Global effect on multi-segment physiological tremors due to localized fatiguing contraction

Physiological tremors within a limb are coupled, whereas between-limb tremors are thought to oscillate independently for a healthy subject. This study was undertaken to reinvestigate bilateral tremor relations and intralimb tremor dynamics for a two-limb task after exhausting a single joint. Fifteen volunteers conducted prolonged tracking with the left (target) shoulder. Neuromuscular functions were monitored before and after the exercise-fatiguing intervention, including tracking displacement, muscle activity of the deltoid, and physiological tremors in the bilateral upper limbs. Localized fatiguing contraction degraded tracking accuracy and movement smoothness, accompanied by an increase in deltoid activation. Segment tremors in the bilateral limbs and inter-limb tremor coherences in 8-12 Hz increased, though coherence peaks in 5-8 Hz waned with fatigue response. Intra-limb tremor relations in the target and non-target limbs were also reorganized with unilateral fatiguing contraction. Tremor coupling in the arm-C7 complexes was enhanced, associated with tremor uncoupling in the forearm-arm and hand-forearm complexes. Tracking error in the pre-fatigue and post-fatigue conditions was predicted by different principal components that had high communalities with tremors of distal and proximal segments of the target limb, respectively. The adaptive changes in tremor dynamics were attributable to fatigue-induced enhancement of common central drive and decline in neural inputs of long-looped reflexes that diverge to contralateral segments.