Sarcomere length behaviour along single frog muscle fibres at different lengths during isometric tetani

Summary A detailed investigation of sarcomere lengthening and shortening during fixed-end tetani has been made along frog muscle fibres stretched over a large range of sarcomere lengths. A variety of sources of error common in such measurements are quantitated and give an uncertainty in sarcomere length of about 53–62 nm. The difference in sarcomere length calculated from the left and right first orders at rest was 21 nm ±16 nm and this is suggested to be a measure of Bragg artefact. The laser diffraction measurements showed that the shortening end regions decrease in size during contraction and that the magnitude of shortening is increased at greater fibre extensions. The average length change and sarcomere length of the central and end regions was 0.10 m (2.85 m) and –0.37 m (2.66 m), respectively. The sarcomere length of the end regions at the end of creep was regularly observed to be <2.1 m. An unexpected finding was the occasional observation of striations in the transition zone between lengthening and shortening regions which remained nearly isometric during a period of tension rise during creep. Measurements of diffraction order linewidth do not suggest increased sarcomere length dispersion in these areas. A smooth transition from shortening to lengthening was always observed. Although our data are in general agreement with the models proposed by Morgan, Mochon and Julian (Biophys. J.39 (1982) 189–96) and Edman and Reggiani(J. Physiol. (Lond.)351 (1984) 169–98), specific differences which do exist are discussed.