Cross-Limb Dynamics of Postural Tremor due to Limb Loading to Fatigue: Neural Overflow but not Coupling.

Abstract

Many experiments have shown independence of the index finger dynamics under bilateral postural tremor protocols. Here we investigated in young adults the dynamics of bilateral multi-directional postural tremor and forearm muscle activity under the progressively fatiguing conditions supporting an external weight to the point of induced postural failure. When no loads were applied, tremor in the vertical (VT) and medio-lateral (ML) directions was similar with prominent peaks within 2-4Hz and 8-12Hz bandwidths. Contrastingly tremor in the anterior-posterior (AP) direction was characterized by a single peak between 0-2Hz. Although no tremor coupling occurred cross-limbs, strong within-limb coupling was found between ML and VT directions when no loads were applied (coherence range:0.77-0.85), implying that these oscillations are related and likely derived from mechanical sources. Applying an external load to the index finger(s) led to significant increases in the amplitude of VT tremor and EMG activity within that limb, but also caused increases in tremor directions not aligned with the gravitational vector (AP and ML). Significant increases in VT and ML tremor and EMG activity in the contralateral (unloaded) limb were also found when a single index finger was loaded, however, this bilateral increase did not align with increases in inter-limb coupling (coherence<0.21). The effects of fatigue caused by prolonged loading were widespread, affecting tremor and muscle activity in both limbs through a combination of neural and mechanical mechanisms. The single- and dual-limb loading to fatigue increased neural overflow but not tremor coupling between the index fingers.