Cyclic H-reflex modulation in resting forearm related to contractions of foot movers, not to foot movement

During rhythmic voluntary oscillations of the foot, the excitability of the H-reflex in the Flexor Carpi Radialis (FCR) muscle of the resting prone forearm increases during the foot plantar-flexion and decreases during dorsiflexion. It is known that, when the two extremities are moved together, isodirectional (in-phase) coupling is the preferred form of movement association. Thus the above pattern of the H-reflex excitability modulation may favor the preferred coupling between the two limbs. To gain some clues about its origin, FCR H-reflex excitability was tested before and after modifying the phase relations between the activation [electromyogram (EMG)] of foot movers and foot movement, either by loading of the foot or by changing the movement frequency. After foot loading, the movement cycle was consistently delayed with respect to the onset of the EMG in Soleus (Sol) or Tibialis Anterior (TA) muscles. Simultaneously, the FCR H-reflex modulation advanced by that same amount with respect to the foot movement, thus remaining phase-locked to the EMG onsets. Similarly, when movement frequency was varied step-wise between 1.0 and 2.0 Hz, the foot movement was progressively delayed with respect to both the EMG onset (Sol and TA) and the FCR H-reflex modulation, so that the phase relation between the motor command to the foot and the H-modulation in the forearm remained constant. These results suggest that modulation of H-reflex in the forearm is tied to leg muscle contraction, rather than to foot kinematics, and point to a central, rather than kinesthetic, origin for the modulation.