OpenGRF: prediction of ground reaction forces and moments during activities of daily living using OpenSim

Abstract

OpenGRF is a new OpenSim-based tool that estimates ground reaction forces and moments (GRF&Ms) and centre of pressure (CoP) directly from joint kinematics, addressing an existing limitation of current OpenSim workflows. The approach combines calibrated foot–ground contact probes with an optimization framework derived from static optimization, and computes CoP by accounting for both kinematic and dynamic contributions. Its performance was validated against force-plate measurements. Validation involved 126 marker-based trials from seven healthy adults, each performing six trials of level walking, stair ascent and stair descent. Results showed the highest accuracy for vertical GRF, with normalized root mean square error (nRMSE) ≤ 1.5% and Pearson's correlation coefficient (ρ) ≥ 0.94, particularly during level walking. Anterior–posterior GRF was also satisfactorily predicted (nRMSE 4.4%–6.1%, ρ = 0.81–0.91). By contrast, medial–lateral GRF was less reliable, especially in stair tasks, with errors up to 11.2% and correlations as low as 0.48. Free moments were the hardest variable to estimate (nRMSE up to 28%), whereas ankle moments showed excellent agreement with force-plate data (nRMSE ≈ 1.7%, ρ ≈ 0.98). Median CoP error was 21–23 mm, with the largest discrepancies during double support. Overall, OpenGRF enables physics-consistent estimation of GRF-related variables, particularly the vertical component, from kinematics alone, although generalizability is limited by the tested sample.