Iliotibial band syndrome (ITBS) is a common problem for many athletes. The ITB is a thick but flat band of fibrous connective tissue. It connects between the pelvic crest and the tibia (lower leg bone). This means it crosses over the hip and the knee.
Constant and repeated hip and knee flexion and extension cause the ITB to rub against the femur (thigh bone). Friction builds up until pain develops from irritation. Most likely you feel the pain when your knee is about 30 degrees away from full extension.
There are several possible causes of ITBS. In some people, this band is just naturally in a shortened position. Stretching is needed to keep it flexible, especially when overuse causes it to tighten up.
There may be other muscles around the ITB that are tight contributing to the problem. This can include the gluteus maximus (buttock muscle), hip adductors, or the tensor fascia lata (TRL). Adductor muscles along the inside of the thigh pull the leg toward the midline. The TFL is another portion of connective tissue that inserts along the ITB.
Besides stretching the ITB, gluteus maximus, and adductors, you can also lower your bicycle seat height. This decreases the amount of knee extension during cycling. The result is less tension across the ITB. Seat position may also be important.
Moving the seat forward may take extra tension off the gluteus maximus. A more upright position and forward seat combined together reduce the amount of hip flexion. This releases tension on the gluteus maximus, which in turn, decreases tension on the ITB.
If none of these suggestions work, then there may be some other problem that needs addressing. You may have to seek out the services of an orthopedic surgeon or physical therapist for a proper diagnosis and treatment suggestions.