Phase control of coupled limb oscillations

We recently proposed that voluntary oscillation of one limb segment may be governed by a position feedback network that adapts the muscular activation to the mechanical context. This model predicts that, in the range 0.4-3.0Hz, the limb oscillation keeps in phase with a sinusoidal voluntary central command. If so, when coupling hand-foot oscillations, synchronism between the two movements may result from a single voluntary command feeding in parallel the two control networks, without interaction between them. To explore this hypothesis, we measured the frequency-dependent phase delay of the limb movement with respect to i) a pacing signal (clk-mov delay) and ii) the motor output (mo-mov delay). Ten subjects oscillated (0.4-3.0Hz) their right prone hand and foot, both alone and coupled in-phase, taking care of synchronising the peak flexion with the metronome beat. Wrist and ankle angular position and EMG from wrist and ankle flexors and extensors were recorded. When the two limbs were oscillated alone, the mo-mov phase delays decayed from –8° at 0.4Hz to –125° at 3.0Hz in the hand and from –8° to –90° in the foot, revealing their different mechanical properties. Instead the clk-mov phase delay of both limbs remained almost constant (about 10°) at all frequencies. This suggests that the two limbs reached a comparable synchronism to the pacer through a compensation process tailored on their respective mechanical properties. Similar results were observed when hand and foot were coupled. The relative phase between hand and foot coupled oscillations, calculated by subtracting data of the foot clk-mov curve from those of the hand curve, was not significantly different from the one expected if no interaction occurred, i.e. calculated on data from oscillations of the isolated limbs. This supports the hypothesis that the same synchronisation mechanism that controls oscillations in each single limb is also able to guarantee limb coupling, excluding the need for any feedback interaction.