10 Hz periodic component influences lower frequency component of the physiological tremor at low force levels.

A positive correlation has been reported between the amplitudes of the 10 Hz and lower frequency components of the physiological tremor (PT) at low force levels, though the generation mechanisms based on motor unit (MU) firing properties are different. This study aimed to investigate the causal relation between these fluctuations. A computer simulation was performed to alter the fluctuation intensity, which enabled manipulation of MU firing properties. Two types of MU contributions to synchronization activity were considered to influence the intensity of the 10 Hz PT: (1) number of MUs involved in synchronization and (2) synchrony between MUs. The impact of oscillatory excitatory input from the central nervous system on the generation of the 10 Hz PT was also evaluated. The results showed that the lower frequency fluctuation (LF fluctuation) was influenced by the number of MUs contributing to the 10 Hz PT amplitude. The synchrony between MUs and the oscillatory excitatory input had no influence on the LF fluctuation. In conclusion, MU synchronization in a certain frequency range increased the fluctuations not only at the synchronizing frequency but also at lower frequencies, and the number of MUs involved in synchronization was a plausible factor to explain the correlation between the 10 Hz and LF fluctuations.