Neural compensation for mechanical loading of the hand during coupled oscillations of the hand and foot

The role of kinaesthetic afferences in controlling coupling of voluntary oscillation of the hand and foot, both in-phase and anti-phase, was investigated by modifying the mechanical properties of one of the two segments (the hand) with applied inertial or elastic loads. Loads consisted of a lead disk, rotating coaxially with the wrist (total inertial momentum 15 g m2), or in two symmetrical rubber bands (elasticity, 4 g deg(-1)) connected 5 cm away from the wrist pivot. Experiments were performed on five male and five female subjects. Both the frequency responses of the hand and foot (i.e. the phase relations between the onset of muscular activation in limb extensors and the onset of the related movement) and the inter-limb phase relations (the phase differences between the hand and foot movement cycles and between the onsets of the electromyographic (EMG) activity in hand and foot extensors) were analysed. The hand frequency-response was fitted with a 2nd-order model, allowing us to describe the loaded and unloaded conditions through the changes in the model response. Inertial loading induced an immediate and steep decay in the frequency response, with a clear-cut reduction of the model resonance frequency, while elastic loading shifted the response to the right and upwards. Inter-limb phase relations were only partially affected by inertial loading of the hand. Despite the fact that the load strongly increased the difference between the frequency-responses of the hand and foot, when hand and foot were oscillated in-phase only about half of this difference remained as an increased phase-lag between hand and foot oscillations. The other half was offset by an advance of the contraction of the hand movers with respect to the foot movers. This compensation mechanism was more effective during anti-phase than during in-phase movements. Elastic loading improved inter-limb synchronisation, since it superimposed the hand frequency-response on that of the foot. In this condition, the requested synchronisation (in-phase or anti-phase) could be achieved by an almost simultaneous (or in strict phase opposition) contraction of the hand and foot movers. In conclusion, the main feedback reaction to the de-coupling effect of hand loading consisted in modifying the timing of activation of the muscles moving the extremities. An advance of hand movers on foot movers is already present in unloaded conditions to compensate for the difference in the natural mechanical properties of the two segments. This advance is enhanced when increasing the inertia of the hand system and attenuated when increasing its elasticity.