Which muscle is NOT primarily involved in maintaining posture?

The pectoralis major is primarily a muscle involved in the movement of the upper body, specifically functions related to the shoulder joint, such as adduction and internal rotation of the arm. While it does contribute to some stabilization of the shoulder, it does not play a significant role in maintaining overall body posture compared to the other muscles mentioned.

In contrast, the soleus, quadratus lumborum, and erector spinae are all crucial for postural stability. The soleus, for instance, is vital for maintaining an upright posture, especially during activities like standing, as it helps control the position of the tibia relative to the ankle and prevents forward lean. The quadratus lumborum stabilizes the lumbar spine and pelvis, providing lateral stability during movement and supporting posture. The erector spinae group is essential for upright posture and is primarily responsible for extending the spine and maintaining spinal alignment.

Thus, while the pectoralis major serves important functional roles in movement, it is not a primary muscle for maintaining posture like the others listed.