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 Ringerʹs 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 μm, and there was no significant difference between ΔSL as a function of clamp location. When aponeuroses were trimmed, sarcomere shortening was not observed at L0 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.