To better understand the role of preventive knee braces in injury prevention, a biomechanical study using fresh frozen cadaveric knees (N = 18) was conducted. Ligament tensions and joint displacements were measured at static, nondestructive valgus forces as well as low-rate destructive forces. After quantifying and establishing individual ligament contributions to valgus restraining function, knees were then braced with two different laterally applied preventive braces, the McDavid Knee Guard and the Omni Anderson Knee Stabler. The effects of lateral bracing were analyzed according to valgus force, joint line opening, and ligament tensions. Valgus applied forces, with or without braces, consistently produced medial collateral ligament (MCL) disruptions at ligament tensions surprisingly higher than the anterior cruciate ligament (ACL) and higher than or equal to the posterior cruciate ligament (PCL). Although large joint displacements were necessary for complete ligament failure, bundle disruption in the MCL, ACL, and PCL was noted at much smaller joint openings. In Part I of this study, no significant protection could be documented with the two preventive braces used. Also, four potentially adverse effects were noted: MCL preload, center axis shift, premature joint line contact, and brace slippage.