Small-angle x-ray scattering from live 5 dpf wildtype and mybphb ^−/− tails. (A–C) The distances between planes in the paracrystalline lattice formed by myofibrillar actin and myosin filaments are measured by the spacing of equatorial x-ray scattering reflections. (A) TEM of zebrafish fast-twitch muscle cross section and schematic showing the distribution of actin and myosin filaments, and major spatial planes created by the array of actin, thin and myosin, thick filaments. (B) Representative x-ray scatter showing 1,0 and 1,1 equatorial reflections caused by planes in A. The circumferential spread of the reflections around the origin is caused by the inhomogeneity of muscle fiber angles within the tail. (C) Representative plot of background-subtracted radial intensity over the angle phi showing position and relative intensity of 1,0 and 1,1 reflections before and after stretch. (D) Spacing of d_1,0 plane in wildtype (n = 7) and mybphb^−/− (n = 8) tails at rest length (L₀), after being stretched to rest length + 10% (L₁₀), and during a 300 Hz, electrically stimulated tetanic contraction at L₁₀ (Tetanus). Note: we previously showed that a tetanic contraction at L₁₀ results in a 10% shortening of the muscle length back to its rest length of ∼L₀ due to tissue compliance at the ends of the preparation (Mead et al., 2020). (E) An increase in the ratio of 1,1 to 1,0 reflection intensities (I_1,1/I_1,0) can indicate a shift of mass from thick to thin filaments (i.e., myosin heads moving towards or attaching to the thin filament). No such shift was seen in either group with stretch or tetanic stimulation. Data are presented as means ± 1 SD.