The semispinalis thoracis is a muscle located in the back, part of the semispinalis group. It extends from the transverse processes of the lower thoracic vertebrae to the spinous processes of the upper thoracic and lower cervical vertebrae. This muscle plays a crucial role in stabilizing and rotating the spine. Functionally, the semispinalis thoracis aids in extending the thoracic region of the spine and contributes to posture. It works in conjunction with other muscles in the back to facilitate movements such as bending and twisting, helping maintain overall spinal health and mobility.