AJP - Cell Physiology, Vol 263, Issue 5 C1065-C1072, Copyright © 1992 by American Physiological Society
Effect of fatigue on rate of isometric force development in mouse fast- and slow-twitch muscles
C. J. Barclay
Department of Physiology, University of Auckland, New Zealand.
Changes in the rate of isometric force development with fatigue were
measured in vitro (25 degrees C) using mouse soleus and extensor digitorum
longus (EDL) muscles. Muscles were fatigued using 30 tetanic contractions.
Rate of force development was determined from the rate constant of an
exponential curve fitted to the rising force phase of a tetanus. For both
muscles, when the intertetanus interval was 3 s, maximum isometric force
and relaxation rate were significantly reduced in the final tetanus
relative to the values in the first tetanus. Rate of force development in
soleus muscles transiently increased and then decreased a small amount. The
final rate was 92.7 +/- 3.3% (n = 4) of the initial rate. In contrast, the
rate of force development in EDL muscles increased to 133.7 +/- 3.3% (n =
4) of the initial rate. This increased rate was evident from the second
tetanus of the series, was fully established after 5 tetani, and the
magnitude of the increase in rate was inversely proportional to
intertetanus interval and was independent of presumed energy expenditure.
The enhanced rate decayed with a time constant of 14.3 +/- 2.0 s and was
independent of presumed energy expenditure. Most of these observations can
be explained by the effects of P(i) on cross bridge kinetics. Other
possible mechanisms, involving more rapid activation, are also suggested.
