The psoas are part of the iliopsoas muscle group, or hip flexors, that run from your lower spine to the lesser trochanter, which is a small, bony protrusion in the inner part of the bone near the groin. They work with other muscles in your abdominal and pelvic region to move your hip joints.
Phases of Contraction
All muscles have three phases of movement: concentric, eccentric and isometric. Concentric contraction is the shortening of the psoas while under tension, such as lifting your knee up toward your ribs from a standing position, according to the National Academy of Sports Medicine. Eccentric contraction is the lengthening of the psoas while under tension, such as what happens when you kneel on the ground and extend a leg behind you. Isometric contraction is holding the psoas in one position for a period without movement. In most exercises, such as squats, hip extension, and lunges, your psoas perform all phases of movement, allowing your hips to move in coordination with other body parts.
Flexion and Extension
Your psoas work with other muscles in your hips and legs to flex and extend the hip joints. These are the primary movement patterns of the hip joints when you walk, run, or kick in a straight line. The psoas also laterally flexes on one side and laterally extends on the opposite side of the the torso when you bend your torso to one one side. When you lie on the ground and lift your torso to an upright position, your psoas assist in pulling your torso up.
Internal and External Rotation
Internal and external rotation involve the psoas to turn with other leg muscles and hip rotators to your left and right. This allows your torso to turn with a wider range of motion than without turning your hip joints. You can sense the way the psoas and buttocks move together by standing with your legs about hip-width apart and then rotating your right leg to your left and right with the balls of your feet and your toes in contact with the ground. This is essential to changing directions when you run, twisting your legs and hips when you dance, or performing kicks across your body.
Tight psoas pull on your lumbar spine, which places excessive stress in the lower back, causing low back pain and hyperactive thighs. This is often caused by either too much sitting or moving with too much hip flexion, such as leg lifts, says fitness professional Anthony Carey, author of "Pain-Free Program." To alleviate tight psoas, perform hip extension exercises, which stretch the psoas to reduce their tightness and strengthen the buttocks.