. However, COP and pelvic movement were significantly later in the diagonal condition. Most of the segments showed reciprocal muscle activation patterns in relation to the perturbation released time. Subsequently, all segments showed cocontraction muscle activation patterns, which was significantly affected by step side, perturbation, and orientation. The results suggest that how the CNS initiated a step was identical with the COP then pelvic movement. The outcome highlights the importance of external perturbation and foot landing orientation effects on postural adjustments, which may provide a different approach to help step initiation.

Characteristics of Postural Muscle Activity in Response to A Motor-Motor Task in Elderly

The purpose of the current study was to evaluate postural muscle performance of older adults in response to a combination of two motor tasks perturbations. Fifteen older participants were instructed to perform a pushing task as an upper limb perturbation while standing on a fixed or sliding board as a lower limb perturbation. Postural responses were characterized by onsets and magnitudes of muscle activities as well as onsets of segment movements. The sliding board did not affect the onset timing and sequence of muscle initiations and segment movements. However, significant large muscle activities of tibialis anterior and erector spinae were observed in the sliding condition (p < 0.05). The co-contraction values of the trunk and shank segments were significantly larger in the sliding condition through the studied periods (p < 0.05). Lastly, heavy pushing weight did not change the timing, magnitude, sequence of all studied parameters. Older adults enhanced postural stability by increasing the segment stiffness then started to handle two perturbations. In conclusion, they were able to deal with a dual motor-motor task after having secured their balance but could not make corresponding adjustments to the level of the perturbation difficulty.

Preparation to a quick whole-body action: control with referent body orientation and multi-muscle synergies

We examined the control of postural stability in preparation to a discrete, quick whole-body sway toward a target and back to the initial position. Several predictions were tested based on the theory of control with referent body orientation and the notion of multi-muscle synergies stabilizing center of pressure (COP) coordinate. Healthy, young adults performed fast, discrete whole-body motion forward-and-back and backward-and-back under visual feedback on the COP. We used two methods to assess COP stability, analysis of inter-trial variance and analysis of motor equivalence in the muscle activation space. Actions were always preceded by COP counter-movements. Backward COP shifts were faster, and the indices of multi-muscle synergies stabilizing COP were higher prior to those actions. Patterns of muscle activation at the motion onset supported the idea of a gradual shift in the referent body orientation. Prior to the backward movements, there was a trend toward higher muscle co-activation, compared to reciprocal activation. We found strong correlations between the sets of indices of motor equivalence and those of inter-trial variance. Overall, the results support the theory of control with referent coordinates and the idea of multi-muscle synergies stabilizing posture by confirming a number of non-trivial predictions based on these concepts. The findings favor using indices of motor equivalence in clinical studies to minimize the number of trials performed by each subject.

Short-Term Effects of Kinesio Taping on Muscle Recruitment Order During a Vertical Jump: A Pilot Study

CONTEXT

Kinesio taping is commonly used in sports and rehabilitation settings with the aim of prevention and treatment of musculoskeletal injuries. However, limited evidence exists regarding the effects of 24 and 72 hours of kinesio taping on trunk and lower limb neuromuscular and kinetic performance during a vertical jump.

OBJECTIVE

The purpose of this study was to analyze the short-term effects of kinesio taping on height and ground reaction force during a vertical jump, in addition to trunk and lower limb muscle latency and recruitment order.

DESIGN

Single-group pretest-posttest.

SETTING

University laboratory.

PARTICIPANTS

Twelve male athletes from different sports (track and field, basketball, and soccer).

INTERVENTIONS

They completed a single squat and countermovement jump at basal time (no kinesio taping), 24, and 72 hours of kinesio taping application on the gluteus maximus, biceps femoris, rectus femoris, gastrocnemius medialis, and longissimus.

MAIN OUTCOME MEASURES

Muscle onset latencies were assessed by electromyography during a squat and countermovement jump, in addition to measurements of the jump height and normalized ground reaction force.

RESULTS

The kinesio taping had no effect after 24 hours on either the countermovement or squat jump. However, at 72 hours, the kinesio taping increased the jump height (P = .02; d = 0.36) and normalized ground reaction force (P = .001; d = 0.45) during the countermovement jump. In addition, 72-hour kinesio taping reduced longissimus onset latency (P = .03; d = 1.34) and improved muscle recruitment order during a countermovement jump.

CONCLUSIONS

These findings suggest that kinesio taping may improve neuromuscular and kinetic performance during a countermovement jump only after 72 hours of application on healthy and uninjured male athletes. However, no changes were observed on a squat jump. Future studies should incorporate a control group to verify kinesio taping's effects and its influence on injured athletes.

Stability of vertical posture explored with unexpected mechanical perturbations: synergy indices and motor equivalence

We explored the relations between indices of mechanical stability of vertical posture and synergy indices under unexpected perturbations. The main hypotheses predicted higher posture-stabilizing synergy indices and higher mechanical indices of center of pressure stability during perturbations perceived by subjects as less challenging. Healthy subjects stood on a force platform and held in fully extended arms a bar attached to two loads acting downward and upward. One of the loads was unexpectedly released by the experimenter causing a postural perturbations. In different series, subjects either knew or did not know which of the two loads would be released. Forward perturbations were perceived as more challenging and accompanied by co-activation patterns among the main agonist-antagonist pairs. Backward perturbation led to reciprocal muscle activation patterns and was accompanied by indices of mechanical stability and of posture-stabilizing synergy which indicated higher stability. Changes in synergy indices were observed as early as 50-100 ms following the perturbation reflecting involuntary mechanisms. In contrast, predictability of perturbation direction had weak or no effect on mechanical and synergy indices of stability. These observations are interpreted within a hierarchical scheme of synergic control of motor tasks and a hypothesis on the control of movements with shifts of referent coordinates. The findings show direct correspondence between stability indices based on mechanics and on the analysis of multi-muscle synergies. They suggest that involuntary posture-stabilizing mechanisms show synergic organization. They also show that predictability of perturbation direction has strong effects on anticipatory postural adjustment but not corrective adjustments. We offer an interpretation of co-activation patterns that questions their contribution to postural stability.

Role of point of application of perturbation in control of vertical posture

The role of point of application of perturbation in the anticipatory (APAs) and compensatory (CPAs) postural control was studied. Twelve healthy participants stood on a sliding board (that was either locked and as such motionless or unlocked and as such free to move in the anterior-posterior direction). The body perturbations were applied either to the shoulders (by a pendulum impact) or the feet (by the movement of the sliding board). Electromyographic activity (EMG) of the trunk and lower extremity muscles was recorded. Latencies, integrals of EMG and muscle co-contraction (C) and reciprocal (R) activation indices were calculated and analyzed within the intervals typical for the APAs and CPAs. Higher EMG integrals were seen in the APAs phase when perturbation was applied to the shoulders. Reciprocal activation of muscles was seen in the APAs phase in the shoulders perturbation condition, while co-contraction was seen in the feet perturbation condition. Co-contraction was observed within the CPA phase in both experimental conditions. Higher C values were found in the feet perturbation condition in the CPA phase. The results suggest that different motor control strategies are employed by the central nervous system when encounter perturbations of similar magnitude but applied to different parts of the body. The outcome highlights the importance of investigation of the role of the point of application of the perturbation.

Standing on wedges modifies side-specific postural control in the presence of lateral external perturbations

Standing on wedges changes the position in the ankle joints and affects postural stability in the medial-lateral direction. The objective of the study was to investigate the role of wedges and external lateral perturbations on anticipatory (APA) and compensatory postural adjustments (CPA). Ten healthy young participants were exposed to perturbations applied to the lateral part of their right shoulder when standing on a planar surface, on a medial or lateral wedges. Bilateral electromyographic activity of dorsal and ventral postural muscles and the center of pressure (COP) displacement were recorded and analyzed during the APA and CPA phases. When exposed to the lateral perturbation, reciprocal activation of shank muscles was seen on the side of the perturbation while co-contraction of shank muscles was seen on the contralateral side during the APA and CPA phases. Standing on a wedge was associated with decreased magnitudes of co-contraction and reciprocal activation of shank muscles. The COP displacements were smaller in the APA phase and larger in the CPA phase while standing on wedges compared to standing on the planar surface. The outcome of the study provides a basis for future investigations of incorporating wedges in balance re-training paradigms for the elderly or individuals with neurological impairment.

APAs Constraints to Voluntary Movements: The Case for Limb Movements Coupling

When rhythmically moving two limbs in either the same or in opposite directions, one coupling mode meets constraints that are absent in the other mode. Isodirectional (ISO) flexion-extensions of the ipsilateral hand and foot can be easily performed with either the hand prone or supine. Instead, antidirectional (ANTI) movements require attentive effort and irresistibly tend to reverse into ISO when frequency increases. Experimental evidence indicates that the direction dependent easy-difficult dichotomy is caused by interference of the anticipatory postural commands associated to movements of one limb with voluntary commands to the other limb. Excitability of the resting wrist muscles is subliminally modulated at the period of ipsilateral foot oscillations, being phase-opposite in the antagonists and distributed so as to facilitate ISO and obstacle ANTI coupling of the hand (either prone or supine) with the foot. Modulation is driven by cortical signals dispatched to the forearm simultaneously with the voluntary commands moving the foot. If right foot oscillations are performed when standing on the left foot with the right hand touching a fixed support, the subliminal excitability modulation is replaced by overt contractions of forearm muscles conforming the APAs features. This suggests that during hand-foot ANTI coupling the voluntary commands to forearm muscles are contrasted by APAs commands of opposite sign linked to foot oscillations. Correlation between the easy-difficult dichotomy and the APAs distribution is also found in coupled adduction-abduction of the arms or hands in the transverse plane and in coupled flexion-extension of the arms in the parasagittal plane. In all these movements, APAs commands linked to the movement of each limb reach the motor pathways to the contralateral muscles homologous to the prime movers and can interfere during coupling with their voluntary activation. APAs are also generated in postural muscles of trunk and lower limbs and size-increase when the movement frequency is incremented. The related increase in postural effort apparently contributes in destabilizing the difficult coupling mode. Motor learning may rely upon more effective APAs. APAs and focal contraction are entangled within the same voluntary action. Yet, neural diseases may selectively impair APAs, which represent a potential target for rehabilitation.

Anticipatory postural adjustments and anticipatory synergy adjustments: preparing to a postural perturbation with predictable and unpredictable direction

We explored two aspects of feed-forward postural control, anticipatory postural adjustments (APAs) and anticipatory synergy adjustments (ASAs) seen prior to self-triggered unloading with known and unknown direction of the perturbation. In particular, we tested two main hypotheses predicting contrasting changes in APAs and ASAs. The first hypothesis predicted no major changes in ASAs. The second hypothesis predicted delayed APAs with predominance of co-contraction patterns when perturbation direction was unknown. Healthy subjects stood on the force plate and held a bar with two loads acting in the forward and backward directions. They pressed a trigger that released one of the loads causing a postural perturbation. In different series, the direction of the perturbation was either known (the same load released in all trials) or unknown (the subjects did not know which of the two loads would be released). Surface electromyograms were recorded and used to quantify APAs, synergies stabilizing center of pressure coordinate (within the uncontrolled manifold hypothesis), and ASA. APAs and ASAs were seen in all conditions. APAs were delayed, and predominance of co-contraction patterns was seen under the conditions with unpredictable direction of perturbation. In contrast, no significant changes in synergies and ASAs were seen. Overall, these results show that feed-forward control of vertical posture has two distinct components, reflected in APAs and ASAs, which show qualitatively different adjustments with changes in predictability of the direction of perturbation. These results are interpreted within the recently proposed hierarchical scheme of the synergic control of motor tasks. The observations underscore the complexity of the feed-forward postural control, which involves separate changes in salient performance variables (such as coordinate of the center of pressure) and in their stability properties.

Sensory Integration during Vibration of Postural Muscle Tendons When Pointing to a Memorized Target

Vibrating ankle muscles in freely standing persons elicits a spatially oriented postural response. For instance, vibrating the Achilles tendons induces a backward displacement of the body while vibrating the tibialis anterior muscle tendons induces a forward displacement. These displacements have been called vibration induced falling (VIF) responses and they presumably are automatic. Because of the long delay between the onset of the vibration and the onset of the VIF (about 700 ms), and the widespread cortical activation following vibration, there is a possibility that the sensory signals available before the VIF can be used by the central nervous system to plan a hand pointing action. This study examined this suggestion. Ten healthy young participants stood on a force platform and initially were trained to point with and without vision to a target located in front of them. Then, they were exposed to conditions with vibration of the Achilles tendons or tibialis anterior muscle tendons and pointed at the target without vision. The vibration stopped between each trial. Trials with vision (without vibration) were given every five trials to maintain an accurate perception of the target’s spatial location. Ankle vibrations did not have an effect on the position of the center of foot pressure (COP) before the onset of the pointing actions. Furthermore, reaction and movement times of the pointing actions were unaffected by the vibration. The hypotheses were that if proprioceptive information evoked by ankle vibrations alters the planning of a pointing action, the amplitude of the movement should scale according to the muscle tendons that are vibrated. For Achilles tendon vibration, participants undershot the target indicating the planning of the pointing action was influenced by the vibration-evoked proprioceptive information (forward displacement of the body). When the tibialis anterior were vibrated (backward displacement of the body), however, shorter movements were also observed. Longer movements would have increased the backward response of the sensed body movement. Thus, it is possible that pointing actions were adjusted on the basis of the expected consequences of the planned pointing action to avoid a response that could have compromised postural stability.

Effects of tibialis anterior vibration on postural control when exposed to support surface translations

The sensory re-weighting theory suggests unreliable inputs may be down-weighted to favor more reliable sensory information and thus maintain proper postural control. This study investigated the effects of tibialis anterior (TA) vibration on center of pressure (COP) motion in healthy individuals exposed to support surface translations to further explore the concept of sensory re-weighting. Twenty healthy young adults stood with eyes closed and arms across their chest while exposed to randomized blocks of five trials. Each trial lasted 8 s, with TA vibration either on or off. After 2 s, a sudden backward or forward translation occurred. Anterior-posterior (A/P) COP data were evaluated during the preparatory (first 2 s), perturbation (next 3 s), and recovery (last 3 s) phases to assess the effect of vibration on perturbation response features. The knowledge of an impending perturbation resulted in reduced anterior COP motion with TA vibration in the preparatory phase relative to the magnitude of anterior motion typically observed during TA vibration. During the perturbation phase, vibration did not influence COP motion. However, during the recovery phase vibration induced greater anterior COP motion than during trials without vibration. The fact that TA vibration produced differing effects on COP motion depending upon the phase of the perturbation response may suggest that the immediate context during which postural control is being regulated affects A/P COP responses to TA vibration. This indicates that proprioceptive information is likely continuously re-weighted according to the context in order to maintain effective postural control.

Tactile and proprioceptive sensory stimulation modifies estimation of walking distance but not upright gait stability: a pilot study

[Purpose] Recently, there has been growing interest in the somatosensory system, but little data exist on the interaction between dynamic postural control and the somatosensory system. The purpose of this study was to determine whether a training program, based on tactile and proprioceptive sensory stimulation of the trunk with the use of perceptual surfaces, improved the estimation of walking distance by healthy subjects, the ability to walk toward a memorized distance without vision, and whether it increases upright gait stability. [Subjects and Methods] Ten healthy subjects with a mean age of 31.9 ± 2.5 years were enrolled and participated in 10 daily sessions of perceptive training using perceptual surfaces, for 45 minutes each session. An experimental indoor test measured the subjects' ability to perceive walking distances to a memorized target in an indoor environment. [Results] After treatment, the distances that were traversed were closer to the target than before treatment. Trunk acceleration did not differ significantly between pre- and post-training and did not increase significantly after training. [Conclusion] Treatment with perceptual surfaces stimulating the trunk midline improves the estimation of walking distance and modifies proprioceptive gait patterns, allowing various corrective strategies to be implemented during ambulation.

Older adults utilize less efficient postural control when performing pushing task

The ability to maintain balance deteriorates with increasing age. The aim was to investigate the role of age in generation of anticipatory (APA) and compensatory (CPA) postural adjustments during pushing an object. Older (68.8 ± 1.0 years) and young adults (30.1 ± 1.4 years) participated in the experiment involving pushing an object (a pendulum attached to the ceiling) using both hands. Electrical activity of six leg and trunk muscles and displacements of the center of pressure (COP) were recorded and analyzed during the APA and CPA phases. The onset time, integrals of muscle activity, and COP displacements were determined. In addition, the indexes of co-activation and reciprocal activation of muscles for the shank, thigh, and trunk segments were calculated. Older adults, compared to young adults, showed less efficient postural control seen as delayed anticipatory muscle onset times and delayed COP displacements. Moreover, older adults used co-activation of muscles during the CPA phase while younger subjects utilized reciprocal activation of muscles. The observed diminished efficiency of postural control during both anticipatory and compensatory postural adjustments observed in older adults might predispose them to falls while performing tasks involving pushing. The outcome provides a background for future studies focused on the optimization of the daily activities of older adults.

Anticipatory and compensatory postural adjustments in conditions of body asymmetry induced by holding an object

The effect of body asymmetry on anticipatory and compensatory postural adjustments was studied. Ten healthy subjects stood on the force platform and held an object in one hand which induced body asymmetry. Subjects were exposed to external perturbations applied to their shoulders while standing with either normal or narrow base of support. Bilateral electromyographic activity (EMG) of dorsal and ventral trunk and leg muscles and center-of-pressure displacements were recorded. Data was analyzed within the intervals typical for anticipatory (APA) and compensatory postural adjustments. Integrals of EMG activity and co-contraction and reciprocal activation of muscles were calculated and analyzed. Reciprocal activation of muscles on the target side and co-contraction of muscles on the contralateral side were seen when standing in asymmetrical stance and being subjected to external perturbations. Decreased magnitudes of co-contraction and reciprocal activation of muscles were seen in the APA phase while standing asymmetrically with narrow base of support. The findings highlight the importance of investigating the role of body asymmetry in maintaining control of vertical posture. The outcome of the study provides a foundation for future studies focusing on improvement in postural control in individuals with body asymmetry due to unilateral weakness.

Effect of experimentally reduced distal sensation on postural response to hip abductor/ankle evertor muscle vibration

This study assessed whether postural responses induced by vibratory perturbations of the hip abductors and ankle evertors, were modified when distal tactile sensation was experimentally reduced through cooling. Sixteen healthy subjects were investigated pre and post cooling. Subjects stood with their eyes closed with a stance width of 4 cm. A 2s vibratory stimulus was applied to the left or right hip abductor or ankle evertor muscle. The order of the site and side of the stimulation was randomised. The postural response to hip abductor and ankle evertor vibration was recorded using 3D motion analysis (Codamotion, Leicestershire). Medio-lateral centre of pressure motion was simultaneously recorded during quiet standing via a force plate (Kistler, UK). Pre-cooling people responded to unilateral ankle vibration with an ipsilateral translation and tilt of the pelvis, and an ipsilateral tilt of the trunk. People responded to unilateral hip vibration with a contralateral translation and tilt of the pelvis, and an ipsilateral tilt of the trunk. Following an experimental reduction in distal tactile sensation there was a significant reduction in the amplitude of pelvic tilt in response to ankle vibration (F(6.2)=P<0.05) and a significant increase in amplitude of pelvic tilt in response to hip vibration (F(5.2)=P<0.05). This suggests that the sensitivity to artificial stimulation of hip proprioception increases with distal cooling, possibly indicating a change in the gain/weighting placed upon sensory information from the hips.

A cutaneous positioning system

Our previous work revealed that torso cutaneous information contributes to the internal representation of the torso and plays a role in postural control. Hence, the aims of this study were to assess whether posture could be manipulated by patterns of vibrotactile stimulation and to determine whether resulting modified postures were associated with specific and consistent spatial attitudes. Ten healthy young adults stood in normal and Romberg stances with six vibrating actuators positioned on the torso in contact with the skin over the anatomical locations corresponding to left and right external oblique, internal oblique and erector spinae muscles at the L4/L5 vertebrae level. A 250-Hz tactile vibration was applied for 5 s either at a single location or consecutively at each location in clockwise or counterclockwise sequences. Kinematic analysis of the body segments indicated that postural responses observed in response to single and sequential stimulation patterns were similar, while the center of pressure remained unaltered in any situations. Moreover, torso inclinations followed rectilinear-like path segments chartered by stimuli loci during sequential stimulations. Comparison of torso attitudes with previous results obtained with co-vibration patterns of the same duration showed that torso inclination amplitudes are equivalent for single (one location) and co-vibration (pairs of locations) patterns inducing the same directional effect. Hence, torso cutaneous information exhibits kinesthetic properties, appears to provide a map of upper body spatial configuration, and could assume the role of an internal positioning system for the upper body.

A wearable proprioceptive stabilizer (Equistasi®) for rehabilitation of postural instability in Parkinson's disease: a phase II randomized double-blind, double-dummy, controlled study

Background

Muscle spindles endings are extremely sensitive to externally applied vibrations, and under such circumstances they convey proprioceptive inflows to the central nervous system that modulate the spinal reflexes excitability or the muscle responses elicited by postural perturbations. The aim of this pilot study is to test the feasibility and effectiveness of a balance training program in association with a wearable proprioceptive stabilizer (Equistasi) that emits focal mechanical vibrations in patients with PD.

Methods

Forty patients with PD were randomly divided in two groups wearing an active or inactive device. All the patients received a 2-month intensive program of balance training. Assessments were performed at baseline, after the rehabilitation period (T1), and two more months after (T2). Posturographic measures were used as primary endpoint; secondary measures of outcome included the number of falls and several clinical scales for balance and quality of life.

Results

Both groups improved at the end of the rehabilitation period and we did not find significant between-group differences in any of the principal posturographic measures with the exception of higher sway area and limit of stability on the instrumental functional reach test during visual deprivation at T1 in the Equistasi group. As for the secondary outcome, we found an overall better outcome in patients enrolled in the Equistasi group: 1) significant improvement at T1 on Berg Balance Scale (+45.0%, p = .026), Activities-specific Balance Confidence (+83.7, p = .004), Falls Efficacy Scale (−33.3%, p = .026) and PDQ-39 (−48.8%, p = .004); 2) sustained improvement at T2 in terms of UPDRS-III, Berg Balance Scales, Time Up and Go and PDQ-39; 3) significant and sustained reduction of the falls rate.

Conclusions

This pilot trial shows that a physiotherapy program for training balance in association with focal mechanical vibration exerted by a wearable proprioceptive stabilizer might be superior than rehabilitation alone in improving patients’ balance.

Trial Registration

EudraCT 2013-003020-36 and ClinicalTrials.gov (number not assigned)

Improvement of anticipatory postural adjustments for balance control: effect of a single training session

Humans use anticipatory and compensatory postural strategies to maintain and restore balance when perturbed. Inefficient generation and utilization of anticipatory postural adjustments (APAs) is one of the reasons for postural instability. The aim of the study was to investigate the role of training in improvement of APAs and its effect on subsequent control of posture. Thirteen healthy young adults were exposed to predictable external perturbations before and after a single training session consisting of catches of a medicine ball thrown at the shoulder level. 3-D body kinematics, EMG activity of thirteen trunk and lower limb muscles, and ground reaction forces were recorded before and immediately after a single training session. Muscle onsets, EMG integrals, center of pressure (COP), and center of mass (COM) displacements were analyzed during the anticipatory and compensatory phases of postural control. The effect of a single training session was seen as significantly early muscle onsets and larger anticipatory COP displacements. As a result, significantly smaller peak COM displacements were observed after the perturbation indicating greater postural stability. The outcome of this study provides a background for examining the role of training in improvement of APAs and its effect on postural stability in individuals in need.

Influence of long-term wearing of unstable shoes on compensatory control of posture: an electromyography-based analysis

PURPOSE

This study investigated the influence of long-term wearing of unstable shoes (WUS) on compensatory postural adjustments (CPA) to an external perturbation.

METHODS

Participants were divided into two groups: one wore unstable shoes while the other wore conventional shoes for 8 weeks. The ground reaction force signal was used to calculate the anterior-posterior (AP) displacement of the centre of pressure (CoP) and the electromyographic signal of gastrocnemius medialis (GM), tibialis anterior (TA), rectus femoris (RF) and biceps femoris (BF) muscles was used to assess individual muscle activity, antagonist co-activation and reciprocal activation at the joint (TA/GM and RF/(BF+GM) pairs) and muscle group levels (ventral (TA+RF)/dorsal (GM+BF) pair) within time intervals typical for CPA. The electromyographic signal was also used to assess muscle latency. The variables described were evaluated before and after the 8-week period while wearing the unstable shoes and barefoot.

RESULTS

Long-term WUS led to: an increase of BF activity in both conditions (barefoot and wearing the unstable shoes); a decrease of GM activity; an increase of antagonist co-activation and a decrease of reciprocal activation level at the TA/GM and ventral/dorsal pairs in the unstable shoe condition. Additionally, WUS led to a decrease in CoP displacement. However, no differences were observed in muscle onset and offset.

CONCLUSION

Results suggest that the prolonged use of unstable shoes leads to increased ankle and muscle groups' antagonist co-activation levels and higher performance by the postural control system.

Effect of a local vibration stimulus training programme on postural sway and gait in chronic stroke patients: a randomized controlled trial

Objective:

To investigate the effect of a local vibration stimulus training programme on postural sway and gait in stroke patients.

Design:

A randomized controlled trial with two groups: a local vibration stimulus training programme group and a sham group.

Setting:

Inpatient rehabilitation centre.

Subjects:

Thirty-one chronic stroke patients.

Interventions:

Both groups underwent a standard rehabilitation programme. The local vibration stimulus training programme group ( n = 16) participated in the local vibration stimulus training programme for 30 minutes a day, five times a week, for six weeks. The sham group ( n = 15) participated in a sham local vibration stimulus training programme for 30 minutes a day, five times a week, for six weeks.

Main measures:

A forceplate was used to measure postural sway under two conditions: standing with eyes open and eyes closed. Gait ability was measured using the GAITRite system.

Results:

In postural sway, greater improvements in the postural sway distance with eyes-open (–11.91 vs. 0.80) and eyes-closed (–20.67 vs. –0.34) conditions and postural sway velocity with eyes-open (–0.40 vs. 0.03) and eyes-closed (–0.69 vs. –0.01) conditions were observed in the local vibration stimulus training programme group, compared with the sham group ( P < 0.05). In gait ability, greater improvement in gait speed (15.06 vs. 2.85), cadence (8.46 vs. 1.55), step length (7.90 vs. 3.64), and single limb support time (0.12 vs. 0.01) were observed in the local vibration stimulus training programme group, compared with the sham group ( P < 0.05).

Conclusions:

These findings suggest that local vibration stimulus training programme is an effective method for improvement of the postural sway and gait ability of chronic stroke patients.

Directional postural responses induced by vibrotactile stimulations applied to the torso

It has been shown that torso-based vibrotactile feedback significantly reduces postural sway in balance-compromised adults during quiet standing and in response to perturbations. This study aimed to determine whether vibrotactile stimulations applied to different torso locations induced directional postural responses and whether torso cutaneous information contributes to body representation. Eleven healthy young adults equipped with an inertial measurement unit (IMU) placed on the torso were asked to maintain an upright posture with closed eyes. Six vibrators (tactors) were placed on the torso in contact with the skin over the left and right external oblique, internal oblique, and erector spinae muscles at the L4/L5 level. Each tactor was randomly activated four times per location at a frequency of 250 Hz for a period of 5 s. The IMU results indicated that vibration applied individually over the internal oblique and erector spinae muscles induced a postural shift of about one degree oriented in the direction of the stimulation, while simultaneous activation of all tactors and activation of tactors over external oblique muscles produced insignificant postural effects. The root mean square of the sway signal was significantly higher during vibration than before or after. However, the center of pressure displacement, measured by a force plate, was uninfluenced by any vibration. These results suggest a multi-joint postural response including a torso inclination associated with vibration-induced changes in cutaneous information. The directional aspect of vibration-induced postural shifts suggests that cutaneous information from the stimulated areas contributes to proprioception and upper body spatial representation.

Corset hypothesis rebutted--transversus abdominis does not co-contract in unison prior to rapid arm movements

BACKGROUND

The aim of this paper is to test "corset" model of spinal stability, specifically the hypothesis that feed forward transversus abdominis activity is bilaterally symmetrical and independent of the direction of perturbation to posture due to arm. This study will assess transversus abdominis electromyographical activity bilaterally.

METHODS

Feed forward intramuscular transversus abdominis electromyographical data and reaction forces on the thorax due to the arm movement were collected and processed for 6 healthy subjects during 6 trials of 8 types of arm movements (randomised order). Reciprocal transversus abdominis indices were calculated as the difference between the normalised integrated feed forward transversus abdominis electromyographical data from each side of the trunk.

FINDINGS

The main finding of the study was that the reciprocal transversus abdominis index was significantly related to the axial rotational forces on the thorax due to arm movement (F=109.991, p<0.001). Right arm movements produced clockwise axial rotation forces on the thorax and dominant left transversus abdominis muscle activity.

INTERPRETATION

The consequence of this finding is that feed forward transversus abdominis activity is not bilaterally symmetrical and is not independent of the direction of perturbation to posture due to arm movement. Transversus abdominis forms part of a synergy of muscles contributing to the generation of axial rotation forces in the core that oppose the forces due to arm movement. These findings indicate that training bilateral pre-activation of the transversus abdominis prior to rapid movement is not justified and may potentially be problematic for the production of normal movement patterns.

Postural control in response to an external perturbation: effect of altered proprioceptive information

The purpose of the study was to investigate the role of altered proprioception on anticipatory (APAs) and compensatory (CPAs) postural adjustments and their interaction. Nine healthy adults were exposed to external perturbations induced at the shoulder level while standing with intact or altered proprioception induced by bilateral Achilles tendon vibration. Visual information was altered (eyes open or closed) in both the conditions. Electrical activity of eight trunk and leg muscles and center of pressure (COP) displacements were recorded and quantified within the time intervals typical for APAs and CPAs. The results showed that when proprioceptive information was altered in eyes-open conditions, anticipatory muscle activity was delayed. Moreover, altered proprioceptive information resulted in smaller magnitudes of compensatory muscle activity as well as smaller COP displacements after the perturbation in both eyes-open and eyes-closed conditions. The outcome of the study provides information on the interaction between APAs and CPAs in the presence of altered proprioception.

Effect of plantar desensitization on postural adjustments prior to step initiation

Plantar cutaneous afferent provides information about the contact between the body and the support surface and could affect the anticipatory postural adjustments (APAs). This study investigated the effect of plantar desensitization on the APAs for step initiation. Twenty-five healthy young adults participated in this study and were instructed to begin walking as fast as possible under 4 plantar desensitization conditions, none (NoneD), and desensitization of the stepping, supporting or bilateral (BilD) plantar surfaces, with eyes open or closed. The desensitization was achieved by cold water immersion of the plantar surface for 15 min. Foot switches recorded the timing of the stepping events. Surface electromyography (EMG) recorded the activation of bilateral tibialis anterior. The center of pressure (COP) and ground reaction force (GRF) data were derived from the force platform on which the subject initiated walking. The results showed that during the anticipation phase, the peak COP displacement toward the stepping leg was significantly smaller in BilD than in unilateral desensitization, which in turn was smaller than in NoneD, regardless of vision. The time to reach the peak COP displacement was significantly sooner with plantar desensitization in the eyes open condition. The GRF, EMG and anteroposterior COP displacement or the timing of the stepping events was not affected by plantar desensitization. These findings indicate that plantar cutaneous afferent contributed to the control of the APAs for step initiation by scaling the displacement of the mediolateral COP displacement and loss of its sensitivity could not be compensated by visual inputs.

Postural control in response to a perturbation: role of vision and additional support

The purpose of the study was to investigate the availability of vision and additional support on anticipatory (APA) and compensatory (CPA) postural adjustments and their interaction. Eight healthy adults were exposed to external perturbations induced at the shoulder level while standing with and without holding onto a walker in full vision and while blindfolded. Electrical activity of the trunk and leg muscles and center of pressure (COPAP) displacement were recorded and quantified within the time intervals typical of APA and CPA. The results showed that with full vision, there was no difference in both APA and CPA in standing with and without holding onto a walker. With subjects holding onto a walker, CPA in standing blindfolded were comparable to CPA in full vision; this was seen in changes in the electrical activity of most of the muscles at the individual muscle, joint, and the muscle group levels as well as in COPAP displacement. The findings suggest that (1) in conditions where vision is available, vision overrules simultaneously available proprioceptive information from the support, (2) while in conditions where vision is not available, proprioceptive information from the support or support itself could be substituted for vision. It is possible to suggest that using a non-stabilizing support could be a valuable strategy to improve postural control when visual information is not available or compromised.

Postural control under visual and proprioceptive perturbations during double and single limb stances: insights for balance training

Single Limb Stance under visual and proprioceptive disturbances is largely used in clinical settings in order to improve balance in a wide range of functional disabilities. However, the proper role of vision and proprioception in SLS is not completely understood. The objectives of this study were to test the hypotheses that when ankle proprioception is perturbed, the role of vision in postural control increases according to the difficulty of the standing task. And to test the effect of vision during postural adaptation after withdrawal of the somesthetic perturbation during double and single limb stance Eleven males were submitted to double (DLS) and single limb (SLS) stances under conditions of normal or reduced vision, both with normal and perturbed proprioception. Center of pressure parameters were analyzed across conditions. Vision had a main effect in SLS, whereas proprioception perturbation showed effects only during DLS. Baseline stability was promptly achieved independently of visual input after proprioception reintegration. In conclusion, the role of vision increases in SLS. After proprioception reintegration, vision does not affect postural recovery. Balance training programs must take that into account.

Combined recruitment of two fixation chains during cyclic movements of one arm

Voluntary adduction-abduction movements of one arm in the horizontal plane discharge a reaction torque which would rotate the trunk in the direction opposite to arm acceleration. Rotation is impeded by muscular fixation chains that exert forces counterbalancing the reaction torque. We examined how two different fixation chains cooperate in stabilising the trunk during the above movements. Standing subjects (n=6), with shoulders ante-flexed, performed cyclic adductions-abductions of the right arm (1.5 Hz) while grasping a fixed handle with the left hand. In this set-up, reaction torque is contrasted by: (1) a leg fixation chain, exerting on the ground a torque around the vertical axis (Tz), recorded by a force platform; and (2) a left arm fixation chain, exerting on the handle a force in the medial-lateral direction (Fh), recorded by a load cell. Subjects performed 20 trials (15 cycles each). It was found that Tz and Fh underwent sinusoidal changes at the same frequency as arm movements and contributed in counteracting the reaction torque. The intensity of the handle grip, monitored by EMG activity in the left Flexor Digitorum Superficialis, was changed from trial to trial and kept constant during each trial. As grip strength increased, Fh amplitude increased linearly while amplitude of Tz linearly decreased. In conclusion, voluntarily strengthening the handle grip progressively deviates the postural actions from the legs to the left arm.

Locomotor adaptation and aftereffects in patients with reduced somatosensory input due to peripheral neuropathy

We studied 12 peripheral neuropathy patients (PNP) and 13 age-matched controls with the "broken escalator" paradigm to see how somatosensory loss affects gait adaptation and the release and recovery ("braking") of the forward trunk overshoot observed during this locomotor aftereffect. Trunk displacement, foot contact signals, and leg electromyograms (EMGs) were recorded while subjects walked onto a stationary sled (BEFORE trials), onto the moving sled (MOVING or adaptation trials), and again onto the stationary sled (AFTER trials). PNP were unsteady during the MOVING trials, but this progressively improved, indicating some adaptation. During the after trials, 77% of control subjects displayed a trunk overshoot aftereffect but over half of the PNP (58%) did not. The PNP without a trunk aftereffect adapted to the MOVING trials by increasing distance traveled; subsequently this was expressed as increased distance traveled during the aftereffect rather than as a trunk overshoot. This clear separation in consequent aftereffects was not seen in the normal controls suggesting that, as a result of somatosensory loss, some PNP use distinctive strategies to negotiate the moving sled, in turn resulting in a distinct aftereffects. In addition, PNP displayed earlier than normal anticipatory leg EMG activity during the first after trial. Although proprioceptive inputs are not critical for the emergence or termination of the aftereffect, somatosensory loss induces profound changes in motor adaptation and anticipation. Our study has found individual differences in adaptive motor performance, indicative that PNP adopt different feed-forward gait compensatory strategies in response to peripheral sensory loss.

Flexible muscle modes and synergies in challenging whole-body tasks

We used the idea of hierarchical control to study multi-muscle synergies during a whole-body sway task performed by a standing person. Within this view, at the lower level of the hierarchy, muscles are united into groups (M-modes). At the higher level, gains at the M-modes are co-varied by the controller in a task-specific way to ensure low variability of important physical variables. In particular, we hypothesized that (1) the composition of M-modes could adjust and (2) an index of M-mode co-variation would become weaker in more challenging conditions. Subjects were required to perform a whole-body sway at 0.5 Hz paced by a metronome. They performed the task with eyes open and closed, while standing on both feet or on one foot only, with and without vibration applied to the Achilles tendons. Integrated indices of muscle activation were subjected to principal component analysis to identify M-modes. An increase in the task complexity led to an increase in the number of principal components that contained significantly loaded indices of muscle activation from 3 to 5. Hence, in more challenging tasks, the controller manipulated a larger number of variables. Multiple regression analysis was used to define the Jacobian of the system mapping small changes in M-mode gains onto shifts of the center of pressure (COP) in the anterior-posterior direction. Further, the variance in the M-mode space across sway cycles was partitioned into two components, one that did not affect an average across cycles COP coordinate and the other that did (good and bad variance, respectively). Under all conditions, the subjects showed substantially more good variance than bad variance interpreted as a multi-M-mode synergy stabilizing the COP trajectory. An index of the strength of the synergy was comparable across all conditions, and there was no modulation of this index over the sway cycle. Hence, our first hypothesis that the composition of M-modes could adjust under challenging conditions has been confirmed while the second hypothesis stating that the index of M-mode co-variation would become weaker in more challenging conditions has been falsified. We interpret the observations as suggesting that adjustments at the lower level of the hierarchy-in the M-mode composition-allowed the subjects to maintain a comparable level of stabilization of the COP trajectory in more challenging tasks. The findings support the (at least) two-level hierarchical control scheme of whole-body movements.

Effects of attentional focus on postural sway in children and adults

The present study examined, in children aged 4-11 and in adults, the postural control modifications when attention was oriented voluntary on postural sway. Since (1) there are less attentional resources in children than in adults, (2) the selective attention processing improves with age, i.e., children use a different strategy to focus their attention than adults, and (3) adults' postural stability decreases when attention is focused on postural sway, we hypothesized that postural stability was less affected in children than in adults when attention was focused on postural sway. Fourty four children aged 4- to 11-year-old and 11 adults participated in the experiments. The postural control task was executed in a Romberg position. Two experimental conditions were presented to the subjects, (1) to look at a video on a TV screen without instruction about the posture, and (2) to fixate a cross placed at the center of the TV screen with the instruction to remain as stable as possible. Postural performance was measured by means of a force platform. Results from this study (1) confirmed a non-monotonic improvement of postural stability during the ontogenetic period without reaching the adults' level at the age of 11, (2) suggested that children, aged 4-11, are able to focus their attention on the control of posture, and (3) showed that the automatic control of posture increases postural stability since the age of 4.

Anticipatory postural adjustments in arm muscles associated with movements of the contralateral limb and their possible role in interlimb coordination

While sitting on a turnable stool, with both shoulders flexed at 90 degrees or, alternatively, with arms parallel to the trunk and the elbows flexed at 90 degrees--the hands being semisupine--subjects performed unidirectional and cyclic movements on the horizontal plane of the right arm (adduction-abduction) or hand (flexion-extension). The left arm was still, in a position symmetrical to that of the right limb and with the hand contacting a fixed support by the palmar or dorsal surface. During both unidirectional and cyclic arm or hand movements, activation of the prime mover muscles (right Pectoralis Major for arm adduction and Infraspinatus for abduction; right Flexor Carpi Radialis and Extensor Carpi Radialis for the hand movements) was accompanied by activation of the homologous muscles of the contralateral arm and inhibition of antagonists. The contralateral activities (1) regularly preceded the burst in the movement prime movers and (2) were organised in fixation chains that, exerting forces on the hand fixed support, will counterbalance the rotatory action exerted on the trunk by the primary movement. Based on these features, these activities may be classified as anticipatory postural adjustments (APAs). The observed APAs distribution is such as to favour the preferential (mirror symmetrical) coupling of upper limb movements on the horizontal plane. The possible role of these APAs in determining the different constraints experienced when performing mirror symmetrical versus isodirectional coupling is discussed.

Variation in Neuromuscular Responses during Acute Whole-Body Vibration Exercise

PURPOSE

Leg muscle strength and power are increased after whole-body vibration (WBV) exercise. These effects may result from increased neuromuscular activation during WBV; however, previous studies of neuromuscular responses during WBV have not accounted for motion artifact.

METHODS

Sixteen healthy adults performed a series of static and dynamic unloaded squats with and without two different directions of WBV (rotational vibration, RV; and vertical vibration, VV; 30 Hz; 4 mmp-p). Activation of unilateral vastus lateralis, biceps femoris, gastrocnemius, and tibialis anterior was recorded using EMG. During RV and VV, increases in EMG relative to baseline were compared over a range of knee angles, contraction types (concentric, eccentric, isometric), and squatting types (static, dynamic).

RESULTS

After removing large, vibration-induced artifacts from EMG data using digital band-stop filters, neuromuscular activation of all four muscles increased significantly (P<or=0.05) during RV and VV. Average responses of the extensors were significantly greater during RV than VV, whereas responses of the tibialis anterior were significantly greater during VV than RV. For all four muscles, responses during static squatting were greater than or equal to responses during dynamic squatting, whereas responses during eccentric contractions were equal to or smaller than responses during concentric and isometric contractions. Neuromuscular responses of vastus lateralis, gastrocnemius, and tibialis anterior were affected by knee angle, with greatest responses at small knee angles.

CONCLUSIONS

Motion artifacts should be removed from EMG data collected during WBV. We propose that neuromuscular responses during WBV may be modulated by leg muscle cocontraction as a postural control strategy and/or muscle tuning by the CNS intended to minimize soft-tissue vibration.

Postural constraints to coupling of ipsilateral hand-foot movements

Ipsilateral hand and foot are easily coupled in isodirectional oscillations, while antidirectional coupling is difficult or even impossible. It was recently suggested that differences between the two types of coupling depend on the interaction of postural mechanisms with voluntary movement. We report here that when standing in an upright position, with the right hand touching a rigid support and the right foot fixed to a tilting platform, fast foot flexions or extensions as well as rhythmical foot oscillations are accompanied by overt electromyogram activities in forearm flexors and extensors. These activities, described here as anticipatory postural adjustments, are distributed to forearm muscles so as to favour isodirectional and hinder antidirectional hand-foot coupling, both when the hand is prone and when it is supine.

Reversals of anticipatory postural adjustments during voluntary sway in humans

We describe reversals of anticipatory postural adjustments (APAs) with the phase of a voluntary cyclic whole-body sway movement. Subjects (n=9) held a standard load in extended arms and released it by a bilateral shoulder abduction motion in a self-paced manner at different phases of the sway. The load release task was also performed during quiet stance in three positions: in the middle of the sway range and close to its extreme forward and backward positions. Larger APAs were seen during the sway task as compared to quiet stance. Although the direction of postural perturbation associated with the load release was always the same, the direction of the APAs in the leg muscles reversed when the subjects were close to the extreme forward position as compared to the APAs in other phases and during quiet stance. The trunk muscles showed smaller APA modulation at the extreme positions but larger modulation when passing through the middle position, depending on the direction of sway, forward or backward. The phenomenon of APA reversals emphasizes the important role of safety in the generation of postural adjustments associated with voluntary movements. Based on these findings, APAs could be defined as changes in the activity of postural muscles associated with a predictable perturbation that act to provide maximal safety of the postural task component.