What is ground reaction force (GRF)?

GRF refers to the force exerted on the athlete by the ground in response to the force exerted by the athlete on the ground. It is defined as mass multiplied by acceleration, and is measured in newton (N). The laws of motion dictate that for every action there is an equal and opposite reaction. The GRF is always present as long as there is physical contact with the ground and works in all directions in which the athlete is exerting force on the ground.

The Motion Catalyst system measures three different types of ground reaction forces: horizontal force, vertical force and rotational force (torque).

Horizontal force refers to the horizontal component of the ground reaction force. This is typically separated in two directions: forces working sideways (mediolateral forces) and forces working forward and backward (anteroposterior forces). The athlete can increase or decrease the horizontal force through movement, by accelerating the body in various directions. Note that horizontal force increases with increased vertical force.

Vertical force refers to the vertical component of the ground reaction force. The vertical force experienced by the athlete is directly affected by the acceleration of gravity, since it works vertically. In addition, the athlete can increase or decrease the vertical force through movement, by accelerating the body away from or toward the ground.

Torque is a measure of rotational force, and is measured in newton meters (Nm).

  • Why do you need rotational and horizontal GRF data?

    Most force plates only provide vertical force data, which is useful when analyzing vertical movements like jumps and lifts. However, having access to only vertical severely limits the types of athletic movements that you are able to assess.

  • Why do you need pressure data?

    A force plate provides the Center of Pressure (CoP) of each foot, but does not tell you whether that pressure is evenly distributed throughout the foot or concentrated in one particular place. In other words you are missing out on essential information on how the athlete is using the ground.

    The pressure data from our combined force and pressure plate shows you exactly how the pressure is distributed for each foot with heat-mapped footprints. This data can help uncover imbalances and provide valuable insights into the athlete’s ability or inability to produce certain ground reaction forces.

  • What separates Motion Catalyst from other sensor plate systems?

    While several companies offer force plates for measuring athletic performance, most of these solutions only offer vertical force. With Motion Catalyst you also get to measure horizontal and rotational force, which give important information for any activities involving rotational or lateral movement. Furthermore, you can combine the force data with detailed pressure data, which is essential for uncovering imbalances that may lead to unstable movements and even injuries.

    In the Motion Catalyst software you can perfectly synchronize all of this force and pressure data with high-speed video images, so you can analyze and understand the whole movement with both visuals and data. Motion Catalyst provides a variety of video analysis tools and organizes your recordings so you can track performance over time.