First up, lets consider the function of the psoas major muscle. As well as being a primary hip flexor, it may also assist with lumbar extension in someone with a normal lordosis by forward tilting the pelvis. It assists flexion of the spine when bending as well as some external rotation of the hip. This has some bearing on how best to stretch the psoas which should not only include extension but also some internal rotation. This muscle has also been considered a stabiliser of the spine and hip joint and contributes to upright posture in sitting and standing.
As a pain referral source we often think of the psoas major referring to the anterior thigh and groin, but must remember that pain may also be projected in a vertical direction in a ‘gutter’ along either side of the lower lumbar spine as well as to the sacroiliac region and buttock (Travell & Simons, 1999)
Image courtesy of Primal Pictures Anatomy for Acupuncture