What’s a Force Plate for?

I find many non-engineers intimidated or confused by force plates. I’ve worked extensively with a variety of force plates, and have built one of my own. I thought I would share some wisdom on how they work, what they tell you, and how you shouldn’t be afraid of them!
So what is a force plate?! 

a force plate is a piece of equipment which measures the ground reaction force (GRF) when it is stepped on. They are typically buried into a floor or walkway to measure under-foot forces during walking. It is very similar to a bathroom scale, except while a scale gives you a magnitude (it is known as a scaler, no pun intended), a force plate gives you a magnitude AND a direction (it is known as a vector).

Force plates demonstrate Newton’s 3rd law: for every reaction there is an equal an opposite reaction because they show that for however much force your put on the ground, there is an equal force that the ground imparts on your foot, in the exact opposite direction.

top: a Kistler force plate which uses piezoelectric crystals
middle: an AMTI force plate which uses strain gauges
bottom: the custom force plate that I built at the Structure and Motion Lab @ the RVC which uses strain gauges
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How they work

There are two main types of force plates, strain-based and piezoelectric. Strain-based force plates are less expensive to produce, while piezoelectric force plates can usually measure over a larger range of forces.

Strain-based force plates measure the tiny amount of deformation that occurs when the plate is loaded. This deformation measurement is usually done with strain gauges, arranged in what are called Wheatstone Bridges. Strain gauges, which are essentially electrical resistors, vary their resistance based on how much they are stretched. By placing groups of these gauges in key spots throughout the force plate structure, and connecting them all through electrical circuits, the whole plate deformation can be understood. Through calibration, this whole-plate deformation can be translated to whole-plate force.

Piezoelectric force plates utilize the unique principles of piezoelectric crystals. Piezoelectric crystals generate electricity based on how much pressure they are under. By placing just a few of these crystals within the plate, the force vector applied to the plate can be calculated by combining the charge generated at each crystal. Piezoelectric force plates are simpler than strain-based force plates in that there are far less piezoelectric crystals than there are strain gauges, however piezoelectric crystals are a rare material and thus are expensive.

What they can tell you

In clinical biomechanics analysis, force plate data is typically used in a few different ways. The most basic is for observing the GRF itself to asses ones “impact” with the ground. This is usually done by looking at a graph of the vertical GRF vs. time, or by displaying the GRF vector as an arrow representative of its magnitude and direction (both shown below).

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The force plate data, when used in tandem with  a motion capture system, can also give information about forces and moments occurring within the musculoskeletal system. While the motion capture system can tell you about the motion of the body segments and joints, force plate data under-foot allows you to calculate forces and moments within those segments and joints. This process is called inverse dynamics.

Finally, force plate data can be used to calculate the center of pressure (CoP) during the stance face of a stride. Force plates which are “6-axis” (can independently measure forces and moments in three orthogonal directions) can accurately perform this calculation. The CoP can be used to track the 2D position of weight bearing throughout stance or to help locate a foot in 3D when a motion capture system isn’t being used.

top: a typical vertical GRF, normalized to body weight
bottom: a subject (me) walking over force plates, with the plate location and vector overlaid.

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