Can velocity increment derived from a single, lumbar-mounted inertial measurement unit serve as a surrogate for propulsion force impulse in healthy and post-stroke individuals?

BACKGROUND

Paretic propulsion is challenging to measure outside the laboratory. Trunk anterior acceleration reflects late-stance velocity increment, potentially corresponding to the net propulsion force generated by the legs. We evaluated whether anterior velocity increment from a lumbar-mounted inertial measurement unit (IMU) can serve as a surrogate for propulsion force impulse in healthy and post-stroke individuals.

METHODS

Seventeen healthy and thirteen post-stroke individuals completed a 2-min self-paced treadmill walking trial. Step-by-step anterior velocity increments were derived from lumbar IMU acceleration and compared with propulsion force impulses obtained from treadmill-embedded force plates. Group-level and individual-level associations (R2) between lumbar velocity increment and propulsion force impulse, and root mean squared errors (RMSE) were calculated using linear mixed models. To test whether step aggregation improves validity, we compared individual-level R2 and RMSE across 1-, 3-, and 5-step conditions using Friedman tests.

FINDINGS: Group-level associations were significant but weak (marginal R2: healthy = 0.32; paretic leg = 0.15; non-paretic leg = 0.05), with corresponding RMSE values of 0.061, 0.073, and 0.064 N·s/kg. Median [IQR] individual-level R2 was 0.42 [0.34-0.48] for healthy individuals, 0.19 [0.02-0.52] for the paretic leg, and 0.03 [0.02-0.25] for the non-paretic leg, and RMSE values were 0.022 [0.020-0.025], 0.030 [0.023-0.035], 0.038 [0.027-0.042], respectively. Step aggregation did not affect R2 but significantly reduced RMSE.

INTERPRETATION: Lumbar anterior velocity increment showed poor validity for estimating propulsion force impulse in both healthy and post-stroke individuals, suggesting that a single, lumbar-mounted IMU is insufficient for accurate assessment of propulsion force.