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# 9.3: Mechanisms of Injury

X-Ray image showing a clavicle (collar bone) fractured during a car accident. Image Credit: Clavicle Fracture Left uploaded by Majorkev via Wikimedia Commons

## Relating Injuries to Stress

We have previously learned that body tissues, like any other material, will undergo permanent deformation when under applied stress beyond their yield point. We also learned that materials will fail and eventuallyfracture or rupture when stressed beyond their ultimate strength. For example the clavicle in the previous image fractured because it experienced a stress that was greater than the ultimate strength of bone. Sprains occur when a ligament (connects bone to bone) is torn by a stress greater than its ultimate strength, or even just stretched beyond its elastic region. The same events occurring in a tendon (connects muscle to bone) are known as strains.[2] We already know that strain has a different, but related, meaning to physicists and engineers, so that terminology discrepancy is something to watch out for.

## Relating Stress to Force and Motion

Typically body tissues do well at handling the internal stresses generated while in static equilibrium or dynamic equilibrium, that is holding still or moving at constant speed. For example the stress applied to the Achilles tendon while standing or slowly raising the heels at constant speed is unlikely to exceed the ultimate strength of the tendon. However, rapid changes in motion (large acceleration) require large net force, as determined by Newton's Second Law. Remembering that the stress applied to a material is the applied force divided by the cross-sectional area, we understand that large forces associated with high accelerations can lead directly to injury level stress on tissues. As a result, medical professionals and first-responders often treat patients who experience mechanisms of injury (MOI) that involve rapid changes in motion as having spinal and/or internal injuries until confirmed otherwise by medical imaging or complete examination. For example, landing on a hard surface after falling from a height greater than a few feet. The remainder of this unit will examine the relations between forces and motion, allowing us to analyze techniques and technologies for controlling motion in order to reduce force, stress, and risk of injury.