In human anatomy, the hip flexors are a group of skeletal muscles that act to flex the femur onto the lumbo-pelvic complex, i.e. "pull the knee upward".
The hip flexors are considered to be the most important muscle in the running process.
The hip flexors are (in order of importance to the action of flexing the hip joint):
Without the iliopsoas muscles, flexion in sitting position is not possible across the horizontal plane.