Which of the following is typically not an underactive muscle within the trunk?

The psoas is typically considered an overactive muscle when assessing muscle function within the trunk, particularly due to its role in hip flexion and lumbar spine stabilization. An overactive psoas can often contribute to an anterior pelvic tilt and promote compensatory patterns that may lead to instability or discomfort in the lower back.

In contrast, the transverse abdominis, multifidus, and rectus abdominis are generally regarded as underactive muscles in individuals who exhibit poor core stability or strength. The transverse abdominis plays a vital role in providing stability by acting like a natural corset around the abdomen, while the multifidus is important for spinal stabilization through its function in segmental movement control. The rectus abdominis, while primarily responsible for flexing the trunk, is also important for maintaining posture and alignment.

Understanding the roles these muscles play in core stability helps in designing corrective exercise programs. Recognizing that the psoas is often overactive, while the others may be underactive allows for targeted interventions to achieve a balanced muscular function that supports proper movement patterns and overall stability in the trunk.