The iliotibial tract or iliotibial band (also known as Maissiat's band or IT Band) is a longitudinal fibrous reinforcement of the fascia lata. The action of the ITB and its associated muscles is to extend, abduct, and laterally rotate the hip. In addition, the ITB contributes to lateral knee stabilization. During knee extension the ITB moves anterior, while knee flexion moves the ITB posterior. It originates at the anterolateral iliac tubercle portion of the external lip of the iliac crest and inserts at the lateral condyle of the tibia at Gerdy's tubercle. The figure shows only the proximal part of the iliotibial tract.
The part of the iliotibial band which lies beneath the tensor fasciae latae is prolonged upward to join the lateral part of the capsule of the hip-joint.
It stabilizes the knee both in extension and in partial flexion, and is therefore used constantly during walking and running. In leaning forwards with slightly flexed knee the tract is the main support of knee against gravity.
It is usually developed by people who suddenly increase their level of activity, such as runners who increase their mileage. Other risk factors for ITBS include gait abnormalities such as overpronation, leg length discrepancies, or bow-leggedness. ITB Syndrome is an overuse condition of the distal ITB near the lateral femoral condyle and at Gerdy's tubercle. The most vulnerable range of knee flexion for this condition is at 30-40 degrees; this is where the ITB crosses the lateral femoral condyle.