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 consisted of 25-28 repetitions of the standard tests that included an 18-s interval of continuous frequency-modulated stimulation preceded and followed by single stimuli evoking twitch contractions. The rate of the continuous part was changed in accordance with a symmetrical double-trapezoidal signal, including three successive phases of constant rate at 10, 40 and 10s(-1); between these phases, each lasting for 4s, the rate changed linearly within a 2-s interval. The following modes of muscle activation were applied: (i) stimulation 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) the distributed stimulation of five similar filaments; and (iii) direct stimulation of muscle through bipolar wire electrodes. A relative drop in tension, the fatigue index, expressed as the ratio at the end of a fatigue session over its value at the beginning of the test, was used to quantify fatigue 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+/-0.02. The high-rate stimulation noticeably increased the mean tension during low-rate 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 beginning of the fatigue session. It was demonstrated that the potentiation was connected with after-effects of the rate-tension hysteresis. The hysteresis 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 processes developed more quickly and intensively in the previously fatigued muscles: 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 stimulation 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 the level of nerve-muscle synaptic transmission. The effects of low- and high-frequency fatigue were studied in isometric conditions of muscle contraction. In addition to the well-known differentiation 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 in the muscle contraction.