The frequency response model of fast and slow twitch muscles at various control strategies

The frequency response models of the soleus (slow) and m. gastrocnemius (fast) muscles subjected to four types of firing rate and recruitment control strategies applied by electrical stimulation were determined under isometric conditions. Orderly recruitment of motor units was applied concurrently with firing rate increase to elicit 50% and 100% of the initial force to simulate realistic physiological conditions in two of the four strategies. The third strategy recruited motor units according to their size to generate 100% of the force while the firing rate was set constant at its maximum value. The fourth strategy elicited muscle force by increasing the firing rate while all the motor units were constantly active. The method of least squares was used to test for the model with the best fit to the combined response (from 0.4 to 6 Hz) of all preparations of the same muscle. It was concluded that the frequency response model of skeletal muscles could be described by a linear, second-order system with double poles at 1.85 Hz and a pure time delay. The dependence of tendon viscoelastic stiffness on its length is noted as a factor which may modify the contraction velocity of a fast muscle.<>