Effect of hand inertial loading on limb synchrony with the external clock. The plots illustrate data obtained during isodirectional (A-B) and antidirectional association (C-D), with the same limb symbols as in Figs. 2 and 3. On each plot, solid green lines connect data points obtained during uncoupled oscillations while symbols refer to coupled oscillations data. Hand loading increased the hand movement delay with respect to the clock (clk-mov curve, filled circles in A and C) in a frequency-dependent manner. This reflected the large increase of the hand mechanical impedance, which caused a steep decay of the mech curve (filled circles in B and D) that was not fully compensated by the advance of motor activation (neur curve, open circles in B and D). Foot curves were unchanged. Note that in either association direction, curves obtained during coupled movements fully overlap those obtained when the hand and foot were moved separately.