Does IMU Step measure Ground Reaction Forces?

No, IMeasureU quantifies tibial acceleration.

IMU Step sensors measure tibial acceleration which allows our sensors to pick up on subtle changes in lower limb movement and allows you to determine how a person changes their loading pattern with fatigue, footwear, recovery, etc.

Ground Reaction Force (GRF) is the result of the summed accelerations of ALL body segments. Because GRF takes the entire body into account, not just the lower limb, it is not possible to extrapolate GRF from tibial acceleration. Or to think about it the other way, Ground Reaction Force is also not a good predictor of tibial load.

Tibial acceleration is a much better surrogate for tibial force which allows us to track loading patterns, monitor injuries, and measure only lower limb movement.