The acceleration signal produced by physiological tremor from four different upper limb segments (the finger, hand, forearm and upper limb) was measured by an acceleration sensor during holding posture and was analyzed by power spectrum analysis. Two prominent peaks appeared in the power spectrum, suggesting that the tremor in the four different limb segments was composed of two frequency components. The frequency of one peak at 8-12 Hz did not change between the different limb segments, while the frequency of the other peak decreased with the increase in the mass of the limb segment. A model with two reflex pathways was developed for the tremor in the four limb segments. The model includes two reflex pathways, a spinal pathway and a supraspinal pathway. The theoretical values of the frequency and the amplitude of the tremor predicted by the model were in good agreement with the experimental results. Analysis of the model revealed that one of the two frequency components of the tremor was of spinal origin and was dependent upon the mass of the limb segment, and the second was of supraspinal origin, corresponding to the frequency at 8-12 Hz. In the normal subject, it is possible that the tremor could be used to evaluate the change in neuromuscular function produced by prolonged work involving just part of a limb segments (e.g., typing). It may also be used to evaluate the neuromuscular function of patients suffering from neurological diseases such as muscular dystrophy and Parkinson's disease.