Towards a “more valid” method of validation: application of effective mass for error resolution of force plate measurements

We introduce effective mass as the preferred alternative to total system mass when analyzing a multi-link model of the human body. Effective mass (EM) 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 (BS) and bench press (BP) exercises. Our data suggest that force-plate-derived metrics such as velocity and power contain nontrivial 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 motion capture systems, LPTs, and IMU-based tools.