In
physics
Physics is the natural science that studies matter, its fundamental constituents, its motion and behavior through space and time, and the related entities of energy and force. "Physical science is that department of knowledge which ...
, and in particular in
biomechanics
Biomechanics is the study of the structure, function and motion of the mechanical aspects of biological systems, at any level from whole organisms to organs, cells and cell organelles, using the methods of mechanics. Biomechanics is a branch of ...
, the ground reaction force (GRF) is the force exerted by the ground on a body in contact with it.
For example, a person standing motionless on the ground exerts a
contact force on it (equal to the person's
weight
In science and engineering, the weight of an object is the force acting on the object due to gravity.
Some standard textbooks define weight as a vector quantity, the gravitational force acting on the object. Others define weight as a scalar qua ...
) and at the same time
an equal and opposite ground reaction force is exerted by the ground on the person.
In the above example, the ground reaction force coincides with the notion of a
normal force
In mechanics, the normal force F_n is the component of a contact force that is perpendicular to the surface that an object contacts, as in Figure 1. In this instance '' normal'' is used in the geometric sense and means perpendicular, as oppos ...
. However, in a more general case, the GRF will also have a component parallel to the ground, for example when the person is walking – a motion that requires the exchange of horizontal (
friction
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. There are several types of friction:
*Dry friction is a force that opposes the relative lateral motion of ...
al) forces with the ground.
The use of the word
reaction
Reaction may refer to a process or to a response to an action, event, or exposure:
Physics and chemistry
*Chemical reaction
*Nuclear reaction
* Reaction (physics), as defined by Newton's third law
*Chain reaction (disambiguation).
Biology and m ...
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.
The component of the GRF parallel to the surface is the frictional force. When slippage occurs the ratio of the magnitude of the frictional force to the normal force yields the coefficient of static friction.
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. This can help create baseline parameters when creating strength and conditioning regimens from a rehabilitation and coaching standpoint. 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. When landing from a safe height in a bilateral comparisons on GRF in relation to landing with the dominant foot first followed by the non-dominant limb, literature has shown there were no significances in bilateral components with landing with the dominant foot first faster than the non-dominant foot on the GRF of the drop-jump or landing on vertical GRF output.
[Ball, NB, Stock, CG, Scurr, JC. Bilateral Contact Ground Reaction Forces and Contact Times During Plyometric Drop Jump. Journal of Strength and Conditioning Research; October 2010 – Volume 24 – Issue 10- pp 2762-2769]
References
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Mechanics
Biomechanics