Introduction
Force plates have been around for over 40-years. They are tools used for the measurement of ground reaction forces during walking, jumping, or any other type of movement (they can be thought of as a fancy bathroom scale). Force plates use a one of several different types of sensors (load cells) to measure forces, and the corresponding values are typically presented in Newtons (N)—the standard measure across physics. Beyond vertical force, some force plates can measure shear forces—i.e. lateral and horizontal forces. These come into play more specifically for healthcare and research applications, such as orthotic bracing and studies of specific neuro-muscular diseases.
Single Force Plate System |
Bilateral Force Plate System
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Force plates measure ground reaction forces. If we know the frequency of the force-data, we can then perform additional measurements that provide us with far more information about human movement. With some basic math we can use this information to measure a whole range of things, including:
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Velocity (m/s)
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Power (Watts)
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Displacement (Meters)
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Temporal parameters (seconds)
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Left/Right Asymmetry (for bilateral systems)
Just think - all of this ONLY from the force-time-curve: The image below shows a force-time curve for a common force plate test, the Countermovement Jump. A Countermovement Jump (CMJ) is used to measure an athlete's maximal vertical jumping ability.
Pressure Plates?
There is a bit of misinformation (or perhaps, just vague marketing) about data that can be collected with a force platform, when in fact the devices in question are pressure plates. Pressure plates use a larger number of sensors (typically in the hundreds, if not thousands, depending on size) to give an idea of the pressure distribution of an athlete or patient. Pressure plates can measure force, but only vertical force through the summation of all the sensors bearing weight. This leads them to have less accuracy, but still allows them to provide valid and usable data for some circumstances. In many cases, pressure measurement is performed at a lower sampling rate than force measurement, which renders them less useful for specific applications like jumping. Essentially, using a pressure plate for jump testing, while painting a clear picture of how pressure is distributed beneath a subject’s feet or shoes, is akin to using a jump mat—i.e. calculations on jump height are reliant on contact/flight time, and not velocity. To be clear, a force plate can’t measure pressure distribution - so no "heat maps" like you might have seen at the pharmacy.