The longissimus portion is a muscle located in the lower back and upper buttock. It is a part of the erector spinae group of muscles, which are responsible for maintaining posture and supporting the spine. The longissimus portion originates at the lower six thoracic vertebrae and the lumbar vertebrae, and inserts at the transverse processes of the lower six thoracic vertebrae and the sacrum. It is responsible for extending and laterally flexing the vertebral column. In daily life, it is used in activities such as standing up from a seated position and bending to pick up objects.