This article will summarize findings from several studies that together allow: 1) the examination of the effect that knee contact angle has on the severity of the resulting impact, 2) examination of the relationship between vertical ground reaction impact forces and leg impact accelerations, and 3) exploration of the adaptations that occur in response to running during changing environmental conditions. Changing the knee flexion angle at contact can alter the effective mass during activities in which the foot impacts the ground. It has been shown that increasing the knee flexion angle at ground contact can reduce the peak vertical ground reaction impact force, but it can also increase the peak impact acceleration at the leg. Attenuation can be calculated from accelerometers on the leg and the head and combined with the leg acceleration values to give a more accurate impression of the severity of the impact. Lower-extremity joint contact angles can be used to examine the kinematic adaptations that take place in response to changing environmental conditions. One common adaptation that can occur when the internal or external environment is not ideal is an increase in the knee flexion angle at contact. More extended knee contact angles can increase the forces experienced by the body and therefore increase injury potential. Increased knee flexion may give the runner a larger margin for dealing with kinematic errors but this benefit likely has an associated metabolic cost that will reduce performance.