Effects of distribution of muscle fiber length on active length‐force characteristics of rat gastrocnemius medialis

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Abstract
Background: The length-force curve of mammalian skeletal muscle is often wider than expected on basis of the optimum length of the muscle fibers. Two important effects may explain this discrepancy: muscle pennation and distribution of fiber lengths in the muscle. In the present study of the effects of a Gaussian distribution of fiber lengths on muscle length-force characteristics were investigated in rat gastrocnemius medialis. Methods: Fiber length-force characteristics and parameter values of the Gaussian distribution were derived from literature data (Stephenson et al., 1989, J. Physiol., 410:351–366; Heslinga and Huijing, 1990, J. Morph. 206:119–132; Zuurbier and Huijing, 1993, J. Morphol., 218:167–180). Three different constructions of the distribution model were compared with experimental data. Pennation effects were incorporated in the model. Results: Two constructions gave reasonably good results: (1) the model with a fiber optimum distribution, in which fibers acted at the same absolute length at a given muscle length; (2) the model in which fiber optimum length was uniform but absolute length at a given muscle length was distributed. Conclusions: In rat gastrocnemius medialis, the magnitude of the effects of these distributions is similar to pennation effects. The current results help to explain the relative wide working range of skeletal muscles in human movement and the differences in specific muscle tension as affected by muscle type, species, and age.