lumborum

Lumborum is a Latin genitive form used in anatomy to denote structures associated with the lumbar region of the back. It appears in the names of several muscles that run along or within the posterior abdominal and lumbar areas, most notably the quadratus lumborum, and in parts of the erector spinae such as the longissimus lumborum and iliocostalis lumborum.

Quadratus lumborum: this is a square or “quadrate” muscle of the posterior abdominal wall. It originates from the iliac crest and iliolumbar ligament and inserts on the 12th rib and the transverse processes of L1–L4. It is innervated by the ventral rami of T12–L4 (often described as the subcostal nerve T12 with L1–L4 branches). Functionally, unilateral contraction produces ipsilateral lateral flexion of the trunk, while bilateral contraction helps extend and stabilize the lumbar spine and assists in fixing the 12th rib during respiration. Strains of this muscle are a common cause of low back pain.

Longissimus lumborum and iliocostalis lumborum: both are components of the erector spinae muscle group that extend along the lumbar region. Longissimus lumborum originates from lumbar transverse processes and other posterior elements and inserts on higher vertebrae and ribs; its actions include spinal extension and lateral flexion. Iliocostalis lumborum originates from the iliac crest and sacrum and inserts on the lower ribs, contributing to extension and lateral flexion of the spine. Both are innervated by dorsal rami of the spinal nerves.

Notes: the term lumborum reflects location in the lumbar region and is used across multiple muscle names rather than designating a single structure.