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Writer's pictureKat Kocurek

The Ground Reaction Force Explained

Updated: Aug 8, 2023

The ground reaction force (sometimes called GRF) is something that we're all constantly living with and responding to at every moment of our lives. And, what most people don't realize, is that the ground reaction force has a major role to play in helping us find ease in our posture!


In this blog post, I provide some context for what the ground reaction force is, how we use it when we workout (probably without even realizing it), and how we can harness it for comfortable and sustainable postural support. I've also filmed an accompanying movement tutorial for those of you who would prefer to watch and hear me explain some of these concepts.


What is the "ground reaction force" anyway?

The most straightforward way to think about the ground reaction force is through the lens of Newton’s 3rd law of motion.

 

Newton's 3rd Law of Motion

For every action (for every force) there is an equal and opposite reaction (an equal and opposite force).

 

We live on a planet where our bodies are constantly exposed to the “pull” of gravity. You might think of gravity as a downward vector of energy that draws objects (including humans!) towards the earth at approximately 9.8 meters per second squared.


Now, we know from Newton’s third law that this force exerted by our bodies on the ground (based on the pull of gravity) has an equal and opposing force. This is the ground reaction force, and it is an inevitability! As long as we live on a planet where we are exposed to gravity, we will always be in contact with the ground reaction force.



 

Newton's 3rd Law of Motion (revisited)

For every action (i.e. force that our bodies exert upon the ground as a product of mass and gravity) there is an equal and opposite reaction (an equal and opposite force - i.e. the ground reaction force).

 

[Side note: It helps me to work with gravity in this specific context (objects being pulled towards our planet at this measured and validated velocity). That said, if you remember your high school physics you probably remember that Newton’s law of universal gravitation states that any object with mass attracts other objects with mass with a force along a line intersecting the two objects. The objects can be as small as particles or as large as planets. The magnitude of the gravitational force is related to the mass of the two objects and the distance between them, so it can vary. Even here on Earth, the actual magnitude of gravity can be experienced differently depending on elevation, air density, and where you are on the earth's not-perfectly-spherical surface. But those differences are minimal. So for our purposes, I’m using the term “gravity” to define the experience in our atmosphere where we are consistently experiencing a vector of force drawing us towards the earth.]


Where in our bodies we encounter the ground reaction force has to do with how our bodies are positioned in relationship with the earth. Are we upright or lying down? If upright, are we sitting or standing?

  • When standing, your feet are the load-bearing points of contact between your body and the earth, which means that any organization that your body attempts in gravity from standing must begin with the feet and legs. Put differently, in order for your nervous system to harness the ground reaction force to help facilitate upward lift of the body (aka a tall posture), that work starts in the feet.

  • When sitting, your pelvis is the load-bearing point of contact between your body and the earth, which means that any organization that your body attempts in gravity from standing must begin with the pelvis. Put differently, in order for your nervous system to harness the ground reaction force to help facilitate upward lift of the body (aka a tall posture), that work starts in the pelvis.



Ground reaction force in fitness

The ground reaction force is a key player in terms of the benefits that are derived from applied fitness activities like plyometrics (jumping up and then landing increases the ground reaction force that the body is exposed to) or weight lifting (increasing the “mass” of the body increases the force that body applies to the earth, thereby increasing the ground reaction force that the body is exposed to).


As the body is exposed to these larger forces, the tissues in the body experience differing degrees and magnitudes of loads, and our amazing biological processes kick in to respond to those loads and the tissue deformations that arise from them. This is how we expand range of motion, resilience, muscle mass, and bone density. It’s also an avenue by which we can encounter injury (i.e. tissue damage resulting from exposure to load that pushes tissues like tendons, ligaments, and muscles beyond deformation and into tearing state).


Ground reaction force in posture

Beyond these applied fitness activities - the ground reaction force is a key player in any efforts you’re making with posture. More specifically, if you’re wanting your body to feel more “upright” when in a standing or sitting position, the ground reaction force is your ally.


Imagine two scenarios, both where you are standing upright in gravity...


Scenario 1 — Harnessing the Ground Reaction Force

In the first scenario, you’re conscious of the ground reaction force. You know that you can recruit that upward vector of energy to inform your uprightness in space, and you practice that work by rooting your feet down into the earth, and inviting a sense of the crown of your head floating skyward (this is the cue I use most often when working with folks in their early stages of postural adjustment).


You avoid the urge to squeeze or tense large muscle groups, and instead focus on cultivating a feeling of easeful, bi-directional length (lengthening the feet down and the crown of the head up). Your conscious act of rooting down with the feet increases the ground reaction force, and your conscious invitation to allow the crown of the head to float skyward also invites that ground reaction force to transmit up the body, turning on the deep, stabilizing (and mostly subconscious) musculature of the legs, pelvis, spine, and neck. These are your tonic muscles, and I've written an entire blog post about their role in posture!


[Standing Upright — This tutorial demonstrates how to stack your body's centers of gravity in order to harness the ground reaction force. The result? Ease in an upright standing posture! Watch the tutorial>>]


Scenario 2 — Fighting with Gravity

In the second scenario, you’re not thinking about or actively working with the ground force reaction. You’re simply standing on the earth, trying to keep yourself upright.


I would submit that this is most of us, most of the time. We find ourselves slouching, feeling heavy and weighted, and when we want to feel “upright” we thrust ourselves into something that feels like a military position - recruiting our full myofascial system to try to counteract the downward pull of gravity. It might not look much different than when we harness the ground reaction force, but it feels qualitatively different. Specifically, it feels like a lot more work... especially when you consider that your nervous system is likely recruiting both tonic AND phasic muscles to the task.


If tonic muscles are your deep, stabilizing postural muscles that can sustain activation without fatigue, then your phasic muscles are the opposite. Phasic muscles are the large, fast-twitch muscles designed for short bursts of rapid activity. They are innervated to contract more quickly than tonic muscles, but they also tire more quickly (because they are designed for rapid bursts of activity rather than sustained postural efforts).


In this scenario, we can put on the guise of “uprightness” - but at a great energetic expense. It’s nearly impossible to keep this postural pattern for long, and we inevitably succumb to the downward pull of gravity, lest we overexert our phasic muscles and wind up creating dysfunctional patterns of tension (something that also happens quite often!).


[Despite our best conscious efforts to "assume good posture," if we can't harness the ground reaction force to activate our deep, stabilizing tonic muscles, then our phasic muscles will inevitably tire and send us back into a passive, slouchy position. Learn how to find a supported seat using the ground reaction force>>]


Gravity and the ground force reaction are old friends

As humans living on Earth, gravity is omnipresent. That means the ground reaction force is too. We are always interacting with these two forces. As infants, we learn our most basic movements - rolling over, crawling, standing, and walking - by learning to interact with gravity and ground. Our tendon strength and bone density grow and maintain based on the exposure to gravity and ground reaction force (astronauts lose tendon strength in as little as 90 days when in zero-gravity environments, and they lose bone mineral density at a rate of 1% - 2% per month!).


I hope this blog post has provided a new cognitive lens into how these forces are at play, and resources to help you feel your own body's response to gravity and ground reaction force! As always, please don't hesitate to reach out with questions.

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