Lower extremity injuries due to automobile accidents are often overlooked, but can have a profound societal cost. Knee injuries, for example, account for approximately 10% of the total injuries. Fracture of the knee is not only an acute issue but may also have chronic, or long term, consequences. The criterion currently used for evaluation of knee injuries in new automobiles, however, is based on experimental impact data from the 70''s using seated human cadavers. These studies involved various padded and rigid impact interfaces that slightly alter the duration of contact. Based on these data and a simple mathematical model of the femur, it appears fracture tolerance increases as contact duration shortens. In contrast, more recent studies have shown mitigation of gross fractures of the knee itself using padded interfaces. The use of padded interfaces, however, result in coincidental changes in contact duration and knee contact area. Therefore, it is difficult to extract the direct effect of loading rate on fracture tolerance of the knee. The object of the current study was to isolate the effect of loading rate alone on fracture tolerance of the human knee joint. Paired experiments were conducted on eight pairs of isolated cadaver knees impacted with a rigid interface to approximately 5 kN at a high (5 ms to peak) or low (50 ms to peak) rate of loading. Gross fracture and occult microfractures of the knee joint were documented. A second part of the study examined some chronic effects of loading rate on "subfracture" injuries in an animal. Thirty-four rabbits were subjected to a "subfracture" knee load at the same rates as used in the human studies. Alterations in the mechanical properties of retropatellar cartilage and thickening of subchondral bone were documented out to one year post "subfracture" trauma to the joint.The current study documented an opposite effect than that expected based on 70''s experiments with seated cadavers. There was an increase in the number of gross fractures and occult microfractures in high versus low rate of loading experiments. A similar effect was also seen in the "subfracture" chronic animal experiments, which showed relatively more degradative change in the mechanical properties of cartilage following high versus low rate of loading experiments. There was also a significant increase in subchondral bone thickening underlying cartilage and increased fissuring of cartilage in high versus low rate of loading experiments. The current study suggests a relative decrease in tolerance of the knee at high versus low rates of loading in acute experiments with human cadavers and in the chronic setting with animals. Therefore, it would appear that rate of knee loading may be an important issue in establishing a future injury criterion for the knee itself.