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J Appl Physiol 69: 740-746, 1990;
8750-7587/90 $5.00
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Journal of Applied Physiology, Vol 69, Issue 2 740-746, Copyright © 1990 by American Physiological Society

ARTICLES

Increased fatigue of isovelocity vs. isometric contractions of canine diaphragm

B. T. Ameredes and T. L. Clanton
Division of Pulmonary and Critical Care Medicine, Ohio State University, Columbus 43210.

A comparison of fatigue as a loss of force with repeated contractions over time was performed in canine respiratory muscle by isometric (nonshortening) and isovelocity (shortening) contractions. In situ diaphragm muscle strips were attached to a linear ergometer and electrically stimulated (30 or 40 Hz) via the left phrenic nerve to produce either isometric (n = 12) or isovelocity (n = 12) contractions (1.5 s) from optimal muscle length (Lo = 8.8 cm). Similar velocities of shortening between isovelocity experiments [0.19 +/- 0.02 (SD) Lo/S] were produced by maximizing the mean power output (Wmax = 210 +/- 27 mW/cm2) that could be developed over 1.5 s when displacement was approximately 0.30 Lo. Initial peak isometric tension was 1.98 kg/cm2, whereas initial peak isovelocity tension was 1.84 kg/mc2 (P less than 0.01) or 93% of initial isometric tension. Fatigue trials of 5 min were conducted on muscles contracting at a constant duty cycle (0.43). At the end of the trials, peak isovelocity tension had fallen to 50% of initial isometric tension (P less than 0.01), whereas peak isometric tension had only fallen by 27%. These results indicate that muscle shortening during force production has a significant influence on diaphragm muscle fatigue. We conclude that the effects of shortening on fatigue must be considered in models of respiratory muscle function, because these muscles typically shorten during breathing.


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J. F. Watchko, M. J. Daood, G. C. Sieck, J. J. Labella, B. T. Ameredes, A. P. Koretsky, and B. Wieringa
Combined myofibrillar and mitochondrial creatine kinase deficiency impairs mouse diaphragm isotonic function
J Appl Physiol, May 1, 1997; 82(5): 1416 - 1423.
[Abstract] [Full Text] [PDF]


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