Proceedings of the international occupational hand-arm vibration conference, October 28-31, 1975, Cincinnati, Ohio. Wasserman DE, Taylor W, Curry MG, eds. Cincinnati, OH: U.S. Department of Health, Education, and Welfare, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, DHEW (NIOSH) Publication No. 77-170, 1977 Apr; :50-59
The effects of low frequency forced vibration (3-30 hertz) applied to the ankle joint are studied in six normal human subjects. The subjects sat in a chair with the right foot strapped to a foot-plate which permitted only dorsiflexion-plantarflexion about the ankle joint. Significant nonlinearities were seen in the angular rotation of the foot over 8 to 12 hertz with torque as the driving input. The effective compliance has sharp resonance near 6.5 hertz when tonic, voluntary muscle activity is present. A two-cycle-average response was obtained for a 10 second data record by taking successive intervals equal to twice the modulation period. Fourier transform of a 10 second data record showed generation of harmonics and subharmonics of the drive frequencies. For the angular rotation, a significant proportion of the power is in harmonic frequencies when the drive frequency is from 8 to 12 hertz. The following phenomena were seen in the data: slowly increasing amplitudes of oscillation (to a limit) at drive frequencies near resonance; self-sustaining oscillations of the ankle joint near the resonant frequency after the modulation signal to the motor is turned off, particularly in the fatigued limb; and distortion (from the sinusoidal) of angular rotation during which there are spontaneous recurrences of oscillation at the drive frequency.