The functional motility of motor units of the gastrocnemius and soleus muscles in the rat under indirect stimulation depending on the stimulus frequency]

With increasing pulse rate (up to 150 square pulses/sec) applied for 1 second at the gastrocnemius and soleus muscle the amplitude of electromyographic response was found to decrease. An amplitude decrease of sum action potentials (AP) recorded from the muscle surface occurred already at pulse rates below 100 pulses/sec. For the gastrocnemius muscle, this amplitude fall in the medium pulse-rate region can be described by the relation: amplitude = -log-pulse rate. At the soleus muscle, the amplitude fall is shifted towards the higher frequencies. After intensive swimming of the animal the amplitude depression of AP at the gastrocnemius muscle occurs already at a lower rate, and departs from the aforementioned relation. The causes are looked for in the differential properties of the various muscle fibres.