Ground Reaction Force & how to use it
What is Ground Reaction Force (GRF)? Newton’s 3rd Law of Motion states that “For every action there is an equal and opposite reaction.” According to that, when we walk or run, every time our foot lands, there is what is called a ‘Ground Reaction Force’ produced.
Some go as far as to claim that “the ground reaction force is an important external force acting upon the human body in motion” and “we use this force as propulsion to initiate and to control the movement.” And so on. You could find plenty of examples of such statements. GRF could be considered a pretty important external force acting upon the body in motion. But what is propelling who and where – is a totally different discussion. Let’s not get into what really initiates or controls the movement of a human body. I guarantee you that Ground Reaction Force has less to do with it than it is often suggested.
Experiment. Stand up and while standing on one foot, raise the other and then land it by driving it down. Try not to hurt yourself and notice the result – did your foot bounce back up? No. One more time – raise your foot and drive it down harder – your foot didn’t bounce back again? What does it tell you (or at least should tell you)? Ground reaction force was applied to the foot, not to the general center of mass (which is the prerogative of gravity), and because of that, by definition, GRF can’t move GCM.
Ground reaction force is a non-propulsive force, i.e it is not an active force that moves your body forward, or backward or in any direction for that matter.
If GRF worked the way some propose – we’d be all walking around in a very different manner and with more care, after all we don’t want to get the “reaction” from the ground when we stomp around or slam our feet down. It might have been advantageous to us if we were on the Moon where gravity is less. But then again, even on the Moon, it would be more useful in jumping from place to place, rather than the actual running. Running over there is not the same as it is over here.
Rodolfo Margaria wrote about it way back, but some of us are still not paying attention. “They (on the Moon) still had the same muscle force as on Earth and the same support on the ground, but they couldn’t run and walk in the same bipedal manner. It means that they couldn’t make a push off. The Moon’s gravity force is only 1/6 of the Earth’s gravitational force, which seemingly immediately “eliminated” forward propulsion from muscle work. Here the question arises, why couldn’t the muscles perform their task? Isn’t it easier to do it in reduced gravity?”
Getting back to our little experiment – did you feel shock (and maybe pain if you hit it hard enough) dissipating through your leg after you landed the foot? I bet you did. What does THAT tell you?
Ground reaction force provides us with feedback. It is a “mirror” that we can and should use to correct our movement. Ground Reaction Force can be successfully used to identify and correct errors in technique, and that has obvious implications and tremendous benefits in Physical Therapy and athletic coaching.
As it often goes, scientific evidence and research are dragging their feet behind the current developments. There is so much information and so many different opinions and just a couple of studies with some data recorded and confusing conclusions made. In practical application on the level useful to health and fitness professionals, runners and athletes, GRF is simpler than it is often presented.