Stain guages are are too stiff to measure stress. piezo Accelerometers can measure G but cables become microphonic. Instrumenting all the stress points with accelerometers would be impractical.
For auto collision tests, with dummies, they can measure certain zones with accelerometers. but I would rely on high speed photography with fiducial marks (+) to measure impact g =Δx/Δt²
If dummy is colliding with floor due to gravity without restraint and examining shock of landing with flexible matts vs rigid matts from different heights, you can easily estimate shock. using ratio of distances assuming linear forces.
g = Δd.h. / Δs.h. ( fall due to gravity = drop height / stop height )
so falling say from 1m corner post into middle of floor that moves say 10 cm is 1.0/0.1 = 10 g ,if the body was rigid.
This could be injure body so the collision must be broken to avoid full impact on subject.