[2] The use of the word reaction derives from Newton's third law, which essentially states that if a force, called action, acts upon a body, then an equal and opposite force, called reaction, must act upon another body.
The force exerted by the ground is conventionally referred to as the reaction, although, since the distinction between action and reaction is completely arbitrary, the expression ground action would be, in principle, equally acceptable.
[3] GRF is often observed to evaluate force production in various groups within the community.
One of these groups studied often are athletes to help evaluate a subject's ability to exert force and power.
Plyometric jumps such as a drop-jump is an activity often used to build greater power and force which can lead to overall better ability on the playing field.