Phase control of hand and foot coupled oscillations

We recently proposed that voluntary oscillation of a limb segment may be governed by a position feedback network that adapts muscular activation to the mechanical context so as to keep the segment oscillations in phase with the voluntary position command. Hypothetically, a position control of each limb should per se guarantee synchronous coupling of the hand and foot, despite their different mechanical properties. To test this hypothesis, ten subjects separately oscillated (0.4-3Hz) their right hand and foot, taking care of synchronising the peak flexion with a metronome beat. For each limb, the phase delay between the pacing signal and the oscillation peak (clk-mov delay) remained constant over the whole frequency range, suggesting that synchronism to the pacer was achieved through a control process tailored on the mechanical properties of each limb. When subjects coupled hand-foot oscillations, either in-phase or anti-phase, the clk-mov delays in the range 0.4-3Hz did not change significantly from those observed in the uncoupled limbs. The clk-mov delays remained similar in the coupled and uncoupled movements (no significant difference) even after enhancing the mechanical difference between the limbs by inertial loading of either extremity. This supports the view that a "private" position control of each limb may guarantee limb coupling, excluding the need for any feedback interaction. In this way, synchronous movements of two mechanically different extremities may be achieved with one single voluntary command.