The erector spinae muscles is not just one muscle, but a group of muscles and tendons which run more or less the length of the spine on the left and the right, from the sacrum or sacral region (the bony structure beneath the lower back [lumbar] vertebrae and between your hips/glutes) and hips to the base of the skull.

The erector spinae muscles are really three muscles: spinalis muscle, longissimus muscle, and iliocostalis muscle. To get more specific, anatomists subdivide each of these three muscles into smaller muscles based on the region of the spine they are in

They are also known as the sacrospinalis group of muscles. These muscles lie on either side of the vertebral column spinous processes (the bony points up and down the middle of the back) and extend throughout the lumbar, thoracic, and cervical regions (lower, middle, and upper back and the neck). The erector spinae is covered in the lumbar and thoracic regions (lower back and lower middle back) by the thoracolumbar fascia, and in the cervical region (neck) by the nuchal ligament.