Oscillation of the human ankle joint in response to applied sinusoidal torque on the foot

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Abstract
Low-frequency (3-30 Hz) oscillatory rotation of the ankle joint in plantarflexion-dorsiflexion was generated with a torque motor. Torque, rotation about the ankle and electromyograms EMG for the gastrocnemius-soleus and the anterior tibial muscles were recorded. Fourier coefficients at each drive frequency were used to calculate the effective compliance (ratio of rotation and torque). The compliance had a sharp resonance when tonic, voluntary muscle activity was present. The resonant frequency of compliance was between 3-8 Hz. The location of the resonant frequency and the magnitude of the compliance at resonance depended upon the degree of tonic muscle activity and the amplitude of the driving torque. The resonant frequency increased with increasing tonic activity. With tonic muscle activity, the compliance in the frequency range below resonance increased with increasing amplitudes of driving torque. The EMG when evoked by the rhythmic stretch, lagged the start of stretching by between 50-70 ms. When tonic muscle activity was present, the resonant frequency of the stretch reflex was between 5-6.5 Hz. Following the start of driven oscillation at frequencies near resonance, slowly increasing amplitudes of angular rotation (to a limit) were observed. Distortion (from the sinusoidal wave shape) of angular rotation was frequently observed with drive frequencies between 8-12 Hz during which there sometimes occurred spontaneous recurrences of oscillation at the drive frequency. For the angular rotation, a significant portion of the power may be in subharmonic frequency components of the drive frequency when that frequency is between 8-12 Hz. Self-sustaining oscillation (clonus) near the resonant frequency of the compliance was sometimes observed after the modulation signal to the motor was turned off. This was most often seen when the gastrocnemius soleus muscles were fatigued. Clonus may be evoked by driven oscillation at any frequency. The hypothesis that physiological tremor, which occurs between 8-12 Hz, is a consequence of stretch reflex servo properties seems to be at odds with the observations of resonance in the compliance and of self-generated clonus both occurring in the 5-8 Hz region.