Postural constraints to coupled movements of hand and foot

Aim: 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. In a less stable postural context (upright standing) and using the hand for postural support, those subliminal changes are transformed into overt activities, featured as Anticipatory Postural Adjustments (APAs). We report here evidence for the reverse correlation: oscillations of one hand in the parasagittal plane are accompanied by overt APAs in the foot flexors and extensors. Methods: Ten subjects standing with their right forearm directed forward (arm vertical, elbow at 90°), either prone or supine, performed series of 15-20 impulsive flexions or extensions of the wrist (movement rate 0.2Hz) as well as series of 30 cyclic flexions-extensions at 1Hz. 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. Results: During impulsive upward rotations of the wrist, 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. 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. Conclusion: These activities, together with those observed in the forearm muscle during foot movements, may favour isodirectional and hinder antidirectional coupling of ipsilateral hand and foot.