A stretch-to-match ruler hypothesis. (A) Structural simulation of stretching one nebulin superrepeat (modeled by ROSETTA) to 0, 43, 80, 100%. Note that the 0 and 43% extension models are enlarged by 50% for clarity. The 80% extension may be representative of the remaining structure at the detachment points in the current experiments (our unpublished data). (B) Full length nebulin associates along and around actin filaments in the sarcomere only when stretched to >1 μm, with the concomitant optimization of matching periodicity to helical parameters of actin. Each super-repeat spans one arc length around actin, with a single stretched full-length nebulin spanning and associating 26 actin axial repeats along the 1 μm long thin filament. Nebulin may exert significant compression on actin both longitudinally and radially. The exact structure of nebulin on the thin filament is unknown at present.