With population aging and the widespread use of abdominal CT，opportunistic assessment of skeletal muscle using existing imaging data has received increasing attention. The paraspinal muscles are an important component of the active stabilizing system of the lumbar spine and contribute to segmental stability，postural control，and load transmission. With advancing age，these muscles may exhibit atrophy，reduced CT attenuation，and increased fatty infiltration； changes in their structure and tissue composition have been associated with low back pain，lumbar degenerative changes，and functional impairment. The third lumbar vertebral （L3） level is generally included in abdominal CT scans and can be readily identified. At this level，the cross-sectional area，mean CT attenuation，and fatty infiltration of the posterior paraspinal muscles （hereafter referred to as the L3-level paraspinal muscles） may reflect local muscle degeneration in terms of muscle quantity and tissue composition in middle-aged and older adults. This review summarizes the anatomical definition of the L3-level paraspinal muscles，CT-based quantitative parameters，scanning and segmentation methods，and requirements for methodological standardization； describes age-，sex-，and body size-related differences in these parameters； reviews evidence regarding their associations with low back pain，lumbar degenerative changes，and bone mass parameters； and discusses their potential value in risk identification and opportunistic assessment. It aims to provide a reference for standardizing CT-based quantitative assessment of the L3 paraspinal muscles，facilitating comparisons across multicenter studies，and promoting clinical translation.