Physiological action tremor of the ankle.

Comparisons were made between physiological tremor (PT), which is the involuntary oscillation produced when the leg is held in a plantarflexed position, and physiological action tremor (PAT), which is an involuntary oscillation associated with the voluntary slow raising and lowering of the heel. Frequency analysis using spectral techniques revealed a major frequency range of 6-9 Hz for PT and a major frequency range of 4-8 Hz for PAT. Computed root-mean-square (rms) acceleration values for each tremor type showed that the amplitude of PAT was significantly higher (2-5 times) than that of PT. This significant increase in amplitude occurred at specific stages of the voluntary movement. Major peaks in the spectral plots of the demodulated electromyogram signals of the soleus and tibialis anterior muscles corresponded to the frequency peaks seen with PAT.