The dependence of the short-range elasticity on sarcomere length in resting isolated frog muscle fibres.

The tension response of a resting muscle fibre to a sudden small stretch (ca. 1% of the fibre length) at constant velocity was analyzed according to a mechanical model in which two components are arranged in parallel: (i) The parallel elastic component (PEC) which is responsible for the resting tension of the fibre. (ii) The short-range elastic component (SREC) which has a highly non-linear tension response: Initially, the tension increases linearly with stretch, but with further stretch the increase in tension subsides and a steady tension level is attained. Both the initial short-range elastic stiffness and the steady state tension level increased with increasing sarcomere length up to 2.9-3.3 micrometer where both variables reached maximum values. The maximum value of the steady tension level was about 1% of the maximum twitch tension. With further increase of sarcomere length both the short-range elastic stiffness and the steady tension level decreased and approached zero at a sarcomere length of about 3.7 micrometer. The structural basis for the SREC is considered to be myosin heads which are cross-linking the thick and the thin filaments even in the resting state of the fibre.