* Corresponding author. Tel.: # 1-858-552-8585; fax: #1-858-552- 4381. E-mail address: rlieber@ucsd.edu (R.L. Lieber). Journal of Biomechanics 33 (2000) 1249}1255 Interaction between series compliance and sarcomere kinetics determines internal sarcomere shortening during "xed-end contraction Yasuo Kawakami , Richard L. Lieber * Department of Life Sciences, University of Tokyo, Komaba 3-8-1, Meguro, Tokyo 153-8902, Japan Departments of Orthopaedics and Bioengineering, Veterans Awairs Medical Center, University of California, San Diego, San Diego, CA 92161, USA Accepted 6 April 2000 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 "xed-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 di!raction. The MTU was clamped either at the bone (n"6), or at the proximal and distal ends of the aponeuroses (n"6). Muscle "bers were also trimmed along with aponeuroses down to 5}20 "bers 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 m, and there was no signi"cant di!erence between SL as a function of clamp location. When aponeuroses were trimmed, sarcomere shortening was not observed at ¸  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 a!ecting 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.  2000 Elsevier Science Ltd. All rights reserved. Keywords: Frog; Semitendinosus muscle; Contractile and elastic components 1. Introduction Theoretically, muscle is divided into contractile and elastic components, the latter of which can be further divided into elements arranged in series with or parallel to muscle "bers. Muscle "bers and tendinous tissues have been proposed to represent anatomical bases of contrac- tile and elastic components, respectively (Ettema and Huijing, 1989; Lieber et al., 1992; Zajac, 1989). Compli- ance of the series elastic component (SEC) a!ects force- transmission kinetics from muscle "bers to bone. Its mechanical importance has long been recognized (since Hill, 1951) and understanding in-series compliance is a key factor in modelling muscle mechanics (Herzog et al., 1992; Lieber et al., 1992; van Soest et al., 1995). In spite of a number of studies on the mechanical properties of muscle "bers and tendinous tissues per se, little is known how these structures interact with each other. Such information is quite important since these compo- nents function as a unit in intact skeletal muscle. Besides, the structural constituents of in-series compliance have not been clearly determined. Based on the two component model, Lieber et al. (1992) simulated "xed-end muscle contraction. They the- orized that "ber shortening occurs internally resulting from elongation of SEC by muscle force development. This theory clearly demonstrated the interaction between contractile and elastic components, but it was not experi- mentally tested. In this study we carried out experiments on "xed-end contraction of frog muscle}tendon unit to actually test the proposed theory. We hypothesized that internal sarcomere shortening results from the interac- tion between muscle force (i.e., more shortening for larger BM=1313=Binni=Venkatachala=BG 0021-9290/00/$ - see front matter  2000 Elsevier Science Ltd. All rights reserved. PII: S 0 0 2 1 - 9 2 9 0 ( 0 0 ) 0 0 0 9 5 - 6