A “central” theory for the biomechanics of brain injury is proposed that includes the construct that acceleration of the head, per se, is not the proximate cause of injury. Rather, rapid motion of the skull causes displacement of the hard bony structures of the head against the soft tissues of the brain, which lag in their motion due to inertia and loose coupling to the skull. Relative displacement between brain and skull produces deformation of brain tissue and stretching of bridging veins, which contribute to the tissue-level causes of brain injury.The first step in an accurate interpretation of brain injury risk in dummies involves the measurement of the three-dimensional components of translational and rotational acceleration of the head. The In-Line accelerometry method provides an accurate three-dimensional dynamic measurement in the Hybrid III dummy and uses a row of uniaxial accelerometers to interpret the gradient of rotational acceleration by a linear least-square fit through acceleration responses. Utilizing three orthogonal In-Line rows, and a computer program to post-process the accelerometer data, the six independent components of rigid body dynamics are accurately measured during impact.In this preliminary study, 2D head dynamics data are used as input to a brain compliance model, which interprets the effect of rapid skull motion as tissue-level deformations of the brain. The brain compliance approach interprets brain deformation by the Viscous response (VC or the product of strain and strain rate at the tissue-level) which recent experiments show may be the underlying cause of neural trauma. It excludes the effects of skull deformation. The approach identifies the time of injury risks within the brain, and offers a different and potentially better assessment of head injury than currently available with the HIC criterion.