ABSTRACT

The human skeletal motor system functions normally to control posture and movement, 1,2 with control of both occurring during such activities as walking, running, and skating. The study of control of movements can be simplified somewhat by the recognition that many are repetitive or cyclic. Certain of these cyclic movements can be considered to be voluntary (i.e., intended, or "self-initiated and self-terminated").3 These include cyclic movements of the jaw during chewing or speech, cyclic leg motion during walking or running, cyclic finger and hand motion during writing or typing, and so on. In contrast, certain cyclic movements may be regarded as involuntary (e.g., the tremors of our body parts). Other repetitive movements, such as cyclic breathing, can be under either voluntary or involuntary control. Also, certain cyclic movements are difficult to classify as voluntary or involuntary (e.g., the vibrato of a violinist).4 While it appears that the violinist's vibrato is a voluntary oscillation, it nevertheless may occur at a frequency similar to that of the tremor of the same muscle-limb system. It is unclear whether the cat's cyclic paw shake5 at 10 to 13 Hz is an oscillation analogous to a component of human physiological tremor or is analogous to a voluntary oscillation resulting from activity of a central oscillatory system (central pattern generator). Can the system that produces the cat' s purr,6 an oscillation with a dominant frequency of 30 to 35 Hz, be considered as a model system for a voluntary or an involuntary oscillation? Tremor, which generally is regarded as an involuntary oscillation, occurs not only during maintenance of the posture of most body parts, but also during times of voluntary movement of those parts (see Fig. 4.1). However, the same kind of tremor may occur during times of complete muscle relaxation (i.e., with no neural control).7,8 The suggestion has been made that cyclic voluntary movements such as locomotion or handwriting may either result from, or be aided by, a muscle-load, mechanical-resonance (tremor) mechanism.9-14 Freund15 has suggested that a common peripheral mechanism, largely dependent on the muscle's twitch contraction

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time, determines the frequency of both physiological tremor and our fastest (highestfrequency) voluntary movements.