Hi Everyone

We have completed an experiment measuring impact acceleration at both the tibia and sacrum (using Shimmer inertial sensors, Dublin) while recreational runners ran for 15 minutes on a treadmill. The sensors were attached with double sided tape, elastic tape over the sensors onto the skin and with an elastic belt wrapped firmly around the runner's waist. The sensors were aligned with the longitudinal axis of the segment (tibia, sacrum)

While for the majority of runners the impact acceleration at the sacrum was lower than at the tibia, for a few individuals we found the opposite, the sacrum acceleration is actually higher.

No studies to date appear to have reported this phenomenon, in stead suggesting the signal will always be lower because it is dampened as it propagates through the body.
for example:
Reenalda, Jasper, Erik Maartens, Jaap H. Buurke, and Allison H. Gruber. "Kinematics and shock attenuation during a prolonged run on the athletic track as measured with inertial magnetic measurement units." Gait & posture 68 (2019): 155-160. (measured at the tibia and sacrum)

Shorten, Martyn R., and Darcy S. Winslow. "Spectral analysis of impact shock during running." International Journal of Sport Biomechanics 8, no. 4 (1992): 288-304. (measured at the tibia and head)

We have filtered the signal at 60 Hz (with a bidirectional fourth-order Butterworth filter) and we removed any linear trend from the signal (Shorten and Winslow, 1992)

Has anyone else experienced this situation in assessing runners?
Does anyone have an explanation to why the impact acceleration would be greater at the sacrum?

Many thanks


SHORTEN MR, WINSLOW DS: Spectral analysis of impact shock during running. Int J Sport Biomech 8: 288, 1992.