title: Interaction between series compliance and sarcomere kinetics determines internal sarcomere shortening during fixed-end contraction.
abstract: 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 microm, 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.
authors.affiliation: Department of Life Sciences, University of Tokyo, Komaba 3-8-1, Meguro, 153-8902, Tokyo, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Kawakami
authors.forename: R L
authors.initials: RL
authors.lastname: Lieber
mesh_terms: D000818:Animals; D003187:Compliance; D006614:Hindlimb; D066298:In Vitro Techniques; D007700:Kinetics; D009119:Muscle Contraction / Q000502:physiology*; D018482:Muscle, Skeletal / Q000502:physiology; D011894:Rana pipiens; D012518:Sarcomeres / Q000502:physiology*