Deformation and three-dimensional displacement of fibers in isometrically contracting rat plantaris muscles

In this study, the deformation of different fibers of the rat m. plantaris during "isometric" contractions at different muscle lengths was considered. Because the m. plantaris has an obviously inhomogeneous architecture, its fibers on the medial side of the muscle belly are judged to be shorter than those on the lateral side of it. It was expected that longitudinal deforma tion of different fibers would vary accordingly. A 3D video analysis of con tracting muscle showed that longitudinal strain of fibers as a function of muscle length does not differ between fibers on different sides of the musc le. Apart from longitudinal shortening, the fibers were also displaced late rally during a contraction. The fibers displaced during a contraction in a direction perpendicular to their longitudinal axis. The displacement of the fibers occurred asymmetrically, resulting in a helical deformation of the whole muscle. It is concluded that the asymmetric displacement and the heli cal deformation must result from transversal forces between the fibers. It is hypothesized that these transversal forces cancel out differences in lon gitudinal strains that might exist between fibers. (C) 2001 Wiley-Liss, Inc .