**4. Biomechanics**

The biomechanistic aspect of head trauma is composed of two forces: translation or linear (LA) and rotational (RA) accelerations. The former results from direct impact measured in gravitational force unit (g), whilst the latter results from indirect or whiplash impact and is measured as radians per second squared (rad/s2 ). Upon sudden impact with a surface, the head experiences deformation and deceleration in the same direction as the initial force and result in LA. The bending of the skull produces a wave-like pattern which causes tension propagating from the outer to inner skull. Tension propagation magnitude and direction determine the ensuing fracture initiation.

Intracranial damage occurs as a consequence of either brain motion or pressure gradient established by the LA. Brain motion is proposed to potentiate focal hematoma directly. Other authors proposed that the focal site of an impact is exposed to positive gradient resulting in focal injury and the distal site is exposed to negative gradient resulting in shear stress and cavitation. Previous researches reported a strong correlation between LA and ICP, and ICP with subsequent neurologic dysfunction.

Holbourn was the pioneer researcher who stated that RA-mediated brain injury was caused by shear stress and strain. Impact duration should also be taken into account as different combinations of impact duration and magnitude result in different injury types. Longer duration at a lower magnitude of RA generates diffuse axonal injury, and the opposite generates subdural hematoma. Although LA and RA are often described separately, the inherent coupling of both forces is inevitable in reality [16, 17].
