METRICS THAT MATTER: What Is Ground Reaction Force (GRF)?
From Newton’s Laws to Next-Gen Run Tech
Long before biomechanics became a buzzword, Sir Isaac Newton gave us the foundation with his third law: for every action, there is an equal and opposite reaction. Apply that to your foot hitting the pavement—and boom—you’ve got ground reaction force (GRF).
Every time you run, your foot pushes down into the ground. The ground pushes back. That pushback is the GRF. It's a critical measure of how much force your body has to absorb and redirect during movement.
Why It Matters in Running
GRF is a key indicator of:
Impact load on your body — higher GRF = more force your joints must manage.
Injury risk — excessive or poorly distributed GRF can contribute to stress fractures, shin splints, and joint strain.
Performance potential — elite sprinters like Usain Bolt often generate massive GRF, but in highly efficient, well-distributed ways.
What the Science Says
A study in The Knee found that runners with a softer landing and increased knee flexion at impact exhibited lower GRFs and reduced injury rates. Meanwhile, forefoot strikers often display shorter contact times but higher peak GRFs—revealing a tradeoff in style vs. strain.
GRF also varies with speed, terrain, footwear, and fatigue. Monitoring changes over time is essential for performance and injury prevention.
Couro’s Take
At Couro, Ground Reaction Force isn’t just a data point—it’s a pillar of how we evaluate athletic performance. GRF is one of the key metrics in the Kinetics category of our Couro Running Score, which makes up 35% of your total score. That means how much force you apply to the ground—and how efficiently you do it—has a direct impact on how your run is graded.
Why? Because GRF reflects your power potential, but also your injury risk. Too low, and you may not be generating the force needed for speed or acceleration. Too high—especially with poor form—and you risk overloading your joints. That’s why Couro doesn’t just measure your force output, we contextualize it: comparing it against your movement patterns, symmetry, and alignment to help you understand whether your stride is powerful, efficient, and safe.
In short: high force alone isn’t good or bad—it’s how you apply it, and how your body handles it, that matters. Couro captures that nuance and reflects it directly in your personalized run score.
Stay tuned. As our understanding of biomechanics evolves, we’ll continue updating this post with the latest research and tools to help you run smarter.