Differential control of in-phase and anti-phase coupling of rhythmic movements of ipsilateral hand and foot

Summary

Rhythmic flexion-extensions of ipsilateral hand and foot are easily performed (“easy” association) when the two segments are moved in phase (isodirectionally), whereas great care and attention are required (“difficult” association) to move them in phase opposition. We searched for features distinguishing the two types of coupling by analyzing, on ten subjects: 1) the frequency limit in each association; and, 2) if coupling is modified by inertial or elastic loading of the hand. 1) Subjects were asked to oscillate hand and foot at various paced frequencies, in the easy or in the difficult association for one minute at least. In the easy coupling, the task was performed up to 2.0–2.5 Hz, the duration being thereafter shortened by muscular fatigue. In the difficult coupling when the frequency was increased above 0.7–1.7 Hz, the performance rapidly shortened, not because of fatigue but because of an inevitable reversal to the in-phase movement. The frequency-duration curve always followed a similar decay, although it covered different frequency ranges in the various subjects. 2) The effect of charging the hand with inertial or elastic loads was studied at the subject's preferred frequency, chosen when the hand was unloaded. Without loading, in the easy association the hand cycle slightly lagged the foot cycle while in the difficult one an almost perfect phase opposition was maintained. Under inertial load (inertial momentum: 9 gm²), in the easy association the hand lag was increased by 10° to 45°, despite a compensatory advanced activation of the forearm EMG; in the difficult association, instead, the hand lag was small (less than 10°), thanks to an even earlier onset of the forearm EMG. The elastic load (torque: 4 gm) had negligible effects on the phase relation between movements but improved the phase relation between EMGs. These findings show that coupling is tighter in the difficult than in the easy association, a feature that is emphasized by the effect of the load. This supports the idea that kinaesthetic afferences have more pronounced influences on control of the anti-phase than the in-phase coupling.