Early Anticipatory Postural Adjustments Research Articles

Individuals with stroke improve anticipatory postural adjustments after a single session of targeted exercises

Impairment of anticipatory postural adjustments (APAs) is an important source of postural instability in older adults and individuals with neurological disorders. The aim of the study was to investigate whether APAs could be improved in people with stroke as a result of targeted exercise involving their unaffected upper extremity. Nine individuals with stroke and five healthy control individuals participated in the laboratory tests before and after a single session of practice consisting of pushing a medicine ball attached to the ceiling and stopping the ball with their palm when it returns. The tests included self-initiated perturbations induced by fast, discrete shoulder flexion movements and external perturbations induced by a pendulum impact. Bilateral EMG activity of eight trunk and leg muscles was recorded and analyzed during the anticipatory phase of postural control. Significantly early APAs onsets (p < .05) were seen in trunk and leg muscles after a single session of practice as compared to pre-practice in both the groups and individuals with stroke improved their ability to generate APAs more than control group. While the improvement was more pronounced on the unaffected side of the body, enhanced APAs were recorded on the affected side as well. The observed practice-related earlier activations of muscles confirm a possibility of APA enhancement in individuals with stroke. The outcome provides a background for the development of balance rehabilitation protocols focused on improvement of anticipatory postural adjustments in individuals with neurological disorders.

The Effect of a Four-Week Balance Training Program on Anticipatory Postural Adjustments in Older Adults: A Pilot Feasibility Study

Balance deterioration in older adults limits their activities of daily living, community participation, and is a significant risk factor for falls. One contributory element to this functional decline is impairment of anticipatory postural adjustments (APAs). To evaluate the role and feasibility of a novel training program in improving APAs for balance control of older adults. Six older adults (73.3±5.0 years) were randomly assigned into the Experimental (EG) and Control groups (CG). The EG participated in four weeks of APA-focused training involving catching a medicine ball while standing. All subjects were exposed to predictable external and self-initiated perturbations before and after training. EMG activity of eight trunk and leg muscles was recorded bilaterally and muscle onsets were analyzed during the anticipatory phase of postural control. Clinical tests of balance (Timed-Up and Go, Single limb stance, and Activities-specific Balance Confidence scale) were implemented. Early onsets of APA activity prior to the external perturbations were seen in the EG following training. Moreover, early APA activity was observed in the EG prior to bilateral arm flexion, a task that was not a part of training, indicating a transfer of the learning effect of training. The improvement of APAs in the EG also resulted in improved performance on clinical outcome measures. There were no improvements seen in the CG for both the APA activity and the clinical balance tests. The study outcome suggests that a four-week APA-based training program is feasible and could be effective in improving postural control, functional balance, mobility, and quality of life in older adults.

Higher Precision in Pointing Movements of the Preferred vs. Non-Preferred Hand Is Associated with an Earlier Occurrence of Anticipatory Postural Adjustments.

It is a common experience to exhibit a greater dexterity when performing a pointing movement with the preferred limb (PREF) vs. the non-preferred (NON-PREF) one. Here we provide evidence that the higher precision in pointing movements of the PREF vs. NON-PREF hand is associated with an earlier occurrence of the anticipatory postural adjustments (APAs). In this aim, we compared the APAs which stabilize the left or the right arm when performing a pen-pointing movement (prime mover flexor carpi radialis (FCR)). Moreover, we analyzed the elbow and wrist kinematics as well as the precision of the pointing movement. The mean kinematics of wrist movement and its latency, with respect to prime mover recruitment, were similar in the two sides, while APAs in triceps brachii (TB), biceps brachii (BB) and anterior deltoid (AD) were more anticipated when movements were performed with the PREF than with the NON-PREF hand (60–70 vs. 20–30 ms). APAs amplitudes were comparable in the muscles of the two sides. Earlier APAs in the preferred limb were associated with a better fixation of the elbow, which showed a lower excursion, and with a less scattered pointing error (PREF: 10.1 ± 0.8 mm; NON-PREF: 16.3 ± 1.7). Present results suggest that, by securing the more proximal joints dynamics, an appropriate timing of the intra-limb APAs is necessary for refining the voluntary movement precision, which is known to be scarce on the NON-PREF side.

Effects of anticipation certainty on preparatory brain activity and anticipatory postural adjustments associated with voluntary unilateral arm movement while standing

We examined the effects of anticipation certainty concerning which voluntary movement is required in response to a stimulus while standing on preparatory brain activity and anticipatory postural adjustments (APAs). Ten right-handed adults abducted their left or right arm rapidly in response to a visual imperative stimulus, based on the type of stimulus. A warning cue, which did or did not contain information about the side of arm abduction, was presented 2000ms before the imperative stimulus. Preparatory brain activity before arm abduction was quantified by the mean amplitude of the contingent negative variation 100ms before the imperative stimulus (late CNV amplitude). Compared with the low anticipation condition, in the high anticipation condition the following results were obtained only in the case of right arm abduction: (1) larger late CNV amplitude, (2) earlier postural muscle activities with respect to the focal muscle of arm abduction, and (3) smaller peak displacement of center of pressure during the abduction. These findings suggest that high anticipation of voluntary movement of dominant arm to a stimulus while standing influences preparatory brain activity before the movement, resulting in earlier APAs and thus smaller disturbance of postural equilibrium during the movement.

Fatigue-induced early onset of anticipatory postural adjustments in non-fatigued muscles: support for a centrally mediated adaptation

Muscle fatigue has been shown to result in early onset of anticipatory postural adjustments (APAs) relative to those produced in a non-fatigued state. This adaptation is thought to reflect an attempt to preserve postural stability during a focal movement performed in a fatigued state. It remains unclear, however, whether this adaptation is of central (e.g., central nervous system motor command) or peripheral (e.g., muscle contractile properties), origin. One way to confirm that this adaptation is centrally driven is to identify fatigued-induced early APA onsets in non-fatigued muscles. In this study, APAs were obtained using a rapid bilateral reaching maneuver and recorded via surface electromyography before and after conditions of rest (n = 25) or fatigue (n = 25). Fatigue was generated using isokinetic exercise of the right leg. Results showed that fatigue-induced early APA onsets occurred in fatigued and non-fatigued muscles, confirming that fatigue-induced early APA onset is a centrally mediated adaptation.

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.

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.

Fatigue-induced adaptive changes of anticipatory postural adjustments

To examine the fatigue-induced adaptive changes (e.g., timing) of anticipatory postural adjustments (APAs), APAs of 30 research participants were recorded before (baseline) and after (post-test) conditions of either rest (control group, n=15) or fatigue (fatigue group, n=15). Muscle fatigue was generated using a dead-lift exercise performed to exhaustion. Self-initiated postural perturbations were induced using a rapid unilateral arm-raising maneuver (focal movement), and APAs were obtained using electromyography (EMG) recorded bilaterally in the lumbar and thoracic paraspinal muscles as well as the hamstring muscles. Postural stability during the focal movement was assessed using a force plate. Results showed that fatigue had no effect on postural stability during the focal movement, and yet caused earlier APA onsets in three of the six muscles evaluated. In spite of early APA activation, the APA EMG integrals of two of the three postural control muscles which exhibited fatigue-induced early APA onsets (T9 and L4 contralateral paraspinals) did not differ between baseline and post-test measures. The findings suggest that early APA onset may enhance postural stability by permitting a longer duration APA which can counteract fatigue-induced decreases in the force-producing capability of muscles that contribute to postural stability.