A force platform, or force plate, measures the resultant forces transmitted through its surface; multicomponent platforms also measure moments about defined axes. It is used in gait and balance experiments to observe external loading during contact. A manufacturer's six-output description specifies three force and three moment components from which a center of pressure can be calculated.[ref]
A platform combines a load-bearing surface, force-sensitive transducers, signal acquisition and a calibration relating sensor outputs to mechanical quantities. Applying known loads at known locations tests both force response and moment sensitivity. Off-diagonal calibration terms can matter when a nominal load produces responses in several channels; the instrument indication must be interpreted in its specified coordinate frame.
An original calculation chooses the origin on the top surface, with normal to it and a positive normal resultant . For a resultant represented at plus a residual moment normal to the plate,These signs follow from in the stipulated right-handed frame. A manufacturer origin below the surface or a different force-sign convention requires the corresponding transformation; the formulas are not coordinate-free instructions.
For chosen readings , and , the calculated point is . If the same moment uncertainty is divided by a much smaller , position uncertainty increases. At zero normal load this ratio does not define a meaningful contact location.
The center of pressure is a summary of the measured loading, not the body's center of mass. Motion observations can supply additional evidence for an inverse-dynamics analysis, but the force platform alone does not directly measure every joint force or muscle action. If several contacts share one plate, its resultant can also conceal their separate contributions. Recording which contacts occurred and synchronizing the platform with motion measurements makes the mechanical interpretation testable.
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