Figure 2

Synchrony of separate or isodirectionally coupled hand and foot oscillations with the external clock. Relationships between the three relevant phase delays (clk-mov, mech and neur) and the limbs oscillation frequency (0.4–3.0 Hz). A continuous line connects data from the uncoupled limbs, data from coupled movements are illustrated by symbols (hand, blue circles; foot, red triangles). A. In both limbs the clk-mov delay, measured between the clock beat and the flexion peak of the oscillations, remained constant over the whole frequency range (no significant effect of frequency). Negative clk-mov values when the oscillation peak followed the clock beat. B. The mech curves (same limb symbols as in A) trace the increasing lag of the hand and foot movements with respect to the EMG activation, caused by the segments mechanical impedance. Despite the increasing lag of mechanical origin, in either extremity the clk-mov synchrony was constant because of the progressive phase-advance of the motor activation with respect to the clock signal, (neur curves, open symbols in B). Note that the zero reference for the mech curves is the EMG onset while for the neur curves it is the clock beat. All curves obtained during isodirectionally coupled hand-foot oscillations virtually overlap the corresponding curves obtained during separate oscillations of the two segments (green lines). Vertical bars in A and B = Standard Error of the Mean (covered by the symbol in most instances).