I concur with Adam's advice to use small accelerometers. Also, try to attach them at sites where there is no soft tissue between skin and bone, e.g. the anterior-medial side of the tibia. Have a large enough area of contact so the sensor can't wobble.

I was wondering how you define "impact acceleration at the sacrum". Is there a clear impact peak? If not, you could be measuring the vertical motion of the pelvis under the influence of muscles and gravity, probably around 1 g amplitude. In that case, the acceleration at the sacrum does not necessarily need to be smaller than at the tibia.

And a pet peeve of mine, please use the term "damping", not "dampening". The latter is about moisture, not mechanics.

Ton van den Bogert