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 digitorum longus (EDL) and gastrocnemius medialis (GM) muscle-tendon complex was measured using quick length decreases (0.2 mm within 3 ms) during isometric contractions. Extension of tendinous structures at maximal isometric force level was measured by means of photography. These data allowed us to distinguish between series elastic compliance of tendinous structures and muscle fibres (cross-bridges). Using mathematics, similar as in the Alpha method (MORGAN, 1977), a force dependent component and a constant component of series elastic compliance could be distinguished. Extension values of SEC in the cross-bridges of about 1.5% of fibre length were found, which is close to values found for isolated frog muscle fibres. The force dependent compliance was significantly higher (3%). Therefore, it is concluded that part of the force dependent compliance resides within the tendinous structures, even beyond the toe-region. Thus, at high force levels compliance of tendinous structures is not constant. Direct measurements on isolated GM tendon confirmed this conclusion. Functional consequences of these tendinous properties are discussed. For both EDL and GM, about 85% of SEC extension at maximal isometric force (F o ) appeared to be located in the tendinous structures. However, tendinous compliance, normalised with respect to its length, is higher for GM. For GM the free tendon is more compliant than aponeurosis, whilst EDL has a rather uniform distribution of normalised compliance along free tendon and aponeurosis. Differences of tendinous compliance may be related to functional differences between the muscles.