Fig. 1. Example records of fused isometric tetanus of an isolated muscle fiber of the mouse. (A) Standard `isometric' recording (fixed fiber ends). (B) Force production during length clamp of a fiber segment. Upper trace in each panel shows force; lower trace, segment length recording with sarcomere spacing indicated. A downward deflexion of segment length signal indicates shortening. An initial transient at the onset of the length-clamp manoeuvre in B caused the early tension step. Note, however, that the time required to reach plateau force is quite similar during contraction with fixed fiber ends and during segment length-clamp recording. The intrinsic shortening during the standard isometric recording in A was 5.0% of fibre length. Temperature, 23.5°C.