Model of muscle-tendon interaction during frog semitendinosis fixed-end contractions.

A structural model was developed to explain sarcomere shortening at the expense of tendon lengthening in the frog semitendinosis (ST) muscle-tendon system. The model was based on the data of Lieber et al. [Am. J. Physiol. 261, C86-C92 (1991)], who determined the relationship between the sarcomere length, tendon load (as a fraction of maximum isometric tension) and tendon, bone-tendon junction (BTJ), and aponeurosis strain. The model was generated assuming a finite time-course of cross-bridge attachment [Huxley, Prog. Biophys. 7,255-318 (1957)], an ideal sarcomere length-tension relationship [Gordon et al., J. Physiol. 184, 170-192 (1966)] and an ideal force-velocity relationship [Katz, J. Physiol. 96, 45-64 (1939); Edman, J. Physiol. 291, 143-159 (1979)]. Functionally, sarcomeres operated on three distinct regions of the length-tension curve: (1) regions where the muscle force decreased as sarcomeres shortened (the shallow and steep ascending limbs); (2) regions where the muscle force increased as sarcomeres shortened and there was little passive tension (descending limb, where sarcomere length greater than or equal to 3.0 microns); and (3) regions where the muscle force increased as sarcomeres shortened and there was a significant passive tension (descending limb where sarcomere length greater than 3.0 microns). Using such a physiological model, it was found that the effect of tendon compliance was to 'skew' the sarcomere length-tension curve to the right and to increase the operating range of the muscle-tendon unit.(ABSTRACT TRUNCATED AT 250 WORDS)