Final answer:
The tensor fascia latae muscle lies deep to the iliotibial tract, stabilizing the knee by pulling on the iliotibial band, and works in conjunction with other lateral rotator muscles deep to the gluteus maximus.
Step-by-step explanation:
The muscle that lies deep to the iliotibial tract is the tensor fascia latae muscle. It is a thick, squarish muscle located in the superior aspect of the lateral thigh and is important for stabilizing the knee by making the iliotibial band taut. The tensor fascia latae muscle synergizes with the gluteus medius and iliopsoas muscles to flex and abduct the thigh. Additionally, deep to the gluteus maximus, there are other muscles like the piriformis, obturator internus, obturator externus, superior gemellus, inferior gemellus, and quadratus femoris that play roles in the lateral rotation of the femur at the hip.