Frequency response model of skeletal muscle: effect of perturbation level, and control strategy.

The frequency response model of the soleus muscle of the cat was determined as a function of various firing rate and recruitment control strategies and at various force oscillation levels. We found that the basic frequency response models of the muscle during individual force oscillations at various control strategies in which the motor unit population of the muscle was fully recruited to obtain 50 per cent and up to 100 per cent of the maximum force, either concurrently with firing rate increase or at constant firing rate, were nearly identical. The model consisted of a second-order, linear low-pass filter with double poles at 1.85 Hz and a pure time delay of 16 ms. The model resulting from only firing rate increase from the frequency of fusion of the smallest motor unit to the maximum tetanic rate of the muscle while all the motor units were continuously active was nonlinear, and depended on the force's oscillation level. This nonlinear response of the rate coding process is also identified as the source of the increased harmonic distortion in the model where the initial 50 per cent of the force was generated by fully recruiting all the motor units, and the final 50 per cent of the force was generated by firing rate increase. We concluded that the basic frequency response model of a muscle under conditions similar to voluntary contraction consists of a linear, second-order system which is robust and independent of control strategy and force perturbation level.