Postural constraints to hand-foot coupled movements

When sitting on an armchair and rhythmically oscillating one foot, the excitability of flexor and extensor muscles of the resting ipsilateral forearm is subliminally modulated in a reciprocal way, increasing in each muscle when the ‘isodirectional’ foot mover is activated [1]. In a less stable postural context and using the hand for postural support, those forearm subliminal changes are transformed into overt activities, featured as Anticipatory Postural Adjustments (APAs) [2]. We report here evidence for the reverse correlation, i.e., that oscillations of one hand in the parasagittal plane are accompanied by overt activities in the foot flexors and extensors. Ten subjects standing with their right arm vertical, elbow flexed at 90° and forearm directed forward, either prone or supine, performed series of 15-20 impulsive flexions or extensions of the wrist. Movements rate was 0.2Hz. EMG from the prime movers Extensor and Flexor Carpi Radialis (ECR /FCR) as well as from ipsilateral Tibialis Anterior (TA) and Soleus (Sol) was recorded and averaged, triggering on the onset of the prime mover burst. When the wrist was upward rotated, TA EMG was activated synchronously with the bursts in ECR if the hand was prone and FCR if it was supine. Conversely, during downward wrist rotation, Sol EMG activation accompanied the bursts of FCR (hand prone) or ECR (hand supine). In both situations, those leg muscle were excited that would act as to rotate the foot isodirectionally with the actual hand movement. The hand-related activation of leg muscles was only visible when the background contraction, in either TA or Sol, was kept to a minimal level by properly shifting the centre of pressure. Activation, moreover, underwent progressive adaptation, usually disappearing after some dozen of movements. When the hand (either prone or supine) was oscillated rhythmically, EMG in TA and/or Sol was modulated sinusoidally, the excitatory half-cycle being linked to activation of the ‘isodirectional’ forearm muscle. Leg muscles activation, either impulsive or cyclic, was more consistently observed in TA (9/10 subjects) than in Sol (6/10). Moreover, in 8/10 subjects APAs in a given leg muscle were coupled to one or the other wrist mover burst depending on the prone or supine position of the hand. It is proposed that these activities, together with those observed in the forearm muscle during foot movements [2], may favour isodirectional and hinder antidirectional coupling of ipsilateral hand and foot. References 1. Borroni, P, Cerri, G, Baldissera, F (2004) Excitability changes in resting forearm muscles during voluntary foot movements depend on hand position: a neural substrate for hand-foot isodirectional coupling. Brain Res, 1022, 117-125 2. Baldissera, F, Esposti, R (2005) Postural constraints to coupling of ipsilateral hand-foot movements. Neuroreport, 16, 1615-1619