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.

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.

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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.

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