Mechanical resonance of the human body during voluntary oscillations about the ankle joint.

Viscoelastic models of the musculoskeletal system suggest resonant frequencies of oscillatory movement at which maximal output is realized with minimal energy expenditure. To investigate this, resonance at the human ankle joint was explored by comparing predicted and experimental gain/frequency and phase frequency functions. These functions were predicted from viscous (B) and elastic (K) coefficients of ankle extensors which were determined from the damped sinusoidal force produced after landing with muscles in sustained contraction (B = 3986 kg/s, K = 31,898 kg/s2). Experimental input/output functions were determined from Fourier analysis of force (output) and rectified, filtered EMG (input) obtained during voluntary sinusoidal oscillations of ground reaction force of a specified magnitude and frequency. Correlations between predicted and experimental functions were significant (p less than 0.02) in four of five subjects. The average resonant frequency was 3.33 +/- 0.15 Hz. The mechanical properties of muscle are considered to be as important as the nervous system in determining the choice of both the movement pattern and the strategy which is adopted in response to changing conditions.