Biomechanics is literally the study of the "mechanics" of "life" (or living organisms). Thus, biomechanics is a very broad field of science and expertise that may include many sub-specialties unrelated to traffic accident reconstruction or traffic collisions.
Biomechanics in the arena of traffic collision analysis and accident reconstruction can be narrowed down and simply defined as the study of human motion (also known as occupant kinematics or occupant dynamics). It includes the anatomy, kinematics, kinetics, and kinesiology of vehicle occupants and/or pedestrians involved in collisions. Biomechanical analysis provides an important link between traditional accident reconstruction results and injury potential or injury causation from a medical perspective. Specifically, accident reconstruction results such as vehicle speed, angle of impact, delta-v, principal direction of force, and vehicle acceleration / deceleration characteristics are correlated to the biomechanical response and movement of a vehicle occupant with respect to these collision parameters, and addresses what anatomical parts of the body are affected.
In this context, biomechanical analysis represents the synergy of traditional engineering and physics principles with knowledge of human anatomy, physiology, and injury tolerance.