Nebulin super-repeats, TFL, and the force–sarcomere length relation . (A) Correlation between the number of nebulin super-repeats and TFL in a range of species. Data shown are for perch fast muscle (white trunk muscle), which has 21 super-repeats (Table 1) and a TFL of 940 nm (Granzier et al., 1991), EDL muscle from WT mouse (25 repeats, TFL 1,088 nm), WT − 3 SR mouse (22 super-repeats, TFL 980 nm), WT + 3 SR mouse (28 super-repeats, TFL 1,088 nm; Donner et al., 2004; Kiss et al., 2020; Lindqvist et al., 2020), and human quadriceps (29 super-repeats, TFL 1,270 nm; Kruger et al., 1991; Labeit and Kolmerer, 1995). TFL varies with the number of nebulin super-repeats (R² of linear regression line = 0.99). (B) Mouse EDL immunolabeled with antibody to nebulin N terminus (inverse image). The antibody is localized at the edge of the overlap zone. If thin filaments were to extend beyond the nebulin epitope by ∼50 nm (the approximate Z-disk width), this would be easily detectable. See text for details. Scale bar, 500 nm. (C) Predicted force–sarcomere length relation for perch and human fast muscle fibers (we assumed a constant thick filament length of 1.6 µm and a bare zone width of 0.15 µm). The descending limbs are horizontally shifted by 0.66 µm.