Power Developed By Motor Units Of The Peroneus Tertius Muscle Of The Cat
The mechanical properties of motor units have been extensively studied under isometric conditions. Under dynamic conditions, the relationship between the force developed by single motor units and the muscle shortening velocity was determined for relatively high frequencies of activation. However, the interaction between the force-shortening velocity relation and the force-rate of activation relation was still unknown. We studied the power (which is the product of force and velocity) developed by single or groups of motor units during sinusoidal muscle stretches of 1-, 2-, 4-, 6-, and 8-Hz frequency. Motor units were stimulated with frequencies of 20, 40, 60, 80, 100, and 120 Hz during the shortening phase of the muscle stretch. The relationships, for different shortening velocities, between the power developed by single or groups of motor units and the frequency of stimulation were sigmoidal. However, these relations were not proportional to the shortening velocity. The relationships, for different frequencies of stimulation, between the power and the shortening velocity exhibited a maximum. The shortening velocity at which this maximum occurred increased with the frequency of stimulation. Slow motor units showed the lowest of those shortening velocities, whereas the fast fatigable motor units showed the highest. Groups of slow (or fast fatigue resistant) motor units had similar shortening velocities to those of single slow (or fast fatigue resistant) motor units. A mathematical function was fitted, using regression analysis, for all single and groups of motor units to the relationship among the power, the shortening velocity, and the frequency of activation. This function allowed examination, for different shortening velocity-frequency of activation combinations, of the relationship between the power developed by single and groups of motor units and the maximal isometric tetanic force they developed. These relationships were usually not monotonic but a monotonic relation could be obtained if slow, fast resistant, and fast fatigable motor units were activated at different frequencies. These results suggest that during a movement, the frequency of activation of motor units is mainly adjusted to the movement velocity and that the power developed by a muscle is mainly adjusted by the number of recruited motor units.