Mechanical and electrical correlates of isometric muscle fatigue in skeletal muscle in the cat.

The amplitude and duration of motor unit action potentials, the rise time, peak tension, and half relaxation time of an isometric twitch, the force-velocity relationship, and tetanic tension were measured at the beginning (fresh muscle) and at the end of a fatiguing isometric contraction at a tension of either 40 or 70% of the initial strength in the soleus (a slow twitch muscle) and the medial gastrocnemius (a fast twitch muscle) of the cat. These same parameters were also measured at set intervals following these contractions to assess their rate of recovery to pre-exercise values. At The end of a fatiguing contraction at either tension examined, Vmx, twitch tension and tetanic tension, were all reduced while there was a prolonged twitch duration and duration of the motor unit action potential for both types of muscle. The height of the motor unit action potential was only marginally effected by muscle fatigue. Following the fatiguing contraction, the endurance required several hours to recover in the medial gastrocnemius muscle but recovered fully within 15 min after either tension in the soleus muscle. Tetanic tension and twitch tension both required less than 10 min for full recovery in the medial gastrocnemius muscle but recovered fully to the pre-exercise values within 3 min following fatiguing isometric contractions in the soleus muscles. Vmx, and the height and duration of the motor unit action potential both recovered within 1 min following the end of the exercise.