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

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Abstract
The tension response of a resting muscle fibre to a sudden small stretch (ca. I % 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 pm 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 demased and approached zero at a sarcomere length of about 3.7 pm. The structural basis for the SREC is considered to be myosin heads which are cross‐linking the thick and the thin fdaments even in the resting state of the fibre.