The long fibers of the superficial medial collateral ligament are the primary stabilizers of the medial side of the knee against valgus and rotatory stress. The long fibers as a functional unit have a complex pattern in which the anteriormost fibers tighten as the knee flexes from the position of extension; simultaneously, the fibers just posterior to them slacken. The long fibers arise from a critical point on the medial femoral condyle relative to the instant centers of rotation such that the anterior border is kept under tension from full extension to 90 degrees of flexion. This may be of fundamental importance for surgical reconstruction of torn medial ligaments.