Abdominal muscles are involved in all the following actions except:

The abdominal muscles play a significant role in a variety of movements and functions, primarily related to the stabilization and movement of the trunk. They are actively involved in forceful expiration, where they contract to push air out of the lungs. The abdominal muscles also contribute to pelvic tilting and flexion of the trunk, enabling movements such as bending forward or stabilizing the pelvis during various activities.

In the context of back extension, this action is primarily facilitated by the muscles located in the posterior chain, particularly the erector spinae group, which work to extend the spine. Therefore, abdominal muscles are not directly involved in this action, as their primary functions focus on flexion and stabilization of the trunk, rather than extension. This distinction clarifies why back extension is not an action associated with the abdominal muscles.