Postural constraints to coupling of limb movements

Ipsilateral hand and foot are easily coupled in isodirectional oscillations, while antidirectional movements require great effort and attention. It has been recently observed that, when sitting in a armchair and rhythmically oscillating one foot, corticospinal projections to the resting forearm muscles of the same side are subliminally modulated, in parallel with the overt activation of foot movers. We report here that in a less stable postural context and using the hand for postural support, those subliminal excitability changes are transformed into overt activities, featured as Anticipatory Postural Adjustments. Experimental subjects standing in up-right position and with their right hand touching a rigid support, performed fast flexion or extension movements with the right foot, fixed to a tilting platform. Synchronously with activation of the foot movers, EMG changes were recorded in wrist flexors or extensors of the supporting arm. Changes could be excitatory or inhibitory according to the hand position (prone or supine) and its contact with the support (palmar or dorsal): as a rule, those forearm muscles were excited that would act as to rotate the hand isodirectionally with the actual foot movement, while muscles acting "antidirectionally" were inhibited. When the foot was rhythmically oscillated, EMG in forearm muscles was modulated sinusoidally, the excitatory half-cycle being linked to activation of the "isodirectional" leg muscle. It is thus proposed that the neural mechanism favouring isodirectional and hindering antidirectional coupling of hand and foot is postural in nature.