Early Postural Adjustments Research Articles

Rambling and trembling trajectories in the analysis of postural sway prior to the self-paced and reaction time tasks

Objective: Previous studies suggested that center of pressure (COP) shifts occur before an expected perturbation in the form of early and anticipatory postural adjustments which operate in a short time scale. However, the effect of such perturbations on pre-existing postural set on a longer time scale remained uncovered. The purpose of this study was to investigate whether rambling and trembling components of the COP trajectories depend on postural task or phase of trial before a self-initiated perturbation. Materials and Methods: Twenty-four young healthy participants took part in the study. Subjects performed three postural tasks, namely, (i) quiet stance task: 60 seconds quiet stance, (ii) self-paced task: maximal vertical jump from quiet stance under the self-paced time condition, and (iii) reaction-time task: maximal vertical jump from quiet stance under the reaction-time condition. Postural sway features were examined in two phases, the first and last 20 seconds of the trials. Results: The features of rambling and trembling components of the COP trajectories were affected by postural task or phase of trial. The ellipse area of the COP and rambling trajectories were significantly different among postural tasks. The median frequency was significantly different between the phases of trials for the COP and rambling trajectories. Conclusion: This study indicated task-specific changes in postural sway features. Rambling and trembling trajectories, which would reflect two underlying human postural control mechanisms as maintaining the body's equilibrium with respect to a moving reference point and oscillating around the moving reference point respectively, were affected differently before a whole-body maximum-effort self-initiated perturbation.

Influence of Target Width and Distance on Postural Adjustments in a Fencing Lunge.

The aim of this study was to examine whether target width and target distance influence the planning phase of a fencing lunge (early and anticipatory postural adjustments) as well as the execution phase of a fencing lunge. Eight elite female fencers participated in the study. The displacement of the center of foot pressure, muscle activity of the tibialis anterior, and kinematics of center of mass were recorded using force plates. The results show that target width and distance have no effect on early and anticipatory postural adjustments as well as the acceleration and velocity of the center of mass at the moment of foot-off. However, a greater target distance was associated with a greater max center of mass acceleration and velocity, and larger target width resulted in a greater max center of mass acceleration during lunging (p < 0.05). We suppose that the effect of task parameters on preparing a fencing lunge may be mitigated due to the specific technique adopted by expert fencers and the ballistic nature of a fencing lunge.

Postural control during turn on the light task assisted by functional electrical stimulation in post stroke subjects

Postural control mechanisms have a determinant role in reaching tasks and are typically impaired in post-stroke patients. Functional electrical stimulation (FES) has been demonstrated to be a promising therapy for improving upper limb (UL) function. However, according to our knowledge, no study has evaluated FES influence on postural control. This study aims to evaluate the influence of FES UL assistance, during turning on the light task, in the related postural control mechanisms. An observational study involving ten post-stroke subjects with UL dysfunction was performed. Early and anticipatory postural adjustments (EPAs and APAs, respectively), the weight shift, the center of pressure and the center of mass (CoM) displacement were analyzed during the turning on the light task with and without the FES assistance. FES parameters were adjusted to improve UL function according to a consensus between physiotherapists’ and patients’ perspectives. The ANOVA repeated measures, Paired sample t and McNemar tests were used to compare postural control between the assisted and non-assisted conditions. When the task was assisted by FES, the number of participants that presented APAs increased (p = 0.031). UL FES assistance during turning on the light task can improve postural control in neurological patients with UL impairments.

Influence of base of support on early postural adjustments and fencing lunge performance

ABSTRACT The aim of the study was to examine whether the width of the base of support (BOS) has an influence on early postural adjustments (EPA) and armed upper limb and COM kinematics during lunging. Eight elite female fencers participated in the study. The displacement of the centre of foot pressure (COP) and kinematics of centre of mass (COM) were recorded using force platforms. The kinematics of the wrist of the armed upper limb were obtained using a 3D accelerometer. The results show that EPA were characterised by a very small COP displacement (about 0.6 to 1.2 cm). The COP amplitude showed a tendency to decrease with an increase in BOS width under reaction time conditions. In contrast, a greater BOS width was associated with a significant increase in COM acceleration at foot-off and peak COM acceleration during lunging also under reaction time instructions. The kinematics of the wrist were not influenced by BOS. The results show that in elite fencers, the upper limb movement is dissociated from the whole body movement. We suppose that a small COP amplitude is a postural skill which is integrated into the lunging skill and might influence the acceleration of COM.

Two Aspects of Feedforward Control During a Fencing Lunge: Early and Anticipatory Postural Adjustments.

The present study investigated whether expertise in fencing influences the onset of postural preparation during the fencing lunge and how it changes under different performance conditions. We also questioned if the onset of feedforward control can be categorized into one of the postural phases: anticipatory or early postural adjustment. Eight elite fencers and nine physical education students performed an attack with a lunge in self-paced and reaction time conditions from three different initial stance widths. The onset of the center of pressure (COP) displacement and EMG activities for the tibialis anterior (TA) of both limbs were recorded. The results show that expertise in fencing delays the onset of the activity of TA of the front leg and the onset of COP displacement during fencing lunge performance in comparison to controls. Additionally, in contrast to the control group, fencers produce typical APA patterns in the activation of TA under different performance conditions, delayed reaction time in comparison to self-initiated lunging, and constant time of APA onset under different widths of stance. According to different times and functions of TA activity and COP displacement in lunging, we propose to address them as anticipatory postural adjustment and early postural adjustment, respectively.

Postural Adjustments during Interactions with an Active Partner

Ensuring stability of the human vertical posture is a complex task requiring both anticipatory and compensatory postural strategies when a standing person performs fast actions and interacts with the environment, which can include other persons. How people adjust their preparatory and compensatory postural adjustments in situations when they interact with an active partner is still poorly understood. In this study we investigated the postural adjustments while two healthy persons played a traditional childhood game. While standing facing each other, they were asked to push with their hands against the hands of the opponent only, and to make the opponent to take a step. We explored strategies when pushing the opponent’s hands generated perturbations to the posture of both players and when one of the players withdrew the arms to neutralize the opponent’s pushing action. Electromyograms were recorded from the leg and trunk muscles and used to quantify early (EPAs), anticipatory (APAs) and compensatory (CPAs) postural adjustments, as well as the co-activation and reciprocal changes in the activity of agonist–antagonist pairs. Results showed higher indices of muscle co-activation during EPAs during the game compared to the control conditions. We found that postural preparation strategies defined whether a participant kept or lost balance during the game. Our results highlight the importance of muscle co-activation, the role of anticipation, and the difference in strategies while interacting with an active partner as compared to interactions with passive objects.

Lower-Limb Amputees Adjust Quiet Stance in Response to Manipulations of Plantar Sensation.

Interfering with or temporarily eliminating foot-sole tactile sensations causes postural adjustments. Furthermore, individuals with impaired or missing foot-sole sensation, such as lower-limb amputees, exhibit greater postural instability than those with intact sensation. Our group has developed a method of providing tactile feedback sensations projected to the missing foot of lower-limb amputees via electrical peripheral nerve stimulation (PNS) using implanted nerve cuff electrodes. As a step toward effective implementation of the system in rehabilitation and everyday use, we compared postural adjustments made in response to tactile sensations on the missing foot elicited by our system, vibration on the intact foot-sole, and a control condition in which no additional sensory input was applied. Three transtibial amputees with at least a year of experience with tactile sensations provided by our PNS system participated in the study. Participants stood quietly with their eyes closed on their everyday prosthesis while electrically elicited, vibratory, or no additional sensory input was administered for 20 s. Early and steady-state postural adjustments were quantified by center of pressure location, path length, and average angle over the course of each trial. Electrically elicited tactile sensations and vibration both caused shifts in center of pressure location compared to the control condition. Initial (first 3 s) shifts in center of pressure location with electrically elicited or vibratory sensory inputs often differed from shifts measured over the full 20 s trial. Over the full trial, participants generally shifted toward the foot receiving additional sensory input, regardless of stimulation type. Similarities between responses to electrically elicited tactile sensations projected to the missing foot and responses to vibration in analogous regions on the intact foot suggest that the motor control system treats electrically elicited tactile inputs similarly to native tactile inputs. The ability of electrically elicited tactile inputs to cause postural adjustments suggests that these inputs are incorporated into sensorimotor control, despite arising from artificial nerve stimulation. These results are encouraging for application of neural stimulation in restoring missing sensory feedback after limb loss and suggest PNS could provide an alternate method to perturb foot-sole tactile information for investigating integration of tactile feedback with other sensory modalities.

Early postural adjustments in football players with cerebral palsy

Introduction: The functional limitations associated with Cerebral Palsy (CP) and inactivity are directly related to the deficit in postural control, namely the Early Postural Adjustments (EPAs). On the other hand, same studies showed that the practice of physical exercise can improve stability. Objectives: Thus, the goal of the present study was to evaluate the influence of physical exercise, in this case in a football player, on the EPAs of athletes with CP. Methods: The study included 5 young male adults with CP, aged between 18 and 36 years old, all football players, class 7 and 8.The electromyographic activity was recorded using Delsys Trigno and the ratio of the postural musculature [soleus (SO) and tibialis anterior (TA)] prior to a step motion, as well as the center of pressure displacement amplitude were calculated through the force platforms (Bertec), before and after a 4 month interval of training. The two moments of evaluation were compared using the Wilcoxon test and the Spearman correlation between the CoP amplitude and the muscular activity ratio was calculated.Participants were recruited in a first period in which they had no physical activity at all and in the following 4 months they started training regularly, with a frequency of three times a week for 1h30min, always with the same trainer and in the same physical training space. Results: There was a statistically significant decrease in the anterior CoP amplitude (-4.47cm; p=0.043) after 4 months, with a negative correlation between this amplitude and the ratio of TA / SO (r=-0.90; p=0.037). In the lateral direction, a diminution of amplitude of minimum CoP (-1.49; p=0.893) without statistically significant correlation (r=0.10; p=0.873) was found. Conclusions: Practicing sport, even in pathology, has a significant effect on the improvement of postural oscillation, with an increase in the relationship between muscular activity and postural control.

Early Intervention and Postural Adjustments During Reaching in Infants at Risk of Cerebral Palsy.

To investigate postural effects of the family-centered program, COPing with and CAring for infants with special needs (COPCA), applied at 3 to 6 months' corrected age in infants at high risk of cerebral palsy. Previously, we reported postural differences between the infants at risk of CP in the control group of the current study and a group of infants developing typically. Now we focus on differences between 2 intervention groups. We explored postural adjustments during reaching in seated infants at 4, 6, and 18 months using surface electromyography of arm, neck, and trunk muscles. Infants randomly received the family-centered program or another infant physical therapy. Using videotaped intervention sessions, we investigated correlations between time spent on specific physical therapeutic actions and direction specificity, recruitment order, and anticipatory activation at 18 months. Postural adjustments in both groups were similar, but development of direction specificity and anticipatory activation in COPCA infants better mimicked typical development. These 2 parameters were associated with COPCA-type physical therapeutic actions. Postural control was similar after both interventions. Positive outcomes were associated with fewer intervening actions of the therapist and greater allowance of spontaneous movements.

Commentary on "Early Intervention and Postural Adjustments During Reaching in Infants at Risk of Cerebral Palsy".

Commentary on "Early Intervention and Postural Adjustments During Reaching in Infants at Risk of Cerebral Palsy".

Early and anticipatory postural adjustments in healthy subjects under stable and unstable sitting conditions

Clinicians frequently incorporate unstable sitting devices into training plans for improving proximal postural muscle control; however, the effect of unstable sitting conditions on postural adjustments during dynamic activities has not been fully explored. The aim of this study was to characterize early postural adjustments (EPAs) and anticipatory postural adjustments (APAs) under stable and unstable sitting conditions. Using a cross-sectional laboratory study design, 13 healthy college student volunteers used their dominant hand to reach forward and push a target under stable and unstable sitting conditions; subjects sat on an air-filled rubber cushion for the unstable condition. EPAs and APAs were quantified by recording muscle activation of the trunk and lower extremity muscles using electromyography (EMG). The center of pressure (COP) was measured using a force plate. The resulting EMG integral of the ipsilateral gastrocnemius muscle was larger during the EPA phase and smaller during the APA phase under unstable conditions (p = 0.014 and p = 0.041, respectively). COP amplitude in the anterior-posterior direction, path length, and velocity, was larger during the APA phase (p = 0.035, p = 0.023, and 0.023, respectively). This suggests greater distal muscle activation during EPAs in unstable sitting conditions, specifically in the ipsilateral gastrocnemius muscle. In addition, APAs adjusted by reducing the activity of the ipsilateral gastrocnemius muscle and increasing the anterior-posterior shift in the COP to compensate for the expected additional perturbation due to an unstable surface.

Control of vertical posture while elevating one foot to avoid a real or virtual obstacle.

The purpose of this study is to investigate the control of vertical posture during obstacle avoidance in a real versus a virtual reality (VR) environment. Ten healthy participants stood upright and lifted one leg to avoid colliding with a real obstacle sliding on the floor toward a participant and with its virtual image. Virtual obstacles were delivered by a head mounted display (HMD) or a 3D projector. The acceleration of the foot, center of pressure, and electrical activity of the leg and trunk muscles were measured and analyzed during the time intervals typical for early postural adjustments (EPAs), anticipatory postural adjustments (APAs), and compensatory postural adjustments (CPAs). The results showed that the peak acceleration of foot elevation in the HMD condition decreased significantly when compared with that of the real and 3D projector conditions. Reduced activity of the leg and trunk muscles was seen when dealing with virtual obstacles (HMD and 3D projector) as compared with that seen when dealing with real obstacles. These effects were more pronounced during APAs and CPAs. The onsets of muscle activities in the supporting limb were seen during EPAs and APAs. The observed modulation of muscle activity and altered patterns of movement seen while avoiding a virtual obstacle should be considered when designing virtual rehabilitation protocols.

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.

Fitts’ Law in early postural adjustments

We tested a hypothesis that the classical relation between movement time and index of difficulty (ID) in quick pointing action (Fitts’ Law) reflects processes at the level of motor planning. Healthy subjects stood on a force platform and performed quick and accurate hand movements into targets of different size located at two distances. The movements were associated with early postural adjustments that are assumed to reflect motor planning processes. The short distance did not require trunk rotation, while the long distance did. As a result, movements over the long distance were associated with substantial Coriolis forces. Movement kinematics and contact forces and moments recorded by the platform were studied. Movement time scaled with ID for both movements. However, the data could not be fitted with a single regression: Movements over the long distance had a larger intercept corresponding to movement times about 140ms longer than movements over the shorter distance. The magnitude of postural adjustments prior to movement initiation scaled with ID for both short and long distances. Our results provide strong support for the hypothesis that Fitts’ Law emerges at the level of motor planning, not at the level of corrections of ongoing movements. They show that, during natural movements, changes in movement distance may lead to changes in the relation between movement time and ID, for example when the contribution of different body segments to the movement varies and when the action of Coriolis force may require an additional correction of the movement trajectory.

Early postural adjustments in preparation to whole-body voluntary sway

We studied postural adjustments associated with a quick voluntary postural sway under two conditions, self-paced and simple reaction-time. Standing subjects were required to produce quick discrete shifts of the center of pressure (COP) forward. About 400–500 ms prior to the instructed COP shift, there were deviations of the COP in the opposite direction (backwards) accompanied by changes in the activation levels of several postural muscles. Under the reaction-time conditions, the timing of those early postural adjustments did not change (repeated measures MANOVA: p > 0.05) while its magnitude increased significantly (confirmed by repeated measures MANOVA: p < 0.05). These observations are opposite to those reported for anticipatory postural adjustments under simple reaction time conditions (a significant change in the timing without major changes in the magnitude). We conclude that there are two types of feed-forward postural adjustments. Early postural adjustments prepare the body for the planned action and/or expected perturbation. Some of these preparatory actions may be mechanically necessary. Later, anticipatory postural adjustments generate net forces and moments of force acting against those associated with the expected perturbation. Both types of adjustments fit well the referent configuration hypothesis, which offers a unified view on movement-posture control.

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.

Gait initiation is affected during emotional conflict

The present study examined whether emotion eliciting images affected motor output when a whole body movement was concerned: gait initiation. From a standing posture, 15 young adults were instructed to initiate gait (several steps) towards pleasant or unpleasant images corresponding to a congruent (CO) or an incongruent (IC) task. The movement was performed as soon as possible after the onset of images, using a paradigm of go/nogo. Because an emotional conflict had to be overcome in IC trials, we hypothesized that the longest reaction times and lowest amplitude of early postural adjustments were found during IC trials, as compared to CO trials. We also anticipated that conflict resolution would be improved if the previous trial was IC rather than CO, as previously reported in a typical press button task. Results demonstrated longer reaction times in IC than CO trials revealing that emotional conflict resolution interfered with cognitive resources involved in motor planning. Although the peak of forward velocity reached by the centre of body mass at the end of the first step and length of this step were similar between CO and IC trials, the amplitude of early postural modifications was reduced in IC trials. All together, these findings provided supporting evidence that a defensive response, possibly a freezing-like behaviour, was implemented during IC trials, but in a transient manner. Furthermore, conflict resolution was improved when the previous trial was IC rather than CO in terms of amplitude of early postural response, but not reaction time.

Effect of asymmetrical limb loading on early postural adjustments associated with gait initiation in young healthy adults.

The aim of this work was to show how postural adjustments are modified in healthy subjects when gait initiation is made more difficult by an increase of the supported body weight under the starting limb. We show that the impulse due to the push of the foot against the ground is enhanced by an increase of the amplitude and duration of the initial vertical force variation. The corresponding lateral excursion of the global center of pressure (CPg) is determined mainly by this force variation, but also by lateral displacements of the centers of pressure under each foot. Rehabilitation methods of patients with gait initiation difficulties may be improved by taking into account the importance of even loading, and of the transient initial push of the starting foot.

Early postural adjustments associated with gait initiation and age-related walking difficulties.

To better understand the functional role of early postural adjustments associated with gait initiation, we studied ground reaction forces before the first step in a group of 10 young and 18 elderly normal subjects as well as in a group of 11 patients with marche à petits pas, an age-related gait disorder in which difficulties in executing the first step are encountered. Kinematic data and ground reaction forces were recorded synchronously and processed by an ELITE motion analysis system. Start difficulties in the patients might be explained by impaired amplitudes and directions of anteroposterior forces as well as smaller values of vertical forces than in the controls walking with comparable step lengths. Indeed, the abnormalities of anteroposterior forces lead to defective propulsive forces, whereas the smaller upward reaction forces make the elevation of the starting leg more difficult. These perturbations are likely to reflect impairments of muscular synergies of the lower limbs manifested by disturbances in the agonist-antagonist coupling, and lack of coordination between both limbs.

Anticipatory postural adjustments in stance and grip.

The reactive forces and torques associated with moving a hand-held object between two points are potentially destabilising, both for the object's position in the hand and for body posture. Previous work has demonstrated that there are increases in grip force ahead of arm motion that contribute to object stability in the hand. Other studies have shown that early postural adjustments in the legs and trunk minimise the potential perturbing effects on body posture of rapid voluntary arm movement. This paper documents the concurrent evolution of grip force and postural adjustments in anticipation of dynamic and static loads. Subjects held a manipulandum in precision grasp between thumb and index finger and pulled or pushed either a dynamic or a fixed load horizontally towards or away from the body (the grasp axis was orthogonal to the line of the load force). A force plate measured ground reaction torques, and force transducers in the manipulandum measured the load (tangential) and grip (normal) forces acting on the thumb and finger. In all conditions, increases in grip force and ground reaction torque preceded any detectable rise in load force. Rates of change of grip force and ground reaction torque were correlated, even after partialling out a common dependence on load force rate. Moreover, grip force and ground reaction torque rates at the onset of load force were correlated. These results imply the operation of motor planning processes that include anticipation of the dynamic consequences of voluntary action.