Pattern of summation with fatigue and inhibition of calcium release in rat muscle.

INTRODUCTION: Fatigue disrupts muscle force summation and is associated with a decrease in cytoplasmic Ca(2+) concentration. The purpose of this study was to compare summation during fatigue and recovery with summation during dantrolene-induced inhibition of Ca(2+) release. METHODS: Rat medial gastrocnemius muscles were evaluated before and after fatigue, or during exposure to dantrolene. Summation was quantified by the ratio of the force transient associated with the final activation in a train of stimuli (Twf), obtained by subtraction of the force with one less stimulus, and the force of the twitch (Tw). RESULTS: This ratio (Twf/Tw) decreased from 2.46 ± 0.11 (mean ± SEM) to 0.8 ± 0.1 during intermittent contractions, but was still significantly different from non-fatigued muscle after 10 min of recovery. Dantrolene altered summation, as Twf/Tw was 1.7 ± 0.2 and 1.27 ± 0.15 at a low dose and a high dose, respectively. CONCLUSIONS: Inhibition of Ca(2+) release alters summation, but repetitive stimulation leading to fatigue changes it more substantially.

Impact of length during repetitive contractions on fatigue in rat skeletal muscle.

The magnitude of fatigue resulting from repeated contractions at a short length has been reported to be less than that which occurs with contractions at a long length. However, there have been what appear to be contradictory reports; the rate of fatigue is greater at a short length. The purpose of this study was to evaluate the impact of length on the magnitude and the rate of fatigue resulting from a series of repetitive stimulations. Experiments were done with anesthetized rats and the medial gastrocnemius muscle was stimulated via the sciatic nerve. Submaximal force-length relationships were obtained prior to and 45 min after repeated contractions (50 Hz, 300 ms) at short or long length. Stimulation was applied at 1 Hz or 0.5 Hz for 5 min at the long length or 1 Hz at the short length (difference = 3.6mm). This approach permitted evaluation of the impact of rate of muscle activation as well as length on subsequent contractile response. Repetitive stimulation at a short length resulted in more potentiation and a greater (relative) rate of fatigue but after 5 min the depression of relative active force was similar between the series at long and short length. The submaximal force-length relationship obtained after 45 min of recovery revealed that depression of force was greater after 1 Hz contractions at the long length. These results are consistent with both sides of the apparent contradiction in the literature; rate of fatigue is greater at a short length and magnitude of fatigue is greater at a long length.

The length dependence of muscle active force: considerations for parallel elastic properties.

The purpose of this study was to choose between two popular models of skeletal muscle: one with the parallel elastic component in parallel with both the contractile element and the series elastic component (model A), and the other in which it is in parallel with only the contractile element (model B). Passive and total forces were obtained at a variety of muscle lengths for the medial gastrocnemius muscle in anesthetized rats. Passive force was measured before the contraction (passive A) or was estimated for the fascicle length at which peak total force occurred (passive B). Fascicle length was measured with sonomicrometry. Active force was calculated by subtracting passive (A or B) force from peak total force at each fascicle or muscle length. Optimal length, that fascicle length at which active force is maximized, was 13.1 +/- 1.2 mm when passive A was subtracted and 14.0 +/- 1.1 mm with passive B (P < 0.01). Furthermore, the relationship between double-pulse contraction force and length was broader when calculated with passive B than with passive A. When the muscle was held at a long length, passive force decreased due to stress relaxation. This was accompanied by no change in fascicle length at the peak of the contraction and only a small corresponding decrease in peak total force. There is no explanation for the apparent increase in active force that would be obtained when subtracting passive A from the peak total force. Therefore, to calculate active force, it is appropriate to subtract passive force measured at the fascicle length corresponding to the length at which peak total force occurs, rather than passive force measured at the length at which the contraction begins.