Postural adjustments in arm and leg muscles during coupled movements of the arms on the horizontal plane

Aim: To test the hypothesis that anticipatory postural adjustments (APAs) may concur in generating the directional preference experienced during coupled movements of the arms on the horizontal plane. Methods: We measured the electromyographic and mechanic postural actions elicited when performing fast unidirectional or cyclic movements of: i) one single arm and ii) both limbs, iso- or antidirectionally coupled. Results: During fast adduction of the right arm in the horizontal plane (prime mover, pectoralis Major, RPM) APAs were recorded in the contralateral LPM as well as in the right ischiocruralis (RIC) muscle. This last action was associated to a transient increase of Tz (torque around body vertical axis) in the direction opposite to arm rotation. Both the APAs in RIC and the Tz changes nearly doubled in size when the arms were coupled isodirectionally (adduction of one arm and abduction of the other) while they vanished when both arms were simultaneously adducted (antidirectional coupling). Conformably, during rhythmic arm oscillations APAs and Tz were cyclically modulated when movements were isodirectional, the modulation amplitude being strongly enhanced by increasing the movement frequency. When oscillations were antidirectional neither APAs nor Tz changes were observed, even if frequency was incremented. Conclusion: During antidirectional movements, APAs in prime movers are synergic with voluntary activation and no postural engagement is requested to leg muscles. Conversely, during isodirectional movements, APAs in prime movers conflict with the voluntary commands and a strong, frequency dependent, postural effort is required to leg muscles. These two factors may co-operate in determining the preference for antidirectional vs. isodirectional coupling.