The 4-6 HZ tremor during sustained contraction in normal human subjects.

Continuous elevation of the middle finger for 15-60 min gave rise to modulation of the e.m.g. at 4-6 Hz. A marked peak (50-150% of the amplitude of the coexisting 8-12 Hz peak) at 4-6 Hz was produced in sixteen out of twenty-one subjects. The 8-12 Hz peak was also enhanced (2-14 times with respect to its initial amplitude in eighteen subjects) during the course of the prolonged contraction but its frequency did not change. The 4-6 Hz and the 8-12 Hz peaks were present simultaneously; it is concluded that the two phenomena are separate entities. A step-function mechanical perturbation of the finger generates, time-locked to the stimulus, a train of waves at the frequency of the slow tremor, which can be abolished by local ischaemia. It is proposed that this slow tremor is due to an oscillatory process, possibly involving the reflex arc, but entailing a longer neuronal delay than that responsible for 8-12 Hz tremor.