Feedforward postural stabilization in a distal bimanual unloading task.

The aim of the present study was to investigate postural adjustments and positional stability in a bimanual unloading task, involving essentially the index finger, in order to test whether proactive adjustments are also observed in distal body segments. A second goal of the study was to evaluate the concept of a central command that would be responsible for coupling movement and posture. The positional disturbance of the right load-bearing index finger of healthy human subjects was studied under two types of manipulations: passive, i.e., imposed, unloading and active unloading, by the subject's left index finger. It was found that, in such a distal task, positional stabilization of the load-bearing finger was much better (by a factor of 6) in the active situation than the passive situation. This improvement was greater than previously reported for a proximal task. An electromyogram (EMG) analysis of the mostly implicated dorsal interosseous muscles revealed a typical unloading reflex in the passive situation (reactive mode) and a suppression of EMG before unloading onset in the active situation (proactive mode). Averaged records showed an almost perfect synchronization between the EMG suppression in the load-bearing interosseous muscle and the onset of the EMG burst of the unloading index finger. A trial-by-trial analysis, however, revealed a considerable scatter in intervals of the two EMG events, with a tendency of the activity burst in the left finger to occur slightly before the suppression of EMG in the load-bearing muscle. No positive correlation was found between the precision of synchronization (intervals near zero time) and the accuracy of performance, i.e., positional stability of the unloaded finger. Although the trial-by-trial variability was large, it is suggested that at least some of this variability is caused by a nonsteady state of motoneuronal excitability. In view of the low-pass property of the muscle, the observed variability in synchronization may be sufficiently precise to maintain the hypothesis of a central temporal coupling of the events in the two hands through a common command. However, the lack of a correlation between the degree of synchronization and the performance in stability argues rather in favor of separate commands to the two hands that select the parameters in the spatial domain. Finally, an intermanual EMG or torque analysis is proposed that might be useful in assessing the accuracy in goal achievement, i.e., the maintenance of a stable finger position in spite of the "internal" perturbation.