An unresolved problem in understanding muscular contraction is why the internal resistance to sarcomere shortening increases progressively during contraction1. We have addressed this problem here by investigating the movement of detached acting filaments in the sarcomeres of insect flight muscle. The final position of the detached actin filaments shows that they were able to slide freely into regions where they have the wrong polarity to interact actively with myosin (double-overlap zones) but where they prevent the exertion of force by cross-bridges between myosin and the correctly polarized acting filaments. These observations indicate that the isometric tension at all sarcomere lengths is directly proportional to the number of cross-bridges in the region of single-overlap of correctly polarized actin and myosin filaments. The decrease in tension as sarcomeres shorten is thus the result of the decrease in the number of effective cross-bridges as actin filaments slide into regions where they are of the wrong polarity to form cross-bridges, and where they inhibit the existing cross-bridges.
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TrombitÁs, K., Tigyi-Sebes, A. Cross-bridge interaction, with oppositely polarized actin filaments in double-overlap zones of insect flight muscle. Nature 309, 168–170 (1984). https://doi.org/10.1038/309168a0
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