The longissimus muscle

The longissimus muscle forms the central column of the erector spinae muscle group and is the longest and thickest of this group. It is divided into three regions based on their attachments:
Longissimus capitis
Longissimus cervicis
Longissimus thoracis, which is further subdivided into thoracic and lumbar parts.
Generally speaking, longissimus muscle is a powerful extensor of the vertebral column. However, its exact function depends on the degree of engagement of the different muscle parts. Bilateral contraction of the entire muscle results in extension of the lumbar, thoracic and cervical spine, along with extension of the head and neck. Unilateral contraction of the longissimus muscle results in lateral flexion of the spine on the same side (ipsilaterally). An alternated unilateral contraction of the left and right longissimus aids the walking process by leveling the pelvis and steadying the vertebral column.