The reorganization of tremulous movements in the upper limb due to finger tracking maneuvers

In light of the interplay between limb oscillatory outputs and the outcome performance of movement effectors, this study was undertaken to investigate neuromotor control in the upper limb during position tracking and posture holding. Sixteen volunteers conducted a postural pointing task and two index tracking maneuvers at 0.3 and 0.6 Hz with an outstretched arm. Limb acceleration in the index finger, hand, forearm, arm, and C7 spinal process were monitored to correlate functionally with the accuracy of index rhythmic displacements. The results showed that index oscillatory activity multiplied with tracking speed, but hand oscillatory activity declined during tracking movement. The tracking maneuvers also altered spectral distribution of the tremulous activities in the context of a lower spectral peak in the range of 8-12 Hz and suppression of spectral peaks at 2-4 Hz, in reference to that already presented in posture tremor. Consisting of three local maxima around 2-4, 8-12, and 18-22 Hz, the coherence of tremulous activity between the finger and hand during position tracking was nearly identical, but inferior to high coherence below 12 Hz during posture holding. Functionally, better tracking performance was associated with a smaller tremulous activity in the finger and hand, entailing sophisticated release of mechanical couplings in the finger-hand and hand-forearm. In conclusion, inversely related to tracking performance, limb tremulous movements were task-dependently organized, and speed-invariant coherence of limb tremulous movement specified an inter-segmental coordination, which is physiological evidence of the generalized motor program for position tracking.