The collateral ligaments at the knee prevent __________.

When the knee is fully extended, these two ligaments are tight, and they prevent any rotation at the knee joint. When the knee is flexed, the MCL and LCL are slack. The knee which is a modified hinge joint, is able to internally and externally rotate when the knee is flexed. The medial collateral ligament (MCL) is on the medial side of the knee and prevents valgus forces. This means it keeps the knee from bowing inward, as when someone is “knock-kneed”. The lateral collateral ligament (LCL) is on the lateral aspect of the knee and prevents varus forces which push the knee laterally, like being bow-legged.