Differential load impact upon arm tremor dynamics and coordinative strategy between postural holding and position tracking

This study contrasted the effects of load characteristics on coordinative strategies between postural holding and position tracking. Fifteen healthy adults conducted postural holding and sinusoidal tracking with the index finger while manipulated loads (0, 50, and 100 g) were superimposed on an outstretched arm. Oscillatory activities in the upper limb, target curve, and index position were monitored. The results showed that the effect of loading upon tremor structure was segment-dependent in relation to the task being performed. Load addition resulted in a task-dependent compensatory modulation of tremors between the distal segments (hand and finger), despite tremor augmentation in the proximal segments (forearm or arm). In comparison with postural holding, manual tracking exhibited a greater release of tremor coupling in the finger-hand complex, but a stronger tremor coupling in the forearm-arm complex secondary to added mass. Tremor reorganization in the upper limb following load addition could be characterized by changes in the primary principal component of segment tremors, which was more downward modulated during loaded tracking than loaded holding. Functionally, adding weight reduced tremor-relevant tracking error, while an opposing load did not add to holding steadiness. In summary, tremors associated with loaded holding and tracking were differently reorganized, in support of the hypothesis that coordination strategies against destabilizing loads for postural task and movement task are fundamentally dissimilar.