The longissimus muscle with its three subdivisions is part of the erector spinae muscle group in the back. Alternately known as the sacrospinalis, the erector spinae muscles run parallel to the spine from the base of the skull to the base of the spine. Located laterally to the semispinalis, another erector spinae muscle, the subdivisions of the longissimus include the longissimus thoracis, longissimus cervicis, and the longissimus capitis.
The longissimus capitis is the uppermost of the longissimus muscles. It originates via attaching tendons on the transverse processes of the top four or five thoracic vertebrae as well as those of the bottom three or four cervical vertebrae. From there it runs up the back of the neck to either side of the spinal column and inserts into the mastoid process, a bony prominence on the underside of the temporal bone on each side of the skull. This muscle’s job is to flex the head and neck sideways, toward the side on which the contracting muscle is located.
Next down the vertebral column is the longissimus cervicis, the second largest of the longissimus group. Found just beneath the capitis and slightly medially, it also originates on the transverse processes of the top four or five thoracic vertebrae, emerging from long tendons. The cervicis, however, inserts via tendons into the second through sixth cervical vertebrae. Its action is to flex the head and neck sideways as well as assist in extension of the upper spine.
The largest of the longissimus muscles is the longissimus thoracis, which spans almost the entire length of the back. It shares its point of origin with the iliocostalis lumborum, another sacrospinalis muscle, arising from the transverse processes of the lumbar vertebrae. From there the muscle stretches to the thoracic spine where it attaches to all of the thoracic vertebrae as well as the lower nine or ten ribs. The longissimus thoracis works bilaterally, or on both sides, to extend the entire vertebral column.
Together with the iliocostalis and spinalis muscle groups and their accompanying tendons, the longissimus group’s predominant function is to extend the spine, or pull the torso backward from a forward-hunched position. Because most extension occurs in the lumbo-thoracic region of the spine, or the middle to lower back, these muscles are collectively larger in this region, becoming shorter and narrower as they approach the top of the spine. The erector spinae muscle group can become quite tight among individuals who sit hunched over a desk all day; this effect can be combated by incorporating lower back stretches as well as allover core strengthening moves into a regular exercise program.