The trunk muscle/fat mass measurement protocol in this study has been previously explained [28 (link)]. Ten consecutive computed tomography (CT) images at the mid-L4 vertebral level were acquired using a Philips MX 8000 IDT CT scanner (Philips Medical Systems, Cleveland, OH, USA), with exposure at 200 mAs, tube voltage set at 120 kV, and 1 mm slice thickness. Total muscle mass (TMM, cm3) and total fat mass (TFM, cm3) in the trunk region were obtained using image processing software (Extended Brilliance Workspace version 4.5.3; Philips Healthcare, Best, The Netherlands). Pre-defined radiation attenuation ranges were used to demarcate adipose (from −190 HU to −30 HU) and muscle (from −29 HU to +150 HU) tissues. TMM was subdivided into back muscle mass (BMM, cm3), psoas muscle mass (PMM, cm3), and abdominal muscle mass (AMM, cm3). BMM and PMM were derived manually from the TMM. AMM was obtained by subtracting the PMM and BMM from the TMM. The BMM consisted of the multifidus, iliocostalis lumborum, longissimus, and quadratus lumborum muscles. Visceral fat mass (VFM, cm3) was calculated by manually outlining the inner abdominal wall, and subcutaneous fat mass (SFM, cm3) was calculated by subtracting the VFM from the TFM. To reduce bias, one technician performed all scan and image processing procedures.
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