Fatigue effects in the cat gastrocnemius during frequency-modulated efferent stimulation

Effects of low- and high-frequency fatigue were studied on muscle dynamics in isometric conditions of the cat,gastrocnemius. Fatiguing sessions consis ted of 25-28 repetitions of the standard tests that included an 18-s interv al of continuous frequency-modulated stimulation preceded and followed by s ingle stimuli evoking twitch contractions. The rate of the continuous part was changed in accordance with a symmetrical double-trapezoidal signal, inc luding three successive phases of constant rate at 10, 40 and 10 s(-1); bet ween these phases, each lasting for 4 s, the rate changed linearly within a 2-s interval. The following modes of muscle activation were applied: (i) s timulation of single filaments constituting approximately one-fifth to one- seventh of the total cross-section of the L-7 and S-1 ventral roots; (ii) t he distributed stimulation of five similar filaments; and (iii) direct stim ulation of muscle through bipolar wire electrodes. A relative drop in tensi on, the fatigue index, expressed as the ratio at the end of a fatigue sessi on over its value at the beginning of the test, was used to quantify fatigu e effects. The fatigue indices during low-rate stimulation were 0.56 +/- 0. 03 (mean +/- S.D.) at the first phase and 0.64 +/- 0.02 at the third phase, while during high-rate stimulation this parameter was only 0.32 i 0.02. Th e high-rate stimulation noticeably increased the mean tension during low-ra te stimulation; the ratio between the reactions at the third and the first phases could be as much as two to three times greater than that at the begi nning of the fatigue session. It was demonstrated that the potentiation was connected with after-effects of the rate-tension hysteresis. The hysteresi s decreased with fatigue, the fatigue index for the rate-tension loop areas ranging from 0.39 to 0.52 (0.45 +/- 0.05, mean +/- S.D.). The fatigue proc esses developed more quickly and intensively in the previously fatigued mus cles: the obtained fatigue indices were 0.73 +/- 0.05 and 0.70 +/- 0.10 at the first and third phases, and 0.62 +/- 0.06 (mean +/- S.D.) at the second phase of stimulation, respectively. In the cases of distributed and direct stimulation applied to muscles in a fresh state, fatigue dynamics did not differ significantly from those observed during single-filament stimulation . In experiments with distributed simulation applied to previously fatigued muscles, a powerful depression of the high-rate components was registered in several cases, which seemed to be connected with depressive effects at t he level of nerve-muscle synaptic transmission. The effects of low- and high-frequency fatigue were studied in isometric co nditions of muscle contraction. In addition to the well-known differentiati on between low- and high-frequency fatigue effects, the complex pattern of efferent stimulation used allowed us to identify additional fatigue-related changes in the rate-tension hysteresis. This hysteresis seems to be one of the possible mechanisms directed to compensate for low-frequency fatigue i n the muscle contraction. (C) 2000 IBRO. Published by Elsevier Science Ltd.