Modulation Of Anticipatory Postural Adjustments Research Articles

Predictive posture stabilization before contact with moving objects: equivalence of smooth pursuit tracking and peripheral vision.

Postural stabilization is essential to effectively interact with our environment. Humans preemptively adjust their posture to counteract impending disturbances, such as those encountered during interactions with moving objects, a phenomenon known as anticipatory postural adjustments (APAs). APAs are thought to be influenced by predictive models that incorporate object motion via retinal motion and extraretinal signals. Building on our previous work that examined APAs in relation to the perceived momentum of moving objects, here we explored the impact of object motion within different visual field sectors on the human capacity to anticipate motion and prepare APAs for contact between virtual moving objects and the limb. Participants interacted with objects moving toward them under different gaze conditions. In one condition, participants fixated on either a central point (central fixation) or left-right of the moving object (peripheral fixation), whereas in another, they followed the moving object with smooth pursuit eye movements (SPEMs). We found that APAs had the smallest magnitude in the central fixation condition and that no notable differences in APAs were apparent between the SPEM and peripheral fixation conditions. This suggests that the visual system can accurately perceive motion of objects in peripheral vision for posture stabilization. Using Bayesian model averaging, we also evaluated the contribution of different gaze variables, such as eye velocity and gain (ratio of eye and object velocity) and showed that both eye velocity and gain signals were significant predictors of APAs. Taken together, our study underscores the roles of oculomotor signals in the modulation of APAs.NEW & NOTEWORTHY We show that the human visuomotor system can detect motion in peripheral vision and make anticipatory adjustments to posture before contact with moving objects, just as effectively as when the eye movement system tracks those objects with smooth pursuit eye movements. These findings pave the way for research into how age-induced changes in spatial vision, eye movements, and motion perception could affect the control of limb movements and postural stability during motion-mediated interactions with objects.

An increase in relative contribution of compensatory postural adjustments during voluntary movement while standing in adolescents and young adults with bilateral spastic cerebral palsy.

Previous studies have revealed several deficits in anticipatory postural adjustments (APAs) during voluntary movements while standing in individuals with bilateral spastic cerebral palsy (BSCP). However, it remains unclear whether compensatory postural adjustments (CPAs) during movement increase to compensate for APA deficits. We investigated the anticipatory and compensatory activities of postural muscles during voluntary movement while standing in adolescents and young adults with BSCP. The study included seven participants with BSCP with level II on the Gross Motor Function Classification System (GMFCS), seven with BSCP with level III on the GMFCS, and fourteen healthy controls. The participants stood on a force platform and lifted a load under two weight conditions (light and heavy). The electromyographic activities of postural muscles were analyzed at time intervals typical for APAs and CPAs. The percentage of muscle activity in the CPA time epoch against the total muscle activity during the APA and CPA time epochs was higher in the two BSCP groups than in the control group. In the control group, a load-related modulation was observed only in the APA time epoch, whereas in the BSCP-II group, the load-related increase was observed in both the APA and CPA time epochs. No load-related modulations were observed in the BSCP-III group. These findings suggest that adolescents and young adults with BSCP exhibit an increase in the relative contribution of CPAs during voluntary movement and that there exist severity-related differences in the modulation of APAs and CPAs.

Anticipatory postural adjustments are modulated by substantia nigra stimulation in people with Parkinson's disease and freezing of gait

BackgroundA precise understanding of the neuronal circuits involved in the control of anticipatory postural adjustments (APAs) for gait initiation is missing. Neurostimulation in Parkinson's disease (PD) provides a method of modulating APAs to gain insight into the underlying circuitry. ObjectiveOur objective was to investigate if APA kinematics for step initiation could be modulated by high frequency stimulation of the subthalamic nucleus (STN) or substantia nigra pars reticulata (SNr) in people with PD and freezing of gait (FoG). MethodsWe studied 14 people with PD and FoG using neurostimulation of the STN and SNr areas after overnight withdrawal of dopaminergic medication on the instrumented stand and walk test. We tested patients in the following randomized conditions: ‘off stimulation’, ‘STN’ stimulation (only), and ‘SNr’ stimulation (only). Patients were blinded to the stimulation condition. The APAs were recorded with inertial sensors and processed offline. Moreover, we assessed clinical scores with respect to motor symptoms, non-motor symptoms, executive function, and FoG. ResultsSNr but not STN stimulation modulated the anterio-posterior size of APA. The SNr modulation of APA was associated with the stimulation effect on FoG (trend; r = 0.580, P = 0.102). The APA modulation was not correlated with any other cognitive or clinical measures. ConclusionNeuromodulation of the SNr but not of the STN modulated APAs in PD patients with FoG. The different effects of STN or SNr on the kinematic parameters of APA support the concept of segregate targets in order to address diverse kinematic components of PD gait.

Modulation of anticipatory postural adjustments using a powered ankle orthosis in people with Parkinson’s disease and freezing of gait

BackgroundFreezing of gait (FOG) during gait initiation in people with Parkinson’s disease (PD) may be related to a diminished ability to generate anticipatory postural adjustments (APAs). Externally applied perturbations that mimic the desired motion of the body during an APA have been demonstrated to shorten and amplify APAs; however, no portable device has been tested. In this study, a portable powered ankle-foot orthosis (PPAFO) testbed was utilized to investigate the effect of mechanical assistance, provided at the ankle joint, on the APAs during gait initiation. Research questionDoes mechanical assistance provided at the ankle joint improve APAs during gait initiation in people with PD and FOG? MethodsThirteen participants with PD and FOG initiated gait across five test conditions: two self-initiated (uncued) conditions in walking shoes [Baseline-Shoes], and the PPAFO in unpowered passive mode [Baseline-PPAFOPassive]; three “go” cued conditions that included an acoustic tone with the PPAFO in unpowered passive mode [Acoustic + PPAFOPassive], the mechanical assistance from the PPAFO [PPAFOActive], and the acoustic tone paired with mechanical assistance [Acoustic + PPAFOActive]. A warning-cue preceded the imperative “go” cue for all the cued trials. Peak amplitudes and timings of the vertical ground reaction forces (GRFs) and center of pressure (COP) shifts from onset to toe-off were compared across conditions. ResultsMechanical assistance significantly increased the peak amplitudes of the GRFs and COP shifts, reduced APA variability, and decreased the time to toe-off relative to the passive conditions. SignificanceThese findings demonstrate the potential utility of mechanical assistance at the ankle joint (with or without an acoustic cue) as a method to generate more consistent, shortened, and amplified APAs in people with PD and FOG.

Neurophysiological and clinical effects of blindfolded balance training (BBT) in Parkinson's disease patients: a preliminary study.

Recent evidence supports the hypothesis that rehabilitative strategies based on sensorimotor stimulation in the neurorehabilitation of Parkinson's disease (PD) may be useful to improve gait in PD patients. We supposed that sensorimotor stimulation produces modulation of anticipatory postural adjustments (APAs) arising from the supplementary motor area (SMA). We aimed to investigate the clinical and neurophysiological effects of a blindfolded balance training (BBT). Randomized controlled trial. Italian hospital. Sixteen PD patients. The patients were randomized in two groups, one group treated with two-weeks BBT and one group treated with two-weeks of physical therapy (PT). We assessed gait parameters (swing, stance, double stance phase of cycle gait) and neurophysiological measurement (functional connectivity between SMA and motor area M1) before and after treatments. We found a decrease of stance and double stance phase and increase of swing phase respect to gait cycle, in BBT group compared to PT group, paralleled by a selective modulation in functional connectivity between M1 and SMA for BBT group. Our findings support that BBT represents a complementary rehabilitative strategy, based on visual deprivation and proprioceptive perturbation in recovery of gait in PD patients, in short time window, likely involving vestibular system and its connections with motor areas. The use of vestibular system stimulation, involving SMA-M1 circuits, may be useful to improve gait control in PD patients.

Modulation of Anticipatory Postural Adjustments in the Anticipation-Coincidence Task

ABSTRACT This study was designed to (a) verify whether the time available for movement preparation and execution modulates anticipatory postural adjustments/focal movement coordination and (b) determine to what extent the coordination in an anticipation-coincidence (AC) timing task is specific. Ten subjects performed an arm-raising movement from standing position in the reaction time, self-initiated (SI), and AC conditions. In the latter condition, subjects had to synchronize movement initiation or the end of the movement to the passage of a visual mobile on a target. In AC trials, time to contact (TC), which is the time before the mobile reached the target, was varied (720, 1,200, 3,000 ms). Electromyography, kinetic, and kinematics data were collected. Results showed that the coordination patterns were modified by TC, the velocity of the mobile, and the condition in which the movement was executed. It also showed that the behavior in the AC condition came closest to the 1 observed in SI condition when TC increased. These results support the existence of different control modes triggered by the temporal pressure.

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.

Modulation of Anticipatory Postural Adjustments in a Complex Task under Different Temporal Constraints

The aim of this experiment was to explore the behavioral effects of various temporal pressures on the anticipatory postural adjustments (APAs) in a complex task. Eighteen handball players performed a handball direct throw in three conditions of temporal pressure: (1) a reactive condition (RC), the throw was initiated as quickly as possible following a visual stimulus; (2) an anticipation-coincidence condition (AC), by synchronizing the impact of the ball with the passage of a visual mobile on a target; and (3) a self-initiated (SI) throw. The whole-body postural oscillation and the acceleration of the wrist were measured before and during the throwing action. Results showed that the delays between the onsets of the postural and focal activities were significantly different between RC and both the SI and the AC conditions. Movement time, time to peaks (negative and positive), are shorter in the RC, intermediate in the AC, and longer in the SI condition. Variability was significantly larger in AC in comparison with RC and SI. These results support the existence of different control modes triggered by the temporal pressure; they demonstrate that these control modes can be generalized to complex intentional movements such as the throwing skill and to an anticipation-coincidence situation.

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.

Environmental constraints on foot trajectory reveal the capacity for modulation of anticipatory postural adjustments during rapid triggered stepping reactions

This study used environmental restrictions on foot movement to challenge the capacity of the central nervous system (CNS) to counter the lateral instability that arises after foot-lift during rapid triggered stepping reactions evoked by unpredictable postural perturbation. The objective was to determine the extent to which lateral stability could be regulated via modulation of the mediolateral (m-l) anticipatory postural adjustment (APA) that precedes foot-lift. A high frontal obstacle was used to double the required swing duration, and thereby increase the potential for the center of mass (COM) to fall laterally toward the unsupported side, during forward-step reactions. The capacity to use lateral step placement to recover lateral stability was restricted by means of lateral barriers. Six healthy young adults were tested. In obstacle-only trials, the APA was insufficient to prevent increased lateral COM motion during the prolonged swing phase; hence, lateral step placement was necessitated. However, when lateral stepping was obstructed, the CNS was able to upregulate the APA amplitude so as to prevent this increase in lateral COM motion. The swing foot was placed medially, with no detriment to clearing the frontal obstacle or recovering equilibrium. There was no change in step timing or anteroposterior (a-p) COM motion. While previous studies have suggested that the a-p COM progression may determine the extent to which the m-l APA is expressed or truncated during triggered stepping reactions evoked by unpredictable perturbation, the present findings demonstrate that prior knowledge of environmental demands can lead to predictive efforts to modulate the APA during such reactions. An apparent preference to underscale anticipatory efforts when lateral step placement is permitted suggests that the CNS may be acting to avoid some potential risk or cost associated with the execution of a large APA.

Task-specific modulation of anticipatory postural adjustments in individuals with hemiparesis

Objectives: To study adaptation of anticipatory postural adjustments (APAs) in paretic and non-paretic muscles of individuals with hemiparesis to changes in the direction of the self-initiated perturbation and additional manual support.Methods: Electrical activity of leg and trunk muscles on both sides of the body and ground reaction forces were recorded in 10 patients with hemiparesis and a group of matched control subjects. Subjects released a standard load, held in the hand of the extended relatively unimpaired arm. The load was released either in front of the body or to the side, with or without the impaired arm touching an external stable surface.Results: APAs were reduced in individuals with hemiparesis, especially on the paretic side. In paretic muscles, the modulation of APAs with the direction of the perturbation was decreased or showed atypical patterns. Also the effects of touch were decreased in patients. Center of pressure displacements shortly after load release were similar in control subjects and patients.Conclusions: The results suggest that the ability of individuals with hemiparesis to prepare for a self-initiated predictable perturbation is reduced and that they may use alternative strategies of postural stabilization.

The roles of proximal and distal muscles in anticipatory postural adjustments under asymmetrical perturbations and during standing on rollerskates

Objectives: The study addresses two controversial issues surrounding the nature of anticipatory postural adjustments (APAs). The first deals with the reproducible APA patterns in proximal postural muscles and variable APA patterns reported for the muscles controlling the ankle joint (TA-SOL). We hypothesized that the TA-SOL muscles participate mainly in the compensation of lateral and rotational perturbations, in particular those associated with asymmetrical movements. The second issue deals with decreased APAs reported during both very stable and unstable standing. We hypothesized that APA changes during unstable standing might depend on the actual mechanical nature of instability. Methods: Eight healthy subjects were recruited who had had no prior experience with rollerskates. They performed series of bilateral and unilateral, flexion and extension movements during regular standing and bilateral movements during standing on rollerskates. EMG changes and shifts of the center of pressure were analyzed within a time window typical of APAs. Results: We found that APAs in proximal muscles were decreased during unilateral shoulder movements as compared to APAs during bilateral movements but did not show right–left differences. In contrast, the distal muscles (TA-SOL) showed little involvement during bilateral movements, while a clear right–left asymmetry was seen during unilateral movements. Bilateral movements performed while standing on rollerskates were accompanied by unchanged APAs in the proximal muscle pairs and increased APAs in the TA-SOL pair. Conclusions: We conclude that the proximal muscles provide a general pattern counteracting expected perturbations in the anterior-posterior direction while the distal muscles deal with asymmetrical perturbations and the modulation of APAs in unusual conditions such as standing on rollerskates. There seems to be no unambiguous relation between the magnitude of APAs and the stability of standing: Depending on the exact mechanical nature of postural instability, it could be associated with qualitatively different changes in the APAs.

Modulation of anticipatory postural adjustments in a reactive and a self-triggered mode in humans

Five subjects raised their preferred arm until horizontal in one of two experimental conditions: (1) in response to a visual signal (reactive condition), and (2) in a self-triggered condition. In both conditions, subjects were instructed to execute the movement at approximately 80% of their maximal velocity, such that all movements were nearly similar. EMG activity was recorded in a focal muscle (Anterior Deltoidus) and in three postural muscles (ipsi-lateral Gastrocnemius, contra-lateral Tensor Fascia Latae and ipsi lateral Semitendinosus) known to be activated prior to the focal muscle. Results showed greater anticipatory postural adjustments in the self-triggered than in the reactive condition for the three postural muscles. It is suggested that similar intentional movements can be triggered by different timing strategies.