In addition to their synergistic functions, cruciate and collateral ligaments exercise basic antagonistic function during rotation. A, In external rotation, it is collateral ligaments that tighten and inhibit excessive rotation by becoming crossed in space. B, In neutral rotation, none of four ligaments is under unusual tension. C, In internal rotation, collateral ligaments become more vertical and are more lax, whereas cruciate ligaments become coiled around each other and come under strong tension. ACL, Anterior cruciate ligament; LCL, lateral collateral ligament; MCL, medial collateral ligament; PCL, posterior cruciate ligament.