CSAs of trunk muscles Transverse MR spin-echo T1-weighted images were obtained at the L3-4 level parallel to the lumbar disc space (repetition time, 760 ms; echo time, 20 ms; matrix, 256 × 265; field of view, 320 mm; slice thickness, 5.0 mm). The image was traced onto paper and the traced image was then transferred to a computer in order to measure CSAs. CSAs were calculated using image analysis software (Scion Image Beta 4.02, Scion Corp., Frederick, MD, USA), and grouped into 5 large areas because they had poorly defined borders. Each of the 5 areas was represented by the same CSA on the left and right sides of the transverse image (rectus abdominis, obliques, psoas, quadratus lumborum, and lumbar erector spinae plus multifidus). The 5 areas were summed to determine the total area. Three of the CSAs included multiple muscles: obliques, psoas, and lumbar erector spinae). Oblique muscles comprise the internal and external obliques and transversus abdominis. Psoas muscles comprise the psoas major and minor muscles. The lumbar erector spinae includes the iliocostalis, longissimus, and spinalis. All CSAs were normalized by dividing the values by the athlete's body weight. This method was used because most of the combat sports adopt the weight category system, and a previous study reported the correlation between CSAs of the trunk muscles in athletes according to their body mass.