Coding of specific muscle groups. Computed tomography scan of the level of the third lumbar vertebra. Skeletal muscle and intermuscular adipose tissue of specific muscle groups have been identified in different (color) coding. Muscle groups from anterior to posterior: 1) rectus abdominis muscle; 2) lateral muscles: external- and internal oblique muscles and transversus abdominis muscle; 3) psoas muscle; and 4) back muscles: erector spinae muscle and quadratus lumborum muscle. A: right side; B: left side.