Biomechanics, the study of the structure and function of biological systems by means of the methods of mechanics, is applied to injuries of the ankle and shoulder. We compare injury patterns to the magnitude, duration, and direction of forces occurring as the result of both accidental and criminal insults. The ability to compare common injury mechanisms to the expected body movements during a precipitating event is important in evaluating the likelihood of a causal relationship between the two. Knowledge of the common mechanisms of various injuries, combined with a review of other evidence related to the injury claim, will aid the biomechanist and/or accident reconstructionist in determining the relationship, or lack thereof, between the precipitating event and the injury claim.