Acting unilaterally, which of the erector spinae muscle group/s perform lateralflexion of the spine? Why?

The erector spinae muscle group that performs unilateral lateral flexion of the spine is the Iliocostalis. This muscle group is divided into three parts: Iliocostalis lumborum, Iliocostalis thoracis, and Iliocostalis cervicis.

Here's why:

1. The erector spinae muscles run vertically along the back of the spine. They are responsible for the extension and lateral flexion of the spine and neck.

2. When acting unilaterally, these muscles can cause lateral flexion, which is the action of bending to one side.

3. Among the erector spinae muscles, the Iliocostalis is the most lateral group. This means it's positioned on the outer sides of the back, making it the primary muscle group responsible for lateral flexion.

4. When the Iliocostalis contracts on one side, it pulls the spine to the same side, causing it to bend laterally. This is why it's responsible for unilateral lateral flexion of the spine.