Fig. 1. Schematic diagram illustrating the process of supercontraction. (A) Here, the muscle is completely extended, resulting in no overlap between thick and thin filaments. (B) Muscle in the contracted state, overlap between thick and thin filaments is optimal. (C) Muscle in an even further contracted state; thin filaments overlap and thick filaments abut on the Z-disk. In normal vertebrate cross-striated muscle, no force can be generated at this stage. (D) Muscle in supercontraction. The thick filaments have passed through the holes in the Z-disks and are starting to engage in bonding with the thin filaments of the adjacent sarcomere. (E) Muscle in complete supercontraction. After this stage, the thick filaments will physically interact with each other and the muscle can no longer generate force. The green bars represent the thick filaments, the blue bars the thin filaments and the red bars the elements of the Z-disk. In the sarcomere, polarities are such that the dark green part of the thick filament can engage in binding with the dark blue thin filament and the light green part of the thick filament with the light blue thin filament. Hypothetical cross-bridges formed between filaments of adjacent sarcomeres (in black) are indicated in D and E. Modified after Osborne (1967) and Hardie (1976).