Fig. 3. (A) Coordinate systems of the Z-stack (x_z, y_z, z_z) in blue and of the fish (x, y, z) in red. (B) Schematic representation of a portion of a muscle fibre (thick red line) that extends between two parallel optical sections (shown pink) in the (x, y, z) system at a mutual distance z. The azimuth and elevation angles and β and the projection of the fibre on the horizontal plane are indicated, as well as the differences between begin and end points in the x- and y-directions x and y. See main text for further explanation.