The effect of aging on anticipatory postural control

The aim of the study was to investigate the differences in anticipatory postural adjustments (APAs) between young and older adults and its effect on subsequent control of posture. Ten healthy older adults and thirteen healthy young adults were exposed to predictable external perturbations using the pendulum impact paradigm. Electromyographic activity of the trunk and leg muscles, the center of pressure (COP), and center of mass (COM) displacements in the anterior–posterior direction were recorded and analyzed during the anticipatory and compensatory postural adjustments (CPAs) phases of postural control. The effect of aging was seen as delayed anticipatory muscle activity and larger compensatory muscle responses in older adults as compared to young adults. Moreover, in spite of such larger reactive responses, older adults were still more unstable, exhibiting larger COP and COM peak displacements after the perturbation than young adults when exposed to similar postural disturbances. Nonetheless, while APAs are impaired in older adults, the ability to recruit muscles anticipatorily is largely preserved; however, due to their smaller magnitudes and delayed onsets, it is likely that their effectiveness in reducing the magnitude of CPAs is smaller. The outcome of the study lends support toward investigating the ways of improving anticipatory postural control in people with balance impairments due to aging or neurological disorders.     

Three components of postural control associated with pushing in symmetrical and asymmetrical stance

A number of occupational and leisure activities that involve pushing are performed in symmetrical or asymmetrical stance. The goal of this study was to investigate early postural adjustments (EPAs), anticipatory postural adjustments (APAs), and compensatory postural adjustments (CPAs) during pushing performed while standing. Ten healthy volunteers stood in symmetrical stance (with feet parallel) or in asymmetrical stance (staggered stance with one foot forward) and were instructed to use both hands to push forward the handle of a pendulum attached to the ceiling. Bilateral EMG activity of the trunk and leg muscles and the center of pressure (COP) displacements in the anterior–posterior (AP) and medial–lateral (ML) directions were recorded and analyzed during the EPAs, APAs, and CPAs. The EMG activity and the COP displacement were different between the symmetrical and asymmetrical stance conditions. The COP displacements in the ML direction were significantly larger in staggered stance than in symmetrical stance. In staggered stance, the EPAs and APAs in the thigh muscles of the backward leg were significantly larger, and the CPAs were smaller than in the forward leg. There was no difference in the EMG activity of the trunk muscles between the stance conditions. The study outcome confirmed the existence of the three components of postural control (EPAs, APAs, and CPAs) in pushing. Moreover, standing asymmetrically was associated with asymmetrical patterns of EMG activity in the lower extremities reflecting the stance-related postural control during pushing. The study outcome provides a basis for studying postural control during other daily activities involving pushing.     

Anticipatory postural adjustments in individuals with multiple sclerosis

Individuals with multiple sclerosis (MS) frequently exhibit difficulties in balance maintenance. It is known that anticipatory postural adjustments (APAs) play an important role in postural control. However, no information exists on how people living with MS utilize APAs for control of posture. A group of individuals with MS and a group of healthy control subjects performed rapid arm flexion and extension movements while standing on a force platform. Electromyographic (EMG) activity of six trunk and leg muscles and displacement of center of pressure (COP) were recorded and quantified within the time intervals typical of APAs. Individuals with MS demonstrated diminished ability to produce directional specific patterns of anticipatory EMGs as compared to control subjects. In addition, individuals with MS demonstrated smaller magnitudes of anticipatory muscle activation. This was associated with larger displacements of the COP during the balance restoration phase. These results suggest the importance of anticipatory postural control in maintenance of vertical posture in individuals with MS. The outcome of the study could be used while developing rehabilitation strategies focused on balance restoration in individuals with MS.     

Pain reported during prolonged standing is associated with reduced anticipatory postural adjustments of the deep abdominals

Within the context of low back pain, the measurement of deep abdominal anticipatory postural adjustments (APAs) during rapid limb movement has received much interest. There is dispute about the association between APAs and back pain. Moreover, there is limited evidence examining compensatory postural adjustments (CPAs) in back pain. This study examined the relationship between APAs and CPAs with pain reported in the low back during 2 h of prolonged standing. Twenty-six participants with no history of severe back pain performed 2-h prolonged standing. APAs and CPAs of the deep abdominal muscles (transverse abdominis/internal obliques) were measured by surface electromyography during rapid shoulder flexion and extension. APAs and CPAs measured pre-standing revealed symmetrical anticipatory activity, but an asymmetry between the different sides of the abdominal wall for CPAs. APAs and CPAs measured pre-standing were not associated with pain reported during standing. For the whole group, APA amplitudes were reduced post-standing during shoulder flexion (p = 0.005). Pain reported during standing was associated with the changes in APA amplitudes post-standing (rs = 0.43, p = 0.002). These findings support previous research using hypertonic saline injections to induce back pain that showed reduced APA amplitudes, and extends findings to suggest pain does not effect compensatory postural adjustments.     

Anticipatory postural adjustments in children with typical motor development

Anticipatory postural adjustments (APAs) play an important role in the performance of many activities requiring the maintenance of vertical posture. However, little is known about how children utilize APAs during self-induced postural perturbations. A group of children, aged 7–16 years, with typical motor development, performed various arm movements while standing on a force platform. APAs were measured by recording the electromyographic activity of six trunk and leg muscles on both sides of the body and displacement of center of pressure (COP). Anticipatory bursts of activity in the dorsal muscle groups of the trunk and legs and suppression in the ventral muscle groups as well as posterior COP displacement were found during the performance of bilateral shoulder flexion. Conversely, during bilateral shoulder extension, the COP displacement was anterior, and APAs were reversed showing bursts of activity in the ventral muscle groups and suppression in the dorsal muscles. During right and left reciprocal arm movements, COP displacement was minimal and APAs were generated in the dorsal muscle groups on the side of the forward moving arm and in the ventral muscle groups on the side of the arm moving into extension. However this pattern reversed for lower leg muscles, where APAs were generated in the ventral muscles on the side of forward moving arm and in the dorsal muscle on the side of the arm moving into extension. The results of this study indicate that children with typical motor development are able to generate APAs, produce task-specific sequencing of muscle activity and differentiate between perturbations in the sagittal and transverse planes. The results of this study indicate that by at least age 7, children who are typically developing demonstrate the ability to generate patterns of anticipatory muscle activation and suppression, along with center of pressure changes, similar to those reported in healthy adults.     

Deficits in task-specific modulation of anticipatory postural adjustments in individuals with spastic diplegic cerebral palsy

We examined whether individuals with spastic diplegic cerebral palsy (SDCP) have the ability to utilize lower leg muscles in anticipatory postural adjustments (APAs) associated with voluntary arm movement while standing, as well as the ability to modulate APAs with changes in the degree of postural perturbation caused by arm movement. Seven individuals with spastic diplegia (SDCP group, 12-22 yr of age) and seven age- and sex-matched individuals without disability (control group) participated in this study. Participants flexed both shoulders and lifted a load under two different load conditions, during which electromyographic activities of focal and postural muscles were recorded. Although the timing of anticipatory activation of the erector spinae and medial hamstring (MH) muscles was similar in the two participant groups, that of the gastrocnemius (GcM) muscle was significantly later in the SDCP group than in the control group. An increase in anticipatory postural muscle activity with an increase in load was observed in MH and GcM in the control group but not in GcM in the SDCP group. The degree of modulation in MH was significantly smaller in the SDCP group than in the control group. An additional experiment confirmed that these differences in APAs between the two participant groups were unlikely to be attributable to their differences in initial standing posture before load lift. The present findings suggest that lower leg muscles play a minor role in APAs in individuals with spastic diplegia. In addition, it is likely that these individuals have difficulty modulating anticipatory postural muscle activity with changes in the degree of postural perturbation.     

Can prepared anticipatory postural adjustments be updated by proprioception?

Stepping over an obstacle is preceded by a center of pressure (CoP) shift, termed anticipatory postural adjustments (APAs). It provides an acceleration of the center of mass forward and laterally prior to step initiation. The APAs are characterized in the lateral direction by a force exerted by the moving leg onto the ground, followed by an unloading of the stepping leg and completed by an adjustment corresponding to a slow CoP shift toward the supporting foot. While the importance of sensory information in the setting of the APAs is undisputed, it is currently unknown whether sensory information can also be used online to modify the feedforward command of the APAs. The purpose of this study was to investigate how the CNS modulates the APAs when a modification of proprioceptive information (Ia) occurs before or during the initiation of the stepping movement. We used the vibration of ankle muscles acting in the lateral direction to induce modification of the afferent inflow. Subjects learned to step over an obstacle, eyes closed, in synchrony to a tone signal. When vibration was applied during the initiation of the APAs, no change in the early APAs was observed except in the case of a cutaneous stimulation (low frequency vibration); it is thus possible that the CNS relies less on proprioceptive information during this early phase. Only the final adjustment of the CoP seems to take into account the biased proprioceptive information. When vibration was applied well before the APAs onset, a postural reaction toward the side of the vibration was produced. When subjects voluntarily initiated a step after the postural reaction, the thrust amplitude was set according to the direction of the postural reaction. This suggests that the planned motor command of the APAs can be updated online before they are triggered.     

Static and dynamic visual cues in feed-forward postural control

Anticipatory postural adjustments (APAs) play an important role in the performance of many activities requiring the maintenance of vertical posture. However, little is known about how variation in the available visual information affects generation of APAs. The purpose of this study was to investigate the role of different visual cues on APAs. Ten healthy young subjects were exposed to external perturbations induced at the shoulder level in standing while the level of visual information about the forthcoming perturbation was varied. The external perturbations were provided by an aluminum pendulum attached to the ceiling. The visual conditions were (1) dynamic cues (full vision and high-frequency strobe light), (2) static cues (low-frequency strobe light) and (3) no cues (eyes open in dark room). Electrical activity of the trunk and leg muscles and center of pressure displacements were recorded and quantified within the time intervals typical for APAs. The results showed that significantly larger APAs were generated in conditions with dynamic visual cues as compared to the conditions with static cues (p < 0.05). Finally, no APAs were observed in the condition where there was complete absence of any visual cues. Principal component analysis further revealed different muscle coupling patterns in the full vision and high-frequency strobe light conditions. These findings suggest the importance of using appropriate visual cues in the generation of APAs.     

Anticipatory postural adjustments while sitting: The effects of different leg supports

The aim of this study was twofold, to analyze the effects of changes in body position and changes in the location of body supports on anticipatory postural adjustments (APAs). Eight healthy subjects were studied while sitting and standing. Subjects exerted upward or downward vertical force against an object attached to a rigid frame and released the object with a fast bilateral shoulder abduction movement. While sitting, four support conditions were studied: with and without feet support, and with anterior or posterior lower-leg supports. The electromyographic activity of leg and trunk muscles was recorded and quantified for APA activity. APAs in sitting with feet support were attenuated in the leg muscles (tibialis anterior, soleus, rectus femoris, and biceps femoris) but not in trunk muscles (erector spinae, rectus abdominis) when compared with standing. In the sitting task, series with and without feet support showed no difference in APAs. Anterior or posterior supports to the lower legs while sitting were associated with enhanced anticipatory activity in biceps femoris and rectus femoris muscles, respectively. However, trunk muscles showed similar anticipatory patterns across all the support conditions. We conclude that the central nervous system uses flexible, adaptive control strategies to adjust APAs to particular mechanical conditions induced by modification of a leg support.     

Effects of muscle vibration on multi-finger interaction and coordination

The purpose of this study was to investigate the effects of changes in the proprioceptive signals induced by muscle vibration on multi-finger interaction and coordination. We hypothesized that unintended force production by non-instructed fingers (enslaving) would increase with muscle vibration while synergy indices during steady-state force production would drop. The framework of the uncontrolled manifold hypothesis was used to quantify indices of multi-finger synergies stabilizing total force during steady-state force production and anticipatory changes in these indices (anticipatory synergy adjustments, ASAs) in preparation to a quick force pulse production with and without hand-muscle vibration at 80 Hz. The dominant hands of twelve healthy right-handed subjects were tested under three conditions: no vibration, vibration of the palmar surface of the hand, and vibration of the forearm applied over the flexor muscles. There were no significant effects of vibration on maximal voluntary force. The magnitude of enslaving was larger during vibration of the hand compared to the other two conditions. During steady-state force production, strong synergies stabilizing total force were seen in all three conditions; however, indices of force-stabilizing synergies were lower during vibration of the hand. Prior to the force pulse initiation, the synergy index started to drop earlier and over a larger magnitude without vibration compared to either vibration condition. Effects of vibration on enslaving and synergy index may be due to diffuse reflex effects of the induced afferent activity on alpha-motoneuronal pools innervating the extrinsic flexor compartments. We conclude that multi-finger synergies are not based on signals from muscle receptors. The smaller synergy indices and ASAs may reflect supraspinal effects of the vibration-induced afferent activity, in particular its interactions with trans-thalamic loops.     

Two aspects of feedforward postural control: anticipatory postural adjustments and anticipatory synergy adjustments

We used the framework of the uncontrolled manifold hypothesis to explore the relations between anticipatory synergy adjustments (ASAs) and anticipatory postural adjustments (APAs) during feedforward control of vertical posture. ASAs represent a drop in the index of a multimuscle-mode synergy stabilizing the coordinate of the center of pressure in preparation to an action. ASAs reflect early changes of an index of covariation among variables reflecting muscle activation, whereas APAs reflect early changes in muscle activation levels averaged across trials. The assumed purpose of ASAs is to modify stability of performance variables, whereas the purpose of APAs is to change magnitudes of those variables. We hypothesized that ASAs would be seen before APAs and that this finding would be consistent with regard to the muscle-mode composition defined on the basis of different tasks and phases of action. Subjects performed a voluntary body sway task and a quick, bilateral shoulder flexion task under self-paced and reaction time conditions. Surface muscle activity of 12 leg and trunk muscles was analyzed to identify sets of 4 muscle modes for each task and for different phases within the shoulder flexion task. Variance components in the muscle-mode space and indexes of multimuscle-mode synergy stabilizing shift of the center of pressure were computed. ASAs were seen ～ 100-150 ms prior to the task initiation, before APAs. The results were consistent with respect to different sets of muscle modes defined over the two tasks and different shoulder flexion phases. We conclude that the preparation for a self-triggered postural perturbation is associated with two types of anticipatory adjustments, ASAs and APAs. They reflect different feedforward processes within the hypothetical hierarchical control scheme, resulting in changes in patterns of covariation of elemental variables and in their patterns averaged across trials, respectively. The results show that synergies quantified using dissimilar sets of muscle modes show similar feedforward changes in preparation to action.     

Does the coordination between posture and movement during human whole-body reaching ensure center of mass stabilization?

The whole-body center of mass (CoM) has been classically regarded as the stabilized reference value for human voluntary movements executed upon a fixed base of support. Axial synergies (opposing displacements of head and trunk with hip segments) are believed to minimize antero-posterior (A/P) CoM displacements during forward trunk movements. It is also widely accepted that anticipatory postural adjustments (APAs) create forces of inertia that counteract disturbances arising from the moving segment(s). In the present study, we investigated CoM stabilization by axial synergies and APAs during a whole-body reaching task. Subjects reached towards an object placed on the ground in front of them in their sagittal plane using a strategy of coordinated trunk, knee, and hip flexion. The reaching task imposed constraints on arm-trajectory formation and equilibrium maintenance. To manipulate equilibrium constraints, differing conditions of distance and speed were imposed. The comparison of distance conditions suggested that axial synergies were not entirely devoted to CoM stabilization: backward A/P hip displacements reduced as head and trunk forward A/P displacements increased. Analysis of upper- and lower-body centers of mass in relation to the CoM also showed no strict minimization of A/P CoM displacements. Mechanical analysis of the effects of APAs revealed that, rather than acting to stabilize the CoM, APAs created necessary conditions for forward CoM displacement within the base of support in each condition. The results have implications for the CoM as the primary stabilized reference for posture and movement coordination during whole-body reaching and for the central control of posture and voluntary movement. Received: 14 July 1998 / Accepted: 23 May 1999     

Role of lateral muscles and body orientation in feedforward postural control

The study investigates the role of lateral muscles and body orientation in anticipatory postural adjustments (APAs). Subjects stood in front of an aluminum pendulum and were required to stop it with their right or left hand. An experimenter released the pendulum inducing similar body perturbations in all experimental series. The perturbation directions were manipulated by having the subjects standing on the force platform with different body orientations in relation to the pendulum movements. Consequently, perturbations were induced in sagittal, oblique, and frontal planes. Ground reaction forces and bilateral EMG activity of dorsal, ventral, and lateral trunk and leg muscles were recorded and quantified within the time intervals typical of APAs. Anticipatory postural adjustments were seen in all experimental conditions; their magnitudes depended on the body orientation in relation to the direction of perturbation. When the perturbation was produced in the lateral and oblique planes, APAs in the gluteus medius muscles were greater on the side opposite to the side of perturbation. Conversely, simultaneous anticipatory activation of the external obliques, rectus abdominis, and erector spinae muscles was observed on the side of perturbation when it was induced in the lateral plane. The results of the present study provide additional information on the directional specificity of anticipatory activation of ventral and dorsal muscles. The findings provide new data on the role of lateral muscles in feedforward postural control and stress the importance of taking into consideration their role in the control of upright posture.     

Two stages and three components of the postural preparation to action

Previous studies of postural preparation to action/perturbation have primarily focused on anticipatory postural adjustments (APAs), the changes in muscle activation levels resulting in the production of net forces and moments of force. We hypothesized that postural preparation to action consists of two stages: (1) Early postural adjustments (EPAs), seen a few hundred ms prior to an expected external perturbation and (2) APAs seen about 100 ms prior to the perturbation. We also hypothesized that each stage consists of three components, anticipatory synergy adjustments seen as changes in covariation of the magnitudes of commands to muscle groups (M-modes), changes in averaged across trials levels of muscle activation, and mechanical effects such as shifts of the center of pressure. Nine healthy participants were subjected to external perturbations created by a swinging pendulum while standing in a semi-squatting posture. Electrical activity of twelve trunk and leg muscles and displacements of the center of pressure were recorded and analyzed. Principal component analysis was used to identify four M-modes within the space of muscle activations using indices of integrated muscle activation. This analysis was performed twice, over two phases, 400–700 ms prior to the perturbation and over 200 ms just prior to the perturbation. Similar robust results were obtained using the data from both phases. An index of a multi-M-mode synergy stabilizing the center of pressure displacement was computed using the framework of the uncontrolled manifold hypothesis. The results showed high synergy indices during quiet stance. Each of the two stages started with a drop in the synergy index followed by a change in the averaged across trials activation levels in postural muscles. There was a very long electromechanical delay during the early postural adjustments and a much shorter delay during the APAs. Overall, the results support our main hypothesis on the two stages and three components of the postural preparation to action/perturbation. This is the first study to document anticipatory synergy adjustments in whole-body tasks. We interpret the results within the referent configuration hypothesis (an extension of the equilibrium-point hypothesis): The early postural adjustment is based primarily on changes in the coactivation command, while the APAs involve changes in the reciprocal command. The results fit an earlier hypothesis that whole-body movements are controlled by a neuromotor hierarchy where each level involves a few-to-many mappings organized to stabilize its overall output.     

Modulation of anticipatory postural adjustments associated with unloading perturbation: effect of characteristics of a motor action

The purpose of the study was to determine whether characteristics of a motor action affect anticipatory postural adjustments (APAs). Standing subjects held a load between their hands with arms extended in front of their body. Next, subjects performed bilateral shoulder abduction movements (motor action) of three amplitudes at three instructed speeds. This motor action led to the release of the same load, inducing unloading perturbation in the sagittal plane. Electromyographic activities were recorded for the leg and trunk muscles. A change in the background muscle activity in these muscles was observed prior to the unloading perturbation and was quantified as APAs. APAs were dependant on instructed speed of the motor action; larger APA activities were observed in the leg and trunk muscles with a faster speed instruction. Meanwhile, the modulation of APAs was not observed by altering the movement amplitude. Moreover, experiments showed that motor action itself without a load release did not generate APA activity. Therefore, we concluded that the central nervous system selects information within a motor action (i.e., speed instruction) to approximate the magnitude of the forthcoming perturbation and modulate APAs, even when the unloading perturbation was unchanged.     

The influence of age and surface compliance on changes in postural control and attention due to ankle neuromuscular fatigue

The reduction in the quality and integration of sensory information with aging could increase the alterations in postural control associated with muscle fatigue observed in younger adults. This study aimed to compare changes in postural control and attentional demands due to ankle muscle fatigue, with intact and reduced proprioceptive information at the ankle, between young and older adults. Eleven young (24 ± 4 years) and 13 older (65 ± 4 years) men stood quietly on a force platform (blindfolded) under four experimental conditions (combinations of firm (FS)/compliant (CS) surfaces and single/dual tasks), before and immediately after a fatiguing exercise. The fatiguing exercise, performed on a dynamometer, consisted of maintaining an isometric contraction of the plantarflexors at 50 % of maximum until exhaustion. Both COP sway area and COP sway velocity were greater on the CS compared to FS and increased with fatigue for both groups in all conditions. COP sway area showed a greater increase with fatigue in older adults when standing on the CS. Reaction time (secondary task) increased significantly after fatigue, but only for older adults when standing on the CS. The effects of fatigue on postural control are more important when proprioceptive information at the ankle is altered. In particular, older adults had more difficulty and may have needed more attention to stand quietly, compared with young adults.     

The effect of short-term changes in the body mass on anticipatory postural adjustments

The purpose of the study was to investigate whether anticipatory postural adjustments (APAs) are modified with short-term changes in the body mass. Nine subjects were asked to catch a 2.2 kg load with their arms extended under conditions of no weight and when additional weights of 10 and 20% of the subject’s body weight (BW) were attached to single body locations or when 20 or 40% BW were attached evenly to two locations. Attaching weights was associated with an increase of the whole body mass, but also involved changes in the vertical position of the center of mass (COM). Electromyographic activity of leg and trunk muscles and ground reaction forces were recorded and quantified within the typical time intervals of APAs. APAs were influenced by the magnitude of the weight attached to the body: an increase in the body mass was associated with anticipatory co-activation of trunk and leg muscles. The level of this co-activation increased with an increase in the magnitude of weight added to the body. At the same time, APAs were affected by the changes in the vertical position of COM. These findings suggest that in the case of short-term changes in the body mass, the CNS might prioritize information regarding the magnitude and location of the additional weight added to the body and utilize a strategy of anticipatory co-activation of postural muscles directed at the stabilization of body segments.     

Decreased variability in postural control strategies in young people with non-specific low back pain is associated with altered proprioceptive reweighting

Optimal postural control is an essential capacity in daily life and can be highly variable. The purpose of this study was to investigate if young people have the ability to choose the optimal postural control strategy according to the postural condition and to investigate if non-specific low back pain (NSLBP) influences the variability in proprioceptive postural control strategies. Young individuals with NSLBP (n?=?106) and healthy controls (n?=?50) were tested on a force plate in different postural conditions (i.e., sitting, stable support standing and unstable support standing). The role of proprioception in postural control was directly examined by means of muscle vibration on triceps surae and lumbar multifidus muscles. Root mean square and mean displacements of the center of pressure were recorded during the different trials. To appraise the proprioceptive postural control strategy, the relative proprioceptive weighting (RPW, ratio of ankle muscles proprioceptive inputs vs. back muscles proprioceptive inputs) was calculated. Postural robustness was significantly less in individuals with NSLBP during the more complex postural conditions (p?<?0.05). Significantly higher RPW values were observed in the NSLBP group in all postural conditions (p?<?0.05), suggesting less ability to rely on back muscle proprioceptive inputs for postural control. Therefore, healthy controls seem to have the ability to choose a more optimal postural control strategy according to the postural condition. In contrast, young people with NSLBP showed a reduced capacity to switch to a more multi-segmental postural control strategy during complex postural conditions, which leads to decreased postural robustness.     

Defective temporal discrimination of passive movements in Parkinson's disease

Perception of limb position and motion is abnormal in Parkinson's disease (PD). Despite the fact that the processing of proprioceptive inputs is inherently temporal, most studies have assessed spatial aspects of proprioception in PD patients. Here, we use a recently described method to test whether deficits also exist in temporal discrimination of proprioceptive inputs. We induced index finger abduction or wrist flexion through percutaneous electrical stimulation of the motor point of the first dorsal interosseous muscle (FDI) or the flexor carpii radialis (FCR), respectively. Twelve patients with unilateral bradykinetic-rigid PD and 12 healthy subjects were asked to report whether pairs of stimuli separated by different time intervals produced single or double index finger abduction movement or wrist flexion. The shortest interval at which subjects reported two separated movements was considered as temporal movement discrimination threshold. Results showed that thresholds were significantly higher in PD patients than in control subjects for both FCR and FDI muscle, thus demonstrating for the first time that temporal proprioceptive processing is altered in PD.