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 bodyʹs 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 bodyʹs vertical axis. Methods: Eight healthy subjects performed bilateral or unilateral shoulder movements in the sagittal or frontal plane that induced rotational perturbation about the bodyʹs vertical axis, while standing on a support which was either fixed or free to rotate about the bodyʹs 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.