Stretch reflex oscillations and essential tremor.

Using a computer-controlled torque motor and manipulandum, 50 ms torque pulses and 70 second trains of binary pseudorandom torque disturbances were applied to the wrists of 10 adult controls and 22 patients with essential tremor in order to study the interaction between mechanically-induced stretch-reflex oscillations and essential tremor. These two oscillations were separated by applying inertial and spring loads to the wrist. There was no evidence of increased or unstable stretch-reflex activity in the essential tremor patients, and stretch-reflex latencies did not correlate with the frequency of essential tremor. Essential tremor and mechanically-induced stretch-reflex oscillations are separate phenomena capable of complex interaction.