Force plates underpin much of applied biomechanics, and the numbers derived from them are frequently reported as though measured directly. What the instrument records is narrower and more specific than the outputs suggest.
The measurement is force against the ground
Sensors beneath the plate register the force applied to its surface, resolved into vertical, forward-backward and side-to-side components, sampled many times per second.
By Newton's third law this equals the force the ground applies to the body. That reaction force is the only external force available to accelerate an athlete horizontally.
The plate also locates where on its surface the net force acts, described as the centre of pressure. This is a calculated location rather than a physical point on the foot. It moves through the foot during contact as weight transfers forward.
Jump height is derived, not observed
A plate cannot see how high someone jumps. Jump height is calculated from the vertical velocity at the instant the athlete leaves the plate, obtained by integrating the force trace.
That integration requires an accurate body mass and a genuinely still period before the movement begins. Errors in either propagate through the entire calculation.
This is why protocols insist on a quiet standing phase, and why a jump performed with an early countermovement can produce a distorted result. Trials are commonly discarded for that reason alone.
Rate of force development depends on where the start is defined
How quickly force rises is a widely used output, and it depends entirely on where the analysis decides the movement began.
Different thresholds for that starting point produce substantially different values from the same trial, which is why comparisons between studies and systems require knowing the processing method. The reported number depends on a choice as much as on the athlete.
Filtering choices have a similar effect, since smoothing the trace changes the steepness of its early portion. Two software packages can report different values from identical raw data.
Two plates answer different questions from one
A single plate records the combined force of both limbs and cannot separate them. Asymmetry measurements require one plate under each foot.
Even then, the plate reports force distribution rather than the joint-level cause of it, which requires motion capture or modelling to infer. An asymmetry can arise at the hip, the knee or the ankle.
What the instrument cannot tell you
Forces inside the body, at a tendon or a joint surface, are not measured. They are estimated through models with assumptions that are difficult to validate in a living athlete.
The plate is therefore excellent for describing external output and consistency over time, and weaker as evidence about internal loading or injury risk. Its strength is tracking one athlete against their own history.
