Background: This study investigates coordination of the upper-limb segments with various external loads by frequency-domain analysis of physiological tremor during the maintenance of limb posture. Physiological tremor is an involuntary oscillation in every segment of a healthy human.
Material/Methods: A subject raised his right upper limb forward while extending his hand and fingers. Physiological tremor was measured by acceleration sensors attached to four segments: the index finger, hand, forearm, and upper arm. A balloon filled with helium gas was attached to the forearm as a minus-load condition. A weight band was attached as a plus-load condition. The measured signals were evaluated by frequency-domain analysis: power spectrum and coherence spectrum.
Results: The amplitude of upper-limb tremor measured from the four segments decreased with the minus-load and increased with the plus-load. However, the degree of the variation depended on the segment. The amplitude of upper-limb tremor measured from the forearm and the hand decreased remarkably with the minus-load, while the amplitude from the upper arm increased with the plus-load. Although adjacent segments were well coordinated, coordination between the segments varied depending on the external load. The minus-load at the forearm led to a lack of coordination between the upper arm and the forearm. To compensate for this, the movements of the forearm and the hand became coordinated.
Conclusions: The experimental protocol of this study allowed implementing a method to estimate the physiological modification of the neuromuscular system under a hypo-gravitational environment.

Keywords: Upper Extremity - physiology, Weight-Bearing - physiology