In the pulling assessment, an excessive low-back arch indicates overactivity in the hip flexors and erector spinae. The NASM CPT7 Study Guide explains that these muscles contribute to anterior pelvic tilt and lumbar extension when overactive.
Underactive muscles in this scenario typically include the intrinsic core stabilizers and gluteus maximus, which are not adequately resisting the pull into lumbar extension. Other muscle groups listed-middle/lower trapezius, upper trapezius, and levator scapulae-are more relevant to scapular movement and postural compensations in the upper body, not lumbar extension patterns. Thus, the overactive muscles here are hip flexors and erector spinae.