Fatigue-related changes in electomyogram activity of the cat gastrocnemiusduring frequency-modulated efferent stimulation

Changes in the compound muscle action potentials of cat gastrocnemius muscl e were studied during low- and high-frequency fatigue. Fatiguing session co nsisted of 25-28 repetitions of the standard single fatigue tests (1.5 min interval between the tests) that included the part of continuous frequency- modulated stimulation preceded and followed by single stimuli evoking twitc h contractions in the muscle. The rate of the continuous part was changed i n accordance with symmetrical double-trapezoidal signal, including three su ccessive phases of constant rate at 10, 40 and 10 s(-1); between these phas es of 4 s duration the rate changed linearly within a 2 s interval. During fatigue relative changes in compound muscle action potential waves were usu ally smaller than changes in tension. Within the same fatigue procedure app lied to a fresh muscle, the drop in tension was as much as 35% for high-rat e 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 pr onounced depression during high-rate stimulation, the decrease proceeding d uring the following phase of low-rate stimulation. The tension changes duri ng long-lasting activation were different for low- and high-frequency fatig ue, 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 p robably associated with a continuous slowing of the action potentials in th e most fatigable motor units and the subsequent disappearance of the reacti ons at least in part of the motor units. Hysteresis effects in muscle contr action seem to be able, at least in part, to compensate for some of the dep ressive effects appearing during conduction of action potentials in muscle fibres. Changes in the compound muscle action potentials were studied during develo pment of the muscle fatigue. These changes showed pronounced dependency on stimulation rate allowing differentiating effects of low- and high-frequenc y stimulation of the efferents supplying muscle under study. At the same ti me the fatigue-related changes in the action potentials were noticeably sma ller than changes in tension, thus supporting existing concepts in the fiel d arguing that fatigue effects are mainly connected with corresponding acti vity-dependent changes in muscle contraction machinery. (C) 2000 IBRO. Publ ished by Elsevier Science Ltd.