SARCOMERE SHORTENING DURING FIXED-END CONTRACTION OF FROG SEMITENDINOSUS MUSCLES

Abstract

1051 Activation of an intact muscle-tendon unit at a fixed length results in sarcomere shortening at the expense of tendon lengthening. Series elasticity permits muscle-tendon units to increase active range, absorb length changes, and to absorb energy thus increasing locomotion efficiency. However, sarcomere shortening at the expense of tendon elongation has not been quantified. The purpose of this study was to measure sarcomere shortening as a function of initial sarcomere length in the intact muscle-tendon-bone (MTB) complex of Rana pipiens semitendinosus (ST) muscles. MTB complexes (n=6) were dissected from R. pipiens after double pithing. Muscles were activated at nine lengths (−3 mm to +6 mm relative to optimal length in 1 mm intervals), while measuring sarcomere length by laser diffraction. Sarcomere shortening with muscles at long sarcomere lengths (∼3.3 μm) was significantly smaller (0.039±.014 μm) compared to sarcomere shortening at short (∼2.0 μm) sarcomere lengths (0.32±.024 μm). The functional consequence of this differential shortening as a function of sarcomere length was to skew the muscle length-tension relationship to longer lengths and to slightly underestimate maximum tetanic tension. The effect was relatively small for the ST muscle but simulations reveal the result would be much greater for muscles with long in-series tendons. This work was supported by NIH grant AR40050 the VA and the Mizuno International Sports Exchange Foundation.