FIGURE 5 | Sarcomere structure at different degrees of extension.

During muscle contraction, sarcomere length decreases, which results in coiling of the I-band part of titin. Simultaneously, inter-filament spacings increase, which is likely to reorientate or stretch Z- and M-line proteins that crosslink thin and thick filaments and myofibrils. During extension, sarcomere length increases and inter-filament spacings decrease. This leads to the extension of titin, while releasing tension on the Z- and M-line proteins. Over-extension of muscle results in the unravelling of titin polypeptide, which starts from the least mechanically stable PEVK region. This is accompanied by compression and reorientation of Z- and M-line proteins. Thin and thick filaments are shown in black, -actinin and spectrin in pink, filamin in purple and M-line proteins in red. Titin is coloured in differently along its length according to regional differences in structure.

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