Journal of Applied Physiology, Vol 55, Issue 3 976-982, Copyright © 1983 by American Physiological Society
Fatigue from incompletely fused tetanic contractions in skeletal muscle in situ
B. R. MacIntosh, W. N. Stainsby and L. B. Gladden
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.
