Neuromuscular control of physiological tremor during elastic load.
BACKGROUND
Physiological tremor is an involuntary and continuous oscillation in every limb segment of a healthy human. This study investigates the neuromuscular control of physiological tremor by an elastic load applied to the middle finger.


METHODS
The subject maintained a stretching position of the middle finger against various tension springs. Physiological tremor was detected by an acceleration sensor attached to the middle finger. The motor-unit activity was estimated by surface electromyogram (EMG) measured from the extensor digitorum communis muscle with physiological tremor. The tremor and the EMG signals were analyzed by power and coherence spectra.


RESULTS
Three frequency components appeared in physiological tremor while the subject extended the middle finger against the tension spring. They were classified as two load-independent components and a load-dependent component. Although the motor-unit activity was enhanced by the extension of the middle finger against a stiffer spring, the tremor amplitude did not significantly change. The extensor digitorum communis muscle produced a larger contraction force to maintain the finger posture against a stiffer spring, while the stiffness of the tension spring restrained the finger movement. The three frequency components of physiological tremor correlated with the motor-unit activity. These results supported the hypothesis of the origin of physiological tremor. The stretch-reflex system caused the load-dependent component, and the mechanical property of the elastic load determined its frequency. The supraspinal system produced the two load-independent components.


CONCLUSIONS
The neuromuscular control to maintain the stretching of the middle finger against the elastic load gave rise to the three frequency components in physiological tremor.
Takanokura
Masato
M
Department of Industrial Engineering and Management, Faculty of Engineering, Kanagawa University, Yokohama 221-8686, Japan. takanokura@ie.kanagawa-u.ac.jp
Sakamoto
Kazuyoshi
K
D000328:Adult; D000437:Alcoholism / Q000503:physiopathology* / Q000652:urine*; D004548:Elasticity; D006225:Hand / Q000294:innervation; D006801:Humans; D008297:Male; D009043:Motor Activity; D018482:Muscle, Skeletal / Q000294:innervation; D009469:Neuromuscular Junction / Q000502:physiology*; D011187:Posture; D014202:Tremor / Q000503:physiopathology*; D016474:Weight-Bearing