Bilateral organization of physiological tremor in the upper limb.

The bilateral patterns of physiological tremor in the upper limb of adults were examined under conditions where eight combinations of the elbow, wrist and index-finger joints of the right arm were braced using individually molded splints. The hypotheses tested were that: (a) coordination of upper-limb tremor involves (compensatory) coupling of intra- but not inter-limb segments, (b) splinting the respective joints of the right arm changes the organization of this synergy in both limbs, and (c) reducing the involvement of joint-space degrees of freedom through restricting their motion (by splinting) results in increased tremor in the distal segments. Under no-splinting conditions, significant relationships were only observed between adjacent (intra-limb) effector units, with the strength of the correlation increasing from proximal to distal. Splinting the right limb resulted in an increase in the strength and number of significant intra-limb relationships in both limbs. No inter-limb tremor relationships were found between any segment during this task, irrespective of the splinting condition. The frequency profile for the tremor in each limb segment showed two prominent frequency peaks (at 2-4 Hz and 8-12 Hz). A third, higher frequency peak (18-22 Hz) was observed in the index fingers only. Splinting the right limb produced a general increase in the amplitude and variability of tremor in the fingertip of both arms. This effect was particularly strong under conditions where the more proximal joints were splinted. The lack of any between-limb relationships, coupled with the fact that splinting one limb influenced both limbs, suggests that some form of linkage does exist between the limbs. It is unlikely that mechanical linkages can explain fully these relationships. It is proposed that the tremor observed in either limb represents the output of a central oscillatory mechanism(s), but that this output is subsequently independently filtered in a parallel fashion on its way to each respective limb. A common bilateral (compensatory) strategy is employed to minimize the tremor in either limb during this multiple-degrees-of-freedom task.