1. We studied the descending limb of the force‐sarcomere length relation in single frog muscle fibres using sarcomere isometric contractions. 2. Sarcomere length was measured simultaneously with two independent methods: a laser diffraction method and a segment length method that detects the distance between two markers attached to the surface of the fibre, about 800 microns apart. Both methods were used to keep sarcomeres at constant length during contraction. 3. Fibres were selected for low resting tension since it was known from previous experiments that for such fibres the force developed by fixed‐end tetani is much higher than that predicted by the degree of filament overlap. 4. With fixed‐end tetani, force decline with increase of sarcomere length was small between 2.0 and 3.0 microns. At a sarcomere length of 3.0 microns, force was about 90% of maximal. 5. With sarcomere isometric tetani, force was considerably lower than with fixed‐end tetani. Force was maximal at about 2.1 microns and decreased to zero at about 3.6 microns. At intermediate lengths the descending limb was within 80 nm of the values predicted from filament overlap. 6. We investigated why force of fixed‐end contractions was much higher than that generated by sarcomere isometric contractions. 7. During the force plateau of fixed‐end tetani at sarcomere lengths longer than about 2.0‐2.2 microns, sarcomeres in the fibres mid‐region were not isometric, but instead stretched slowly. By measuring the force‐velocity relation it was shown that this slow stretch elevates active force well beyond sarcomere isometric force. 8. Stretch of the central region was also observed during the tetanic force rise. This was shown to result in an increase of passive force that grew larger at longer sarcomere lengths. At about 3.6 microns the increase of passive force was similar to the total force generated by fixed‐end contractions at this length. 9. Laser diffraction and segment length methods gave the same results, diminishing the chance that any systematic artifact underlies our findings. 10. While earlier experiments from this laboratory carried out on fibres held at constant length during contraction did not reveal a linear descending limb, the present results support the linear descending limb as a characteristic feature of isometrically contracting sarcomeres.
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