Which of the following is NOT a muscle in the torso group?

The psoas major is a muscle that primarily belongs to the hip flexor group rather than the torso group. Its primary function is to flex the hip joint and assist in stabilizing the spine. While it does connect to the lumbar spine, it is not categorized as part of the abdominal or torso muscles, which primarily focus on movements and stability of the trunk.

In contrast, the transverse abdominis, rectus abdominis, and external oblique are all key muscles located in the torso. The transverse abdominis plays a significant role in providing core stability and maintaining intra-abdominal pressure. The rectus abdominis is responsible for flexing the lumbar spine and contributing to the overall appearance of the abdominal region, and the external oblique aids in trunk rotation and lateral flexion. Each of these muscles is integral to the functioning of the torso as part of core stability and movement, further solidifying the classification of the psoas major as the option that does not belong in the group.