  Wearable devices have been shown to effectively measure the head movement
during impacts in sports like American football. When a head impact occurs, the
device is triggered to collect and save the kinematic measurements during a
predefined time window. Then, based on the collected kinematics, finite element
(FE) head models can calculate brain strain, which is used to evaluate the risk
of mild traumatic brain injury. To find a time window that can provide a
sufficient duration of kinematics for FE analysis, we investigated 118 on-field
video-confirmed head impacts collected by the Stanford Instrumented Mouthguard.
Because the individual differences in brain geometry influence these
calculations, we included six representative brain geometries and found that
larger brains need a longer time window of kinematics for accurate calculation.
Among the different sizes of brains, a pre-trigger time of 20 ms and a
post-trigger time of 70 ms were found to yield calculations of brain strain and
strain rate that were not significantly different from calculations using the
original 200 ms time window recorded by the mouthguard.
