A force platform is a flat, rigid plate embedded with sensors that measures the forces your body exerts against the ground during movement. When you stand, walk, jump, or land on one, it captures the equal and opposite reaction the ground pushes back with, known as the ground reaction force. These measurements are broken into three dimensions and recorded hundreds of times per second, giving clinicians, researchers, and coaches a precise picture of how you move, load your joints, and maintain balance.
How a Force Platform Works
Every time your foot contacts a force platform, sensors inside the plate detect pressure and translate it into data. Most platforms use either piezoelectric crystals (which generate a tiny electrical charge when compressed) or strain gauges (which change resistance when deformed). A typical clinical-grade platform, like those made by Kistler, contains four sensor units, one near each corner. By comparing the output of all four sensors at any given instant, the system calculates not only how much total force is being applied but exactly where on the plate that force is concentrated.
The sensors are oriented in multiple directions, so the platform captures force in three axes simultaneously: vertical (up and down), anteroposterior (front to back), and mediolateral (side to side). Vertical force is the largest component during most activities. When you jump, for example, the platform tracks the downward push of your legs against the plate, and when the reading drops to zero, it knows you’ve left the ground. The front-to-back and side-to-side components reveal subtler details, like whether you’re shifting your weight forward during a squat or drifting sideways while standing still.
What It Actually Measures
The raw output of a force platform includes three force components and three moment components (rotational forces around each axis). From these six channels of data, several useful metrics are derived:
- Ground reaction force (GRF): The three-dimensional force vector the ground applies to your body. During walking, the vertical GRF typically peaks at around 1.0 to 1.2 times your body weight with each step. During running or jumping, it can reach two to five times body weight.
- Center of pressure (CoP): The single point on the platform surface where the total force is effectively concentrated. As you shift your weight, the CoP moves. Tracking its path over time is one of the most common ways to assess balance.
- Impulse: The total force accumulated over a period of time, calculated by integrating the force curve. In a vertical jump, the net vertical impulse before takeoff directly determines how high you’ll fly.
Clinical and research platforms typically sample at 1,000 Hz, meaning they record data 1,000 times per second. Standard gait analysis can be performed at lower frequencies, with useful gait patterns captured at rates above roughly 71 Hz, but the higher sampling rates are important for capturing fast, high-impact events like landing from a jump or a sprinter’s foot strike.
Center of Pressure and Balance Assessment
One of the most widely used applications of a force platform is measuring postural stability. When you stand quietly on the plate, your CoP traces a wandering path as your body makes constant small corrections to stay upright. A healthy person’s CoP path stays within a small area and moves smoothly. Larger, more erratic CoP excursions suggest impaired balance control.
This makes force platforms valuable for assessing conditions that affect stability. They’re used to evaluate balance deficits in people with Parkinson’s disease, stroke survivors, individuals with diabetic neuropathy, and people recovering from lower-limb amputation. In orthopedic settings, CoP measurements help track recovery after hip replacement surgery and monitor postural changes in patients with chronic low back pain. For older adults, force platform testing can identify fall risk by measuring how well someone maintains balance during challenging tasks like walking, turning 180 degrees, and returning to a start point.
Reading a Force-Time Curve
The data from a force platform is most commonly visualized as a force-time curve: a graph with time on the horizontal axis and force (in Newtons) on the vertical axis. Learning to read this curve is central to using the platform effectively.
Take a countermovement jump as an example. Before the jump, the curve sits flat at a level equal to the person’s body weight. When they start to dip down, the curve drops below body weight because they’re momentarily unloading the plate. This is the start of the eccentric (lowering) phase. The curve then rises sharply above body weight as the legs brake and reverse direction, transitioning into the concentric (pushing) phase. Force peaks during this upward drive, then drops rapidly to zero at the instant of takeoff. The shape of that concentric peak varies meaningfully between athletes. Some produce a single large hump, others show a plateau followed by a late peak, and still others display two distinct peaks of roughly equal size. These different waveform shapes reflect distinct jump strategies and muscle coordination patterns.
During walking, the vertical force-time curve has a characteristic “M” shape for each step: a peak at initial contact when your heel strikes, a dip in the middle as your body passes over the foot, and a second peak as you push off with your toes.
Sports Performance Metrics
In athletic settings, force platforms go well beyond simple force measurement. Coaches and sport scientists use them to derive performance metrics that guide training decisions.
Rate of force development (RFD) measures how quickly an athlete can produce force, expressed in Newtons per second. It’s essentially a measure of explosive strength. Higher RFD values are linked to better performance in jumping, sprinting, cycling, weightlifting, and even golf swings. RFD can be calculated as a single average across the entire push, or in narrow time windows (such as the first 50, 100, or 200 milliseconds) to pinpoint whether an athlete is fast out of the gate or builds force more gradually. The time-interval approach and peak RFD using 20-millisecond windows are considered the most reliable measures.
Reactive Strength Index (RSI) is another common metric, calculated by dividing jump height by ground contact time during a drop jump or repeated hop. It captures how well an athlete absorbs and redirects force. Together, these metrics help coaches identify weaknesses, track the effects of a training program, and make return-to-play decisions after injury by comparing the injured limb’s force production to the healthy side.
Clinical and Rehabilitation Uses
In gait laboratories, force platforms are typically mounted flush with the floor so that patients walk across them naturally. Multiple plates may be arranged in sequence to capture several consecutive steps. The force data is often combined with motion capture cameras that track reflective markers on the body, allowing clinicians to calculate the forces and torques acting at individual joints like the ankle, knee, and hip.
This combined analysis is used to plan surgical interventions for children with cerebral palsy, evaluate prosthetic limb alignment in amputees, and assess rehabilitation progress after knee or hip surgery. In fall prevention programs for older adults, platform-based balance testing provides objective, quantifiable scores that clinical observation alone can’t match. While the equipment is more expensive and requires trained operators, it offers precision that complements simpler bedside balance tests.
Calibration and Accuracy
Force platforms require calibration to produce trustworthy data. Manufacturers provide initial calibration procedures, but accuracy can drift over time or change when the plate is installed in a new location, embedded in a floor, or mounted on a treadmill. Post-installation calibration is important because mounting conditions affect sensor behavior.
One validated approach uses an instrumented pole equipped with its own force sensors. The pole is pressed against the platform surface at various locations and angles while a motion capture system tracks its position. This generates reference data that can be compared to the platform’s output, and any systematic errors in force, moment, or center-of-pressure readings are corrected mathematically. When no motion capture system is available, simpler procedures using known vertical loads placed at measured locations on the plate surface can still improve accuracy beyond factory defaults.