In the widely accepted sliding filament theory of muscle contraction, force is assumed to arise from interactions between neighboring thick and thin filaments. It is conventionally held that these interactions are, in fact, the physical attachment of angled cross-bridges which are able to stretch across the rather large distance between the surfaces of the myofilaments (about 13 nm in muscle at rest length). Inasmuch as intact muscle fibers maintain a constant volume, the distance between the myofilaments is not fixed but varies with muscle length. The variation in interfilament distance is not trivial. For example, surface-to-surface distance decreases some 40% when a fiber is stretched from 2.1 µm to 3.4 µm (cf. Schoenberg, Biophys. J. 30: 51–68, 1980). A priori, it seems likely that such changes in working distance would influence cross-bridge performance. The prevailing view is, however, that cross-bridges accomodate these changes in working distance and maintain constant force per bridge over a wide range of muscle lengths by dint of their doubly hinged configuration (Huxley, Science 164: 1356–1366, 1969).
KeywordsThin Filament Muscle Length Intact Fiber Skinned Fiber Rest Length
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