Effect of foot inertial loading on limb synchrony with the external clock. Data obtained during isodirectional (A-B) and antidirectional association (C-D) are illustrated with the same symbols as is Figs. 2 and 3. Foot loading increased the foot movement delay with respect to the clock (clk-mov curve, filled triangles in A and C) in a frequency-dependent manner because of the large increase of the foot mechanical impedance (mech curves, filled triangles in B and D), which was not fully compensated by the advance of motor activation (neur curve, open triangles in B and D). Curves of the hand remained unchanged. Note that in either association direction, curves obtained during coupled movements (symbols) fully overlap those obtained when the hand and foot were moved separately (green lines).