CHANGES IN GEOMETRY OF ACTIVELY SHORTENING UNIPENNATE RAT GASTROCNEMIUS-MUSCLE

Muscle geometry of the unipennate medial gastrocnemius (GM) muscle of the rat was examined with photographic techniques during isometric con tractions at different muscle lengths. It was found that the length of fibers in different regions of GM differs significantly, and proximal aponeurosis length varies significantly from distal aponeurosis lengt h; the angle of the aponeurosis with the muscular action differs signi ficantly among regions at short muscle lengths (full contraction). The se data support the idea that the unipennate GM cannot be represented by a parallelogram in a two-dimensional analysis. As the muscle shorte ns, the area of the mid-longitudinal plane of the GM decreases by 24%, a decrease that may be explained by assuming fiber diameter to increa se in all directions. The angle between fiber and aponeurosis is deter mined by more than fiber length. Hence, such important assumptions as a parallelogram with constant area and fiber angle gamma changes deter mined by fiber length changes, frequently used in the theoretical anal ysis of the morphological mechanism of unipennate muscle contraction, do not hold for the unipennate GM of the rat. Length of the sarcomere within the mid-longitudinal plane of GM varies from 1.92 to 2.14 mum a mong the different muscle regions at muscle optimum length (length at which force production is highest), whereas shortening to 6 mm less th an optimum length produces a range of sarcomere lengths from 0.89 to 1 .52 mum. These data suggest that fibers located in different regions o f the GM reach their optimum and slack lengths at various muscle lengt hs. (C) 1993 Wiley-Liss, Inc.