Figure 1

Schematic representation of the relevant phase measurements. Measurements are illustrated at a limb oscillation frequency high enough to induce a phase delay between EMG and movement. In A and B, the upper traces describe, superimposed on each other, 1) the rectified EMG from extensors (ext EMG, black trace) and flexors (flex EMG, grey trace, reversed) recorded during a movement cycle and, 2) the sinusoidal input to extensors motoneurones (Mn input, MnI, red line) supposed to drive the limb oscillation. Mn th (open arrowhead) is the motoneurone threshold level. MnI to flexor motoneurones has the same course but is sign-reversed. Lower traces describe the ensuing movement (mov), horizontal dashed lines mark the limb passive equilibrium position (eq pos). Upward red arrowheads give timing of the clock signal. In A (oscillations all above the equilibrium) the motoneurone input (MnI) is supraliminal in extensor motoneurones (and subliminal in flexor) the whole cycle along. In B (oscillations crossing the equilibrium) the MnI is first excitatory in flexor (reversed, as the simultaneous flex EMG burst) and then in extensor motoneurones. The MnI lower peak (in A) and the MnI threshold crossing in either antagonist (in B) coincide (omitting the small, fixed conduction time) with the onset of the EMG burst in the related muscle. Three phase differences were measured: i) the clk-mov delay, between the clock and the oscillation peak; ii) the mech delay, between the corresponding points in the MnI (i.e., EMG onset) and movement traces (movement peak in A, equilibrium crossing in B); iii) the neur delay, between the clock and the peak of the motoneurone input. The latter results from the difference (clk-mov -mech): this is directly visible in A and put in evidence in B by the -mech arrow, connecting the peaks of the MnI and movement sine-waves.