Table 1.

Dataset 1 PSM MRI variables included in the analysis for each one of the muscles and spinal segment combination: multifidus (MF; L1–S1), erector spinae (ES; L1–L5), psoas (Ps; L1–L5), and quadratus lumborum (QL; L1–L4). A total of 64 MRI variables are defined (4 for each combination of muscle and spinal segment)

Variable	Image source	Definition
%FCSA_asym	T2	Percentage of lean muscle calculated as the fraction of lean cross-sectional area respect to total area. Asymmetry between left and right muscle calculated as the absolute difference
CSA_asym	T2	Total cross-sectional muscle area. Asymmetry between left and right muscle calculated as the absolute difference
%FCSA_avg	T2	Percentage of lean muscle calculated as the fraction of lean cross-sectional area respect to total area. Average between left and right muscle
CSA_avg	T2	Total cross-sectional muscle area. Average between left and right muscle