  Inertial sensors are commonly used to measure human head motion. Some sensors
have been validated with dummy or cadaver experiments, but methods to evaluate
sensors in vivo are lacking. Here we present an in vivo method using high speed
video to evaluate teeth-mounted (mouthguard), soft tissue-mounted (skin patch),
and headgear-mounted (skull cap) sensors during 6-13g sagittal soccer head
impacts. Sensor coupling to the skull is quantified by displacement from an
ear-canal reference. Mouthguard displacements were within video measurement
error (<1mm), while the skin patch and skull cap displaced up to 4mm and 13mm
from the ear-canal reference, respectively. We used the mouthguard, which had
the least displacement from skull, as the reference to assess
6-degree-of-freedom skin patch and skull cap measurements. Linear and
rotational acceleration magnitudes were over-predicted by both the skin patch
(with 120% NRMS error for a_mag, 290% for alpha_mag) and the skull cap (320%
NRMS error for a_mag, 500% for alpha_mag). Such over-predictions were largely
due to out-of-plane motion. To model sensor error, we found that in-plane
acceleration peaks from the skin patch in the anterior-posterior direction
could be modeled by an underdamped viscoelastic system. In summary, the
mouthguard showed tighter skull coupling in vivo than the other sensors.
Furthermore, the in vivo methods presented are valuable for investigating skull
acceleration sensor technologies.
