What is the primary action of the psoas?

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The main thing the psoas does is flex the hip. It runs from the lumbar spine to the lesser trochanter of the femur, crossing the hip joint. When it contracts, it pulls the thigh up toward the trunk, which is why hip flexion is its primary action. It doesn’t extend the knee, since it doesn’t cross the knee joint, and hip abduction is mainly handled by the gluteal muscles rather than the psoas. While the psoas can influence the spine in other ways when the thigh is fixed, its strongest, most direct role is bringing the thigh forward at the hip.