Postural constraints to coupling of ipsilateral hand-foot movements.

Ipsilateral hand and foot are easily coupled in isodirectional oscillations, while antidirectional coupling is difficult or even impossible. It was recently suggested that differences between the two types of coupling depend on the interaction of postural mechanisms with voluntary movement. We report here that when standing in an upright position, with the right hand touching a rigid support and the right foot fixed to a tilting platform, fast foot flexions or extensions as well as rhythmical foot oscillations are accompanied by overt electromyogram activities in forearm flexors and extensors. These activities, described here as anticipatory postural adjustments, are distributed to forearm muscles so as to favour isodirectional and hinder antidirectional hand-foot coupling, both when the hand is prone and when it is supine.