Erector spinae
With the broad muscles removed, the erector spinae shows as a thick column of muscle on each side of the spine. It starts on the sacrum, the iliac crest and the spines of the lumbar vertebrae and climbs to the ribs, the vertebrae and the skull. In the lower back it can be felt as two firm ridges beside the spine.
Deep muscles of the back, the iliocostalis, longissimus and spinalis of the erector spinae and the shorter muscles beneath them, Gray's Anatomy, 1918, figure 389Wikimedia Commons, Henry Vandyke Carter, Public domain (Gray389.png)
It splits into three strips side by side. The iliocostalis is the outer one and attaches to the ribs. The longissimus in the middle is the longest and reaches the skull behind the ear. The spinalis, the thinnest, runs next to the spines of the vertebrae.
The erector spinae straightens the bent back and holds it upright. When a person bends forward it lets go slowly to control the movement, and in full forward bending it relaxes and the ligaments take the load. Under it, shorter muscles join each vertebra to the next.
Sources
- Susan Standring (ed.), Gray's Anatomy, 42nd ed. (2020), chapter on the back.
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