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)

Relationship between Achilles tendon mechanical properties and gastrocnemius muscle function.

Strain was measured along the length of frog (Rana pipiens) gastrocnemius muscle-tendon units (MTU). Maximum muscle tension (P0) was measured, and the MTU was passively loaded to P0. Strain at P0 was measured at eight intervals along the tendon and aponeurosis and was approximately two percent for all regions except the aponeurosis region closest to the muscle fibers where it was about six percent. A computer model predicted sarcomere shortening of up to 0.5 micron due to tendon lengthening which demonstrates that tendons provide a more complex physiological function than simply transmitting muscle force to bones.

Frog semitendinosis tendon load-strain and stress-strain properties during passive loading.

The mechanical properties of the frog semitendinosis (ST) tendon, bone-tendon junction, and aponeurosis were measured during passive loading to a tension equal to maximum isometric tension (Po). Stiffness and strain in these regions continuously increased as load increased. Tendon stiffness was approximately four times the aponeurosis stiffness. Tendon Young's modulus at Po was only 188 MPa, which is approximately 10 times less than the modulus reported for most mammalian tendons. Similarly, tendon stress at Po was only approximately 3 MPa, which is also less than that predicted for many tendons. Tendon strain at Po was approximately 2% after passive loading. We conclude that different regions of the frog ST tendon have different mechanical properties and that the frog ST tendon operates physiologically in the "toe" region of the stress-strain curve with a variable stiffness that increases with load. Taken together, these results have significant implications in understanding muscle-tendon design and neuromotor control strategies.