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We introduce effective mass as the preferred alternative to total system mass when analyzing a multi-link model of the human body. Effective mass is defined as the mass that would have equivalent motion characteristics at the point of force application if substituted for the whole system. We demonstrate our findings by deriving formulations for loaded back squat and bench press exercises. Our results suggest that force-plate-derived metrics such as velocity and power contain non-trivial errors when total system mass is used to infer kinematics of external loads. We elaborate on the sources of these errors and how they can compromise the assessment of the validity or reliability of other devices, such as linear position transducers and IMU-based tools.