Various pennation angle arrangements of the fascicles/myofibers in skeletal muscle: (A) fusiform, (B) unipennate, (C) bipennante, (D) multipennate. Blue lines indicate the plane for the anatomical cross-sectional area (CSA) (i.e., the CSA that runs perpendicular to the longitudinal axis of the muscle), and green lines indicate the plane for physiological CSA (i.e., the CSA that runs perpendicular to the longitudinal axis of the fascicles/myofibers). Adapted under the Creative Commons Attribution-Share Alike 3.0 Unported license from original work by Uwe Gille (Available online: https://creativecommons.org/licenses/by-sa/3.0/deed.en (accessed on 6 January 2020).