The tensor fasciae latae (TFL) is a muscle of the upper outer thigh. It stretches vertically down the outside of the leg from the hip to the knee via the iliotibial (IT) band. Among other things responsible for the abduction of the hip, or the lifting of the leg to the side, it is significant for its role in stabilizing the knee relative to the hip.
This muscle finds its origin on the outside of the iliac crest, the bony prominence on the ilium that can be felt on the top outer part of the hip, and along the anterior superior iliac spine, the ridge running downward from the crest. The tensor fasciae latae arises from between two muscles: the gluteus medius on the back outer part of the hip and the sartorius, which begins on the outside of the hip and crosses the front of the thigh. It then runs downward from the hip, angling slightly outward with its fibers running parallel, to insert along the IT band about a third of the way down the outer thigh.
A distinction concerning this muscle’s origins is that it is a continuation of the fibers of the fascia lata, a deep layer of connective tissue that encapsulates and separates the muscles of the hip. The fascia lata is interwoven with the tendon into which the large gluteus maximus muscle inserts, and just below this point is where the tensor fasciae latae arises, with the fibers of the fascia lata encasing the TFL in two layers to the inside and outside. As such, this muscle is noteworthy for its role in assisting the glutes in stabilizing the knee joint during extension.
Where the two layers of the fascia lata surrounding the tensor fasciae latae converge, they form the IT band, which is much like a long and narrow tendon running down the outer thigh. Like a chain of rivers and lakes, the sequence of muscles alternating with the fibrous connective tissue of the fascia lata forms the structure that links the hip to the knee joint. As a muscle that abducts the hip in conjunction with the gluteal muscles, the tensor fasciae latae prevents the hip — and therefore the thigh and knee — from buckling inward by pulling slightly outward on the leg when the hip is extended, as in standing position. In doing so, it helps align the knee with the hip and ankle like a column supporting a building.