Early and unintentional release of planned motor actions during motor cortical preparation.

Voluntary movements are often preceded by a movement-related potential beginning as much as two seconds prior to the onset of movement. In light of evidence that motor actions can be prepared and initiated in less than 200 ms, the function of this early activity has remained enigmatic. We hypothesized that the movement-related potential reflects the state of preparation of the planned movement. This was tested by delivering a startling acoustic stimulus during the preparation phase of a load-release task. The cue to release the load was presented either 3.5 seconds after a warning cue (PREDICT condition) or randomly between 4-12 seconds (REACT condition). Electroencephalographic, electromyographic and limb and load kinematic signals were recorded. In a subset of trials, a startle stimulus was delivered at -1500, -1000, -500, -250, -100 or 0 ms before the release cue. A contingent-negative variation (CNV) waveform, with a late phase of slow-rising negativity beginning an average of 1459 ms prior to movement, was observed for the PREDICT condition but not the REACT condition. For both conditions, the startle stimulus frequently evoked the early and unintentional release of the load-release sequence. The incidence of release was significantly (p<0.001) correlated with the late phase of the CNV for the PREDICT condition but not the REACT condition. For the REACT condition, the incidence of movement release was subject-specific, constant across the preparation interval, and uncorrelated with cortical activity. The onset of movement release by the startle stimulus was significantly shorter (p<0.001) for the PREDICT compared to the REACT condition. These findings provide evidence that the late phase of the CNV reflects cortical activity mediating the progressive preparation and storage of the forthcoming movement and that during this phase an intense sensory stimulus can evoke early and unintentional release of the planned action.

Anticipatory postural adjustments in children with hemiplegia and diplegia.

Anticipatory postural adjustments (APAs) play an important role in the performance of many activities requiring the maintenance of standing posture. However, little is known about if and how children with cerebral palsy (CP) generate APAs. Two groups of children with CP (hemiplegia and diplegia) and a group of children with typical motor development performed arm flexion and extension movements while standing on a force platform. Electromyographic activity of six trunk and leg muscles and displacement of center of pressure (COP) were recorded. Children with CP were able to generate anticipatory postural adjustments and produce directionally specific APAs and COP displacements similar to those described in adults and typically developing children. However, children with diplegia were unable to generate APAs of the same magnitude as children with typical development and hemiplegia and had higher baseline muscle activity prior to movement. In children with diplegia, COP was posteriorly displaced and peak acceleration was smaller during bilateral extension compared to children with hemiplegia. The outcomes of the study highlight the role of APAs in the control of posture of children with CP and point out the similarities and differences in anticipatory control in children with diplegia and hemiplegia. These differences may foster ideas for treatment strategies to enhance APAs in children with CP.

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.

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 effect of the amplitude of motor action on anticipatory postural adjustments.

The purpose of this study was to determine whether changes in the amplitude of a motor action triggering the same perturbation affect anticipatory postural adjustments (APAs). Healthy subjects performed releases of the same load with shoulder abduction movements of different amplitudes. Changes in the electrical activity of trunk and leg muscles, as well as displacements of the center of pressure were recorded. Generally, there were no differences in anticipatory activity of muscles and displacements of the center of pressure between series of load releases induced by motor actions of different amplitudes. We suggest that the CNS arranges APAs based on the magnitude of the perturbation if the same muscle groups generate motor actions of different amplitudes.

Anticipatory postural adjustments associated with rotational perturbations while standing on fixed and free-rotating supports.

OBJECTIVE: The role of anticipatory postural adjustments (APAs) has been commonly hypothesized to stabilize the body's center of mass (COM) from reaction forces and torques induced by voluntary movements in a feed-forward manner. This hypothesis was developed from studies which investigated movements that induced anterior-posterior or medial-lateral perturbation to body posture. However, the role of APAs in tasks that induce perturbations about the vertical axis, which are associated with minimal COM displacements, is unclear. The purpose of this study was to examine APAs associated with upper arm movements that induce perturbation about the body's vertical axis. METHODS: Eight healthy subjects performed bilateral or unilateral shoulder movements in the sagittal or frontal plane that induced rotational perturbation about the body's vertical axis, while standing on a support which was either fixed or free to rotate about the body's vertical axis. Changes in the background activity of trunk and leg muscles on both sides of the body, as well as reaction moment about the vertical axis were quantified within the time interval typical of APAs. RESULTS: On the fixed support, clear asymmetry between right and left muscle activity was observed in biceps femoris and soleus during APAs across all tasks. These asymmetries were specific to the movement direction. When the same tasks were performed on a free-rotating support, the asymmetry that was observed on fixed support decreased. CONCLUSIONS: We suggest that the CNS uses asymmetric activity of right and left muscles during APAs to rotate the body segments in the direction opposite to the perturbation. When ground reaction moments did not aid in counteracting the forthcoming perturbation while standing on a free-rotating support, the asymmetric muscle activity decreased to minimize further inter-segmental rotations.

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.

Could a motor action that has no direct relation to expected perturbation be associated with anticipatory postural adjustments in humans?

Anticipatory postural adjustments (APAs) in standing subjects were studied using different motor actions. We tested the hypothesis that a direct relation between the forthcoming perturbation and motor action is needed to trigger robust anticipatory postural adjustments. The same postural perturbation of a standard load release was induced by either a shoulder abduction, utilizing a motor action with a direct relation, or a biting or blowing action that had an indirect relation with the forthcoming perturbation. Electromyogram activity of trunk and leg muscles as well as the displacement of the center of pressure was recorded. Reduced APAs were observed in experimental trials using motor actions with an indirect relation to a perturbation as compared to the control series with perturbations induced by shoulder abduction. These results suggest that a direct relation between motor action and expected perturbation is important in the generation of anticipatory postural adjustments.