How the Psoas Helps Swing a Leg Forward
The psoas major crosses the hip joint, so its pull can move the thigh relative to the pelvis—or help steady the lower back when the leg is planted. It works alongside the iliacus, a neighboring muscle inside the pelvis; together, the pair are often called the iliopsoas, the body’s main hip flexor.
When you lift a knee, the iliopsoas shortens and draws the thigh toward the front of the body. Walking uses that action in a small, repeated swing: after the foot leaves the ground, the muscle helps bring the leg forward for the next step. Climbing stairs calls for a stronger lift because the knee must rise high enough to clear the next step.
A Muscle That Also Helps Steady the Trunk
The psoas begins along the sides of the lower spine and passes through the pelvis to attach near the top of the thighbone. Because it spans both the trunk and the hip, its job changes with what is held still. If the pelvis and spine are steady, it can lift the thigh; if the leg is fixed, it can help control movement around the lower spine and pelvis.
That two-ended arrangement makes the psoas part of coordinated movement, not a single-purpose knee-lifting lever. Each step depends on muscles around the hip and trunk working together to move the leg while keeping the body balanced. The psoas contributes to the lift, but it does not act alone.
