Interaction between series compliance and sarcomere kinetics determines internal sarcomere shortening during fixed-end contraction

The interaction between contractile force and in-series compliance was investigated for the intact skeletal muscle-tendon unit (MTU) of Rana pipiens semitendinosus muscles during fixed-end contraction. It was hypothesized that internal sarcomere shortening is a function of the length-force characteristics of contractile and series elastic components. The MTUs (n = 18) were dissected, and, while submerged in Ringer's solution, muscles were activated at nine muscle lengths(-2 to +6 mm relative to optimal length in 1 mm intervals), while measuring muscle force and sarcomere length (SL) by laser diffraction. The MTU was clamped either at the bone (n = 6), or at the proximal and distal ends of the aponeuroses (n = 6). Muscle fibers were also trimmed along with aponeuroses down to 5-20 fibers and identical measurements were performed (n = 6). The magnitude of shortening decreased as MTU length increased. The magnitude of shortening ranged from -0.08 to 0.3 mu m, and there was no significant difference between Delta SL as a function of clamp location. When aponeuroses were trimmed, sarcomere shortening was not observed at L-0 and longer. These results suggest that the aponeurosis is the major contributor to in-series compliance. Results also support our hypothesis but there also appear to be other factors affecting internal sarcomere shortening. The functional consequence of internal sarcomere shortening as a function of sarcomere length was to skew the muscle length-tension relationship to longer sarcomere lengths. (C) 2000 Elsevier Science Ltd. All rights reserved.