Fatigue from incompletely fused tetanic contractions in skeletal muscle in situ.

The purpose of this study was to investigate the contractile response of skeletal muscle in situ when stimulation results in an unfused tetanic contraction. The left gastrocnemius-plantaris muscle group of anesthetized (pentobarbital sodium) dogs (n = 16) was connected to an isometric lever and stimulated indirectly for 30 min. During 10-Hz stimulation, total tension (the peak of each oscillation in tension) increased during the first 2 min of stimulation (staircase), then decreased during the remaining 28 min of stimulation. Since relaxation was incomplete at this rate of stimulation, the developed tension, the difference between peak tension and the lowest tension between successive contractions, did not follow the same pattern of staircase and fatigue as the peak tension did. Developed tension (delta T) decreased during the staircase response then increased from 2 to 10 min before finally decreasing again during the last 20 min, ending at 56 +/- 15 (mean +/- SE) % of the initial (first contraction) delta T. At 2 min of 10-Hz contractions, half-relaxation time (1/2 RT) was too long to measure (insufficient relaxation between contractions), but later, 1/2 RT decreased from greater than 65 ms to less than 40 ms. Increased 1/2 RT has been associated with reduced energy availability. If an increased 1/2 RT is an indication of insufficient energy, then it can be concluded that fatigue continued in spite of a recovery of energy supplies. This suggests a possible dissociation of fatigue and energy availability.