The mechanical properties, as revealed by minute length changes, of isolated twitch fibers of the frog have been studied at rest and during low-level activation. Resting tension is 77 ± 23 mN/cm2 (mean ± SD) at 2.2 µm sarcomere length.1 The slope of the tension curve (ΔPL) recorded during a constant-speed length change of a resting fiber is initially large. At length changes exceeding about 0.18 % of the initial length of the fiber ΔPL falls abruptly and remains close to zero during the rest of the length change. The amplitude of the tension response is reduced after a length change and returns to normal in about 3 min. Hypertonic sucrose-Ringer solutions cause a small, maintained rise in tension up to 1.4–1.6 times normal osmotic strength. Higher sucrose concentrations cause relatively large, transient tension responses. The initial ΔPL is increased in moderately hypertonic solutions; it may be reduced in more strongly hypertonic solutions. Elevated [K]o (range 10–17.5 mM) causes a marked reduction in ΔPL. In this range of [K]o the reduction is not accompanied by changes in resting tension. Addition of 1–1.5 mM caffeine to the Ringer solution affects the resting tension very little but also reduces ΔPL. The results suggest that stiffness and tension development are not related in a simple way.

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