Neeta Kanekar

The role of anticipatory postural adjustments in compensatory control of posture: 1. Electromyographic analysis

Anticipatory (APAs) and compensatory (CPAs) postural adjustments are the two principal mechanisms that the central nervous system uses to maintain equilibrium while standing. We studied the role of APAs in compensatory postural adjustments. Eight subjects were exposed to external predictable and unpredictable perturbations induced at the shoulder level, while standing with eyes open and closed. Electrical activity of leg and trunk muscles was recorded and analyzed during four epochs representing the time duration typical for anticipatory and compensatory postural control. No anticipatory activity of the trunk and leg muscles was seen in the case of unpredictable perturbations; instead, significant compensatory activation of muscles was observed. When the perturbations were predictable, strong anticipatory activation was seen in all the muscles: such APAs were associated with significantly smaller compensatory activity of muscles and COP displacements after the perturbations. The outcome of the study highlights the importance of APAs in control of posture and points out the existence of a relationship between the anticipatory and the compensatory components of postural control. It also suggests a possibility to enhance balance control by improving the APAs responses during external perturbations.

The role of anticipatory postural adjustments in compensatory control of posture: 2. Biomechanical analysis.

The central nervous system (CNS) utilizes anticipatory (APAs) and compensatory (CPAs) postural adjustments to maintain equilibrium while standing. It is known that these postural adjustments involve displacements of the center of mass (COM) and center of pressure (COP). The purpose of the study was to investigate the relationship between APAs and CPAs from a kinetic and kinematic perspective. Eight subjects were exposed to external predictable and unpredictable perturbations induced at the shoulder level while standing. Kinematic and kinetic data were recorded and analyzed during the time duration typical for anticipatory and compensatory postural adjustments. When the perturbations were unpredictable, the COM and COP displacements were larger compared to predictable conditions with APAs. Thus, the peak of COM displacement, after the pendulum impact, in the posterior direction reached 28+/-9.6mm in the unpredictable conditions with no APAs whereas it was 1.6 times smaller, reaching 17+/-5.5mm during predictable perturbations. Similarly, after the impact, the peak of COP displacement in the posterior direction was 60+/-14 mm for unpredictable conditions and 28+/-3.6mm for predictable conditions. Finally, the times of the peak COM and COP displacements were similar in the predictable and unpredictable conditions. This outcome provides additional knowledge about how body balance is controlled in presence and in absence of information about the forthcoming perturbation. Moreover, it suggests that control of posture could be enhanced by better utilization of APAs and such an approach could be considered as a valuable modality in the rehabilitation of individuals with balance impairment.

Anticipatory postural control following fatigue of postural and focal muscles

OBJECTIVE To investigate the effect of fatigue of postural and focal muscles on anticipatory postural adjustments (APAs). METHODS Nine healthy adults performed rapid bilateral arm raising movements before and after isometric hamstring (postural) and deltoid (focal/prime mover) muscle fatigue. Muscle force and peak acceleration of the arm movements were recorded to assess the presence of fatigue. Ground reaction forces, EMG activity of trunk and leg muscles and center of pressure (COP) displacements were recorded and quantified within the time intervals typical of APAs. RESULTS Early APA onset was seen in erector spinae and semitendinosus muscles post-deltoid fatigue. Anticipatory EMG integrals were reduced in the semitendinosus muscle post-hamstring fatigue, and were increased in the gastrocnemius muscle post-deltoid fatigue. No changes in COP displacement were observed following fatigue of both muscle groups. CONCLUSION A common pattern of APA adaptations seen following fatigue of either muscle groups along with no changes observed in COP displacements emphasizes the efficiency of the CNS in maintaining dynamic postural stability in the presence of fatigue. SIGNIFICANCE The outcome of the study is important for better understanding of the effect of muscular fatigue on feedforward mechanisms of postural control with possible implications for the elderly and individuals with neurological disorders.

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.

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.

Aging and balance control in response to external perturbations: role of anticipatory and compensatory postural mechanisms.

The ability to maintain balance deteriorates with increasing age. Anticipatory and compensatory postural adjustments (APAs and CPAs, respectively), both, are known to be affected in the elderly. We examined the effect of aging on the ability of older adults to utilize APAs and its effect on subsequent control of posture (CPAs). Ten elderly individuals were exposed to external predictable and unpredictable perturbations applied to the upper body in the sagittal plane. Body kinematics, electromyographic activity of 13 muscles, and ground reaction forces were analyzed during the anticipatory and compensatory phases of postural control. The elderly were capable of recognizing an upcoming predictable perturbation and activated muscles prior to it. However, the older adults used different muscle strategies and sequence of muscle recruitment than that reported in young adults. Additionally, when the perturbations were unpredictable, no APAs were seen which resulted in large CPAs and greater peak displacements of the center of pressure (COP) and center of mass (COM) following perturbations. As opposed to this, when the perturbations were predictable, APAs were seen in older adults resulting in significantly smaller CPAs. The presence and utilization of APAs in older adults also improved postural stability following the perturbation as seen by significantly smaller COP and COM peak displacements. Using APAs in older adults significantly reduces the need for large CPAs, resulting in greater postural stability following a perturbation. The results provide a foundation for investigating the role of training in improving the interplay between anticipatory and compensatory postural control in older adults.

Feedforward postural control in individuals with multiple sclerosis during load release

The aim of the present study was to investigate the organization of anticipatory postural adjustments (APAs) in individuals with multiple sclerosis (MS) during self-initiated perturbation in the sagittal plane. Eleven individuals with MS and eleven age-and-gender matched healthy controls were asked to hold a 2.27 kg load in the extended arms and release it using fast arm abduction movements. Electrical activity of six leg and trunk muscles as well as displacements of the center of pressure (COP) were recorded. The results indicate that individuals with MS (1) demonstrate a reduced magnitude of APAs, (2) delayed latency of APAs and (3) smaller anticipatory COP displacement as compared to healthy control subjects. Moreover, in spite of individuals with MS being mildly affected, their balance capacity was significantly diminished. Thus, the outcome of this study demonstrates the underlying impairment in anticipatory postural control of individuals with MS and provides a background for development of rehabilitation strategies focused on balance restoration in this population.

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.

Enhancement of anticipatory postural adjustments in older adults as a result of a single session of ball throwing exercise

The aim of the study was to investigate the role of short-term training in improvement of anticipatory postural adjustments (APAs) and its effect on subsequent control of posture in older adults. Nine healthy older adults were exposed to self-initiated and predictable external perturbations before and after a single training session consisting of throwing a medicine ball. EMG activity of eight trunk and leg muscles and ground reaction forces were recorded before and immediately after the training session. Muscle onsets and center of pressure displacements were analyzed during the anticipatory and compensatory phases of postural control. The training involving throwing of a medicine ball resulted in enhancement of the generation of APAs seen as significantly early onsets of leg and trunk muscle activity prior to the bilateral arm flexion task. Significantly early activation of postural muscles observed prior to the predictable external perturbation, the task that was not a part of training, indicates the transfer of the effect of the single training session. The observed training-related improvements of APAs suggest that APA-focused rehabilitation could be effective in improving postural control, functional balance, mobility, and quality of life in the elderly.

Direction-specific impairments of limits of stability in individuals with multiple sclerosis

OBJECTIVES Impaired postural control in individuals with multiple sclerosis (MS) is associated with falls. The objective was to evaluate the direction-specific limits of stability in people with MS. METHODS Balance control of 18 individuals with relapsing-remitting MS and 18 healthy controls was assessed using instrumented (Limits of Stability (LOS) test) and clinical (Berg Balance Scale (BBS) and Activities-specific Balance Confidence (ABC) scale) tests. RESULTS There were significant differences in reaction time, movement velocity, endpoint excursion, maximum excursion, and directional control measures of the LOS test between individuals with MS and healthy controls. The BBS and ABC clinical balance measures were significantly lower in individuals with MS compared to control subjects. The directional control impairment was seen in the right side and backward diagonals (backward-right and backward-left) directions. A significant difference between the fallers and non-fallers was found on all the components of the LOS test. There was a significant correlation between the BBS and ABC scores and different components of the LOS test. CONCLUSIONS Direction-specific impairment of limits of stability components was observed in individuals with MS. This information could be used in balance rehabilitation of people with MS.