If a client's lower back arches during the Overhead Squat Assessment, this indicates excessive lumbar extension and an anterior pelvic tilt, often due to overactive hip flexors and erector spinae, and underactive core stabilizers-especially the transverse abdominis. NASM identifies the transverse abdominis, internal obliques, and multifidus as key local stabilization system muscles that contract isometrically to stabilize the LPHC during movement. Weakness in the transverse abdominis allows the pelvis to tilt forward and the lumbar spine to overextend, which appears as an arch in the lower back. Strengthening this deep abdominal muscle helps maintain a neutral spine under load.