Just learned that wonderful cut of steak by the name of tenderloin actually comes from the psoas major muscle. The reason it's so tender and juicy is that it's a non-weight bearing muscle. That alone makes it prized but it's not a terribly large muscle so it's limited in quantity. Combine the two and you get an expensive but tasty piece of meat. In humans, it's the major muscle (along with the iliacus) that lifts your thigh up at the hip, aka your hip flexor.