Phasing of muscle activity during rapid finger oscillations

In rapid limb oscillations, bursts of muscle activity occur during certain phases of the movement. The authors examined phase stabilization in rapid oscillations of the index finger in a horizontal plane. Phase stabilization was rapid; therefore, a deviation from the mean phase was, on average, (almost) completely corrected in the subsequent movement cycle. Participants (N = 11) achieved rapid phase stabilization by using phase-dependent modulations of both the frequency of finger oscillation and the duration of interburst intervals. For example, with an early phase of a burst, oscillation frequency was increased immediately and the next burst of muscle activity was delayed. Whereas phase-stabilizing modulations of oscillation frequency are likely to be mechanically mediated, phase-stabilizing modulations of the timing of bursts of muscle activity must be neurally mediated. The present findings therefore strongly suggest that a central pattern generator does not generate rapid finger oscillations in an open-loop mode, but the pattern-generator network integrates, individually or in combination, afferent signals or predictive signals on the current or future states of the oscillating finger.