SERIES ELASTIC PROPERTIES OF RAT SKELETAL-MUSCLE - DISTINCTION OF SERIES ELASTIC COMPONENTS AND SOME IMPLICATIONS

Compliance of the series elastic component (SEC) of rat extensor digit orum longus (EDL) and gastrocnemius medialis (GM) muscle-tendon comple x was measured using quick length decreases (0.2 mm within 3 ms) durin g isometric contractions. Extension of tendinous structures at maximal isometric force level was measured by means of photography. These dat a allowed us to distinguish between series elastic compliance of tendi nous structures and muscle fibres (cross-bridges). Using mathematics, similar as in the Alpha method (MORGAN, 1977), a force dependent compo nent and a constant component of series elastic compliance could bc di stinguished. Extension values of SEC in the cross-bridges of about 1.5 % of fibre length were found, which is close to values found for isola ted frog muscle fibres. The force dependent compliance was significant ly higher (3%). Therefore, it is concluded that part of the force depe ndent compliance resides within the tendinous structures, even beyond the toe-region. Thus, at high force levels compliance of tendinous str uctures is not constant. Direct measurements on isolated GM tendon con firmed this conclusion. Functional consequences of these tendinous pro perties are discussed. For both EDL and GM, about 85% of SEC extension at maximal isometric force (F(o)) appeared to be located in the tendi nous structures. However, tendinous compliance, normalised with respec t to its length, is higher for GM. For GM the free tendon is more comp liant than aponeurosis, whilst EDL has a rather uniform distribution o f normalised compliance along free tendon and aponeurosis. Differences of tendinous compliance may be related to functional differences betw een the muscles.