Iliotibial band syndrome (ITBS) — also known as iliotibial band friction syndrome — is a common and often maddeningly stubborn repetitive strain injury that causes pain mainly on the side of the knee, especially when descending stairs and hills. The injury mostly plagues runners, but it’s not their problem exclusively: a few unlucky cyclists and hapless hikers will get it, and even some relatively inactive people who are more vulnerable in the aftermath of a knee surgery.