A force plate is a specialized measuring instrument used to quantify the mechanical interaction between a body and the surface it is contacting. The device is a rigid platform embedded with sensors that capture the forces exerted by a person standing on, walking across, or jumping from its surface. This technology is foundational in biomechanics and movement science, providing objective data that describes how human motion is generated and controlled.
The Primary Measurement: Ground Reaction Forces
The core function of a force plate is to measure the Ground Reaction Force (GRF), which is the force exerted by the ground back onto the body in response to the body pushing down. According to Newton’s third law, this force is equal and opposite to the force applied by the person to the plate. When a person stands still, the GRF equals their body weight, but during movement, the force dynamically changes based on acceleration.
A force plate measures this interaction in three distinct, perpendicular axes to fully characterize the force vector. The vertical axis (\(F_z\)) is often the largest and relates to weight support, lift, and landing forces, such as those generated during a vertical jump. The anterior-posterior axis (\(F_y\)) runs from front to back, representing the horizontal forces involved in movement, providing insight into braking and propulsive forces during walking or running. The medial-lateral axis (\(F_x\)) runs from side to side, measuring forces related to balance and movement stability.
Key Calculated Metrics
The raw data from the three-dimensional Ground Reaction Force is used to calculate more insightful metrics about the quality and control of movement. The most common of these derived metrics is the Center of Pressure (COP), which is the calculated point on the plate where the total GRF vector is considered to be applied. Analyzing the path and speed of the COP provides a quantitative measure of a person’s postural sway and stability, as it tracks the subtle shifts in weight distribution during standing or dynamic tasks.
Beyond the COP, the force-time data allows for the calculation of Impulse, which is the force applied over a period of time and directly relates to the change in momentum of the body. Calculating the integral of the force-time curve is useful in evaluating the effectiveness of a movement like a squat or jump, showing the net force delivered to the ground. The data also allows for the calculation of moments, which are the rotational forces, or torques, acting on the plate. These rotational forces, particularly around the vertical axis (free torque), are used in combination with GRF and COP data to model forces acting at a person’s joints.
How the Technology Works
Force plates rely on specialized transducers to convert mechanical energy from the applied load into a measurable electrical signal. The two main types of sensors used are strain gauges and piezoelectric sensors. Strain gauge-based systems use thin wires or foils bonded to a resilient structure that slightly deforms when a force is applied. This deformation causes a change in the electrical resistance of the gauge, which is then measured and converted into a voltage signal proportional to the applied force.
Piezoelectric force plates, in contrast, use crystal material that generates an electrical charge when subjected to mechanical pressure. The magnitude of this charge is directly related to the force exerted on the crystal. Multiple sensors are strategically placed beneath the plate’s surface, typically at the four corners, to allow for the simultaneous measurement of forces in all three axes.
Practical Applications
Force plate data is utilized across a wide range of fields to objectively assess and optimize human movement. In sports performance, the technology measures the power and strength characteristics of athletes. Specific tests like the countermovement jump (CMJ) or isometric mid-thigh pull (IMTP) quantify an athlete’s ability to generate force quickly, helping coaches tailor training programs and monitor readiness. Analyzing data from dual force plates can reveal asymmetries in force production between the left and right limbs, which is useful for injury risk screening.
In clinical and rehabilitation settings, force plates provide objective data for gait analysis and assessing balance impairments. Therapists track the COP path to quantify postural stability, a method used to assess recovery following a neurological event or an orthopedic injury. By measuring the magnitude and timing of forces during walking, clinicians can identify deviations in movement patterns and monitor a patient’s progress toward a safe return to activity. Force plate data also serves as a foundation for biomechanical research, allowing scientists to develop and validate complex models of human locomotion.