  Impact induced traumatic brain injury (ITBI) describes brain injury from head
impact not necessarily accompanied by skull fracture. For sufficiently abrupt
head impact decelerations, ITBI results from brain tissue stress incurred as
the brain crashes into the inside of the skull wall, displacing the surrounding
cerebral spinal fluid (CSF). Proper helmet cushioning can damp the impact force
and reduce ITBI. But force is mass times acceleration and commonly used helmet
blunt impact standards are based only on acceleration thresholds. Here I show
how this implies that present standards overestimate the minimum acceleration
onset for ITBI by implicitly assuming that the brain is mechanically decoupled
from the body. I quantify how an arbitrary orientation of the body with respect
to impact direction increases the effective mass that should be used in
calculating the required damping force and injury threshold accelerations. I
suggest a practical method to incorporate the body mass and impact angle into
ITBI helmet standards and provide direction for further work.
