Fatigue-related changes in electomyogram activity of the cat gastrocnemius during frequency-modulated efferent...

Changes in the compound muscle action potentials of cat gastrocnemius muscle were studied during low- and high-frequency fatigue. Fatiguing session consisted of 25-28 repetitions of the standard single fatigue tests (1.5min interval between the tests) that included the part of continuous frequency-modulated stimulation preceded and followed by single stimuli evoking twitch contractions in the muscle. The rate of the continuous part was changed in accordance with symmetrical double-trapezoidal signal, including three successive phases of constant rate at 10, 40 and 10s(-1); between these phases of 4s duration the rate changed linearly within a 2s interval. During fatigue relative changes in compound muscle action potential waves were usually smaller than changes in tension. Within the same fatigue procedure applied to a fresh muscle, the drop in tension was as much as 35% for high-rate stimulation and 59-71% for low-rate stimulation, whereas the decrease of the peak-to-peak compound muscle action potential waves amplitudes did not exceed 10-20%. Compound muscle action potential waves underwent the most pronounced depression during high-rate stimulation, the decrease proceeding during the following phase of low-rate stimulation. The tension changes during long-lasting activation were different for low- and high-frequency fatigue, with more pronounced depression during low-rate stimulation. As a rule, compound muscle action potential waves changes followed opposite patterns. Compound muscle action potential waves progressively split up, which was probably associated with a continuous slowing of the action potentials in the most fatigable motor units and the subsequent disappearance of the reactions at least in part of the motor units. Hysteresis effects in muscle contraction seem to be able, at least in part, to compensate for some of the depressive effects appearing during conduction of action potentials in muscle fibres. Changes in the compound muscle action potentials were studied during development of the muscle fatigue. These changes showed pronounced dependency on stimulation rate allowing differentiating effects of low- and high-frequency stimulation of the efferents supplying muscle under study. At the same time the fatigue-related changes in the action potentials were noticeably smaller than changes in tension, thus supporting existing concepts in the field arguing that fatigue effects are mainly connected with corresponding activity-dependent changes in muscle contraction machinery.