The iliopsoas is a pair of deep muscles on the front side of your hips that work together as one powerful hip flexor. It’s made up of two muscles, the psoas major and the iliacus. The psoas starts along the front of your lower spine and the iliacus starts on the inside of your pelvis. They join together and attach to the top of your thigh bone. Because of where it runs, the iliopsoas connects your torso to your legs and plays a major role in how you move, stand, and stabilize your body.
Its main job is to flex the hip, meaning it helps lift your knee toward your chest and bring your thigh forward when you walk, run, climb stairs, or do exercises like leg raises. It also helps control how your pelvis sits and how your lower back holds position. If it’s stiff or overactive, it can pull your pelvis into an exaggerated forward tilt, which can make the lower back feel tight and can make your lower abs look less flat even if you’re lean. If it’s weak or not coordinating well with your core, you may feel hip pinching, lose power in sprinting and jumping, or struggle to keep a stable pelvis during ab work.
For visible abs, the iliopsoas matters because many “ab exercises” like hanging knee raises, V-ups, and sit-ups involve hip flexion. If your iliopsoas dominates the movement, you’ll feel it mostly in the front of the hips and thighs rather than in the abs. Learning to keep your ribs down, pelvis controlled, and to move from the trunk instead of yanking the legs is what shifts the work back to the abdominal muscles while keeping the hips healthy.