The force bearing capacity of frog muscle fibres during stretch: its relation to sarcomere length and fibre width

1. Single fibres isolated from the anterior tibialis muscle of Rana temporaria were tetanized (0.9-1.8 degrees C) while a marked (similar to 1 mm) segment was held at constant length by feedback control. Force enhancement was produced by applying a controlled stretch ramp to the fibre segment during the tetanus plateau, the steady force reached during stretch being used as a measure of the maximum force that the myosin cross-bridges can hold before they detach. 2. The amplitude of force enhancement during stretch did not vary in proportion to the isometric force as the sarcomere length was changed, maximum force enhancement being attained near 2.4 mu m sarcomere length compared with 2.0 mu m for the isometric force. 3. The influence of fibre width on the force enhancement-sarcomere length relationship was evaluated by normalizing force enhancement to the tetanic (pre-stretch) force in this way allowing for the differences in myofilament overlap at the various lengths. The amplitude of force enhancement (normalized to the tetanic force) increased by approximately 70% as the relative width of the myofilament lattice was reduced from a nominal value of 1.05 at a sarcomere length of 1.8 pm to 0.85 at a sarcomere length of 2.8 mu m. 4. Changes in fibre width equivalent to those produced by altering the sarcomere length were produced by varying the tonicity of the extracellular medium. Force enhancement, normalized to the control isometric force at each tonicity, exhibited a width dependence that agreed well with that described in the previous point. Stretch ramps applied to frog skinned muscle fibres during calcium-induced contracture likewise resulted in a greater force enhancement during stretch after reducing the fibre width by osmotic compression. 5. The results suggest that the strength of binding of the myosin cross-bridges, unlike the isometric force, varies with the lateral distance between the myofilaments.