What is Ground Reaction Force?

What is Ground Reaction Force?

Ground reaction force is one of the most important ideas in strength and power training, and it comes straight from Newton's third law of motion: for every action there is an equal and opposite reaction. When you stand on the floor you push down on it with a force equal to your bodyweight, and the floor pushes back up on you with exactly the same force. That upward push from the floor is the ground reaction force. If it were not there, or were smaller than your weight, you would sink through the floor; if it were larger, you would accelerate upward. Nothing you do in a gym moves your center of mass without it. You cannot pull yourself up by your own bootstraps, so every time you jump, sprint, or stand up out of a squat, the force that actually launches your body comes from the ground, not from your muscles directly. Your muscles create the push, but the ground supplies the reaction that accelerates you. When you are simply standing still, the ground reaction force equals your bodyweight and points straight up through your base of support. The moment you push harder into the floor than your bodyweight, as at the bottom of a jump or the drive out of a squat, the ground pushes back with more than bodyweight and you accelerate upward. Push with less, as when you unweight to begin a countermovement, and the reaction drops below bodyweight and you begin to fall. GRF is a genuine external force you can measure directly with a force plate, in newtons, and it is the single number that best captures how forcefully an athlete interacts with the ground.

How it works

Ground reaction force works through a simple chain: your muscles pull on your skeleton, your feet push on the ground, and the ground pushes back. Because the reaction is equal and opposite, the harder and faster you can push into the floor, the larger the force that accelerates you upward and forward. This is why so much of strength and power training is really about improving how much force you can apply to the ground and how quickly. In a vertical jump the vertical ground reaction force during the push-off often reaches two to four times bodyweight, and the size of the upward impulse, which is force multiplied by the time it acts, decides how fast you leave the floor and therefore how high you jump. A squat is the same physics under a bar: at the bottom you must generate a ground reaction force greater than the combined weight of you and the bar to reverse the movement and stand up, and the sticking point is simply where the force you can produce comes closest to the force required. Sprinting is perhaps the clearest case. Classic force-plate research by Weyand and colleagues showed that faster top running speeds are achieved mainly by applying greater ground forces during each brief foot contact, not by moving the legs more rapidly. The best sprinters hit the ground harder in the fraction of a second the foot is down, and that larger reaction force is what propels them. Ground reaction force is not purely vertical, either. During sprinting and change of direction it has horizontal components that drive you forward or sideways, and the direction of the resultant vector matters as much as its size. Training that improves GRF includes heavy squats and deadlifts to raise maximum force, and jumps, sprints, and plyometrics to improve how much force you can express in the very short time the foot is on the ground. Reactive strength, the ability to absorb the landing force and return it quickly, is essentially skill at handling large ground reaction forces in a stretch-shortening cycle.

The formula

GRF (vertical) = m x (g + a)

The vertical ground reaction force equals your mass times gravity plus your vertical acceleration. Standing still, a = 0 and GRF equals bodyweight (m x g). Accelerating up, a is positive and GRF rises above bodyweight; unweighting, a is negative and GRF drops below it.

How to apply it

• Build maximum force with heavy squats: Heavy back and front squats raise the peak ground reaction force you can produce. A bigger ceiling of force means every jump, sprint, and change of direction has more to draw on. Train in the 3 to 6 rep range for strength.
• Train rate of force development with jumps: The foot is on the ground for a fraction of a second in sprinting and jumping, so you must express force fast. Vertical and broad jumps teach you to build large ground reaction force in that short window, which raw strength alone does not guarantee.
• Sprint to apply horizontal force: Top speed depends on how hard you strike the ground each step. Short maximal sprints of 20 to 40 metres with full recovery train the large, brief vertical and horizontal reaction forces that drive fast running.
• Use plyometrics for reactive strength: Depth jumps and pogo hops train the stretch-shortening cycle: absorb a large landing force and return it quickly. This reactive strength is the ability to handle and reuse ground reaction force, central to sprinting and jumping.
• Push the floor, not the bar, mentally: Cueing yourself to drive your feet through the floor rather than to move the weight often improves force output out of a squat or clean. It aligns your intent with the physics: the ground gives you the force that moves the bar.
• Fix the sticking point with force at the right angle: The sticking point is where required force is closest to available force. Pause squats and partials at that height build the specific ability to keep the ground reaction force high through the hardest joint angles of the lift.

Worked example

This is a simplified vertical ground reaction force trace for an 80 kg athlete doing a countermovement jump. Bodyweight is about 785 N. Watch how GRF drops below bodyweight during the dip, spikes far above it during push-off, briefly hits zero in the air, then peaks again on landing.

The height of the jump is set by the upward impulse, the force above bodyweight multiplied by the time it acts, during push-off. Bigger and faster force into the ground means a bigger reaction and a higher jump.

Maximum force vs rate of force development

A big maximum ground reaction force is useless in a sprint if you cannot express it in the tenth of a second your foot is down. Strength sets the ceiling; rate of force development decides how much of it you actually use in fast actions.

By goal

• Sprinters and jumpers: Prioritise producing large ground reaction force quickly. Combine heavy strength work to raise the ceiling with sprints, jumps, and plyometrics that train you to apply that force in the brief moment the foot contacts the ground.
• Powerlifters: Your total is a ground-reaction-force problem: stand up with more force than the bar plus you weigh. Build maximum force with heavy squats and deadlifts, and attack sticking points where required force nears your limit.
• General fitness and older adults: The ability to produce ground reaction force quickly is what lets you rise from a chair, climb stairs, and catch a stumble. Sit-to-stand work, step-ups, and light jumping preserve this power as you age.

Common misconceptions

• "Your muscles lift you, not the ground." Your muscles create the push, but the force that actually accelerates your center of mass comes from the ground pushing back. You cannot accelerate yourself without an external reaction force, which is exactly what ground reaction force provides.
• "Ground reaction force only points straight up." It points opposite to however you push into the ground. In sprinting and change of direction it has large horizontal components that drive you forward or sideways. The direction of the resultant force matters as much as its size.
• "Faster running comes from moving your legs quicker." Force-plate research found that faster top speeds are achieved mainly by applying greater force to the ground during each foot contact, not by repositioning the legs faster. Elite sprinters hit the ground harder in the same brief contact time.
• "GRF equals your bodyweight in every lift." Only when you are not accelerating. The instant you push harder than your weight, as at the bottom of a jump or the drive of a squat, the ground pushes back with well over bodyweight, often two to four times it during explosive efforts.

Related terms

Ground Reaction Force FAQ

What is ground reaction force in simple terms?

It is the force the ground pushes back on you with when you push into it. Because of Newton's third law it is equal in size and opposite in direction to your push, and it is what actually accelerates your body in every jump, sprint, and squat.

How is ground reaction force related to Newton's third law?

Newton's third law says every action has an equal and opposite reaction. When your feet push down on the floor, the floor pushes back up with the same size force. That reaction from the floor is the ground reaction force, and it is what moves you.

How big is ground reaction force in a jump?

During the push-off of a vertical jump, the vertical ground reaction force often reaches two to four times bodyweight, and landing forces can be higher still. The larger and faster the force into the ground, the higher the resulting jump.

Why does ground reaction force matter for sprinting?

Research by Weyand and colleagues showed faster top running speeds come mainly from applying greater force to the ground during each brief foot contact, not from moving the legs faster. Hitting the ground harder in the same short time is what drives elite speed.

Can I increase my ground reaction force?

Yes. Heavy squats and deadlifts raise the maximum force you can produce, while jumps, sprints, and plyometrics train you to express that force quickly in the short time the foot is on the ground. You usually need both together.

Is ground reaction force always vertical?

No. It points opposite to however you push into the ground. Standing still it is vertical and equals bodyweight, but in sprinting and cutting it has large horizontal parts that propel you forward or sideways as well as up.

What does ground reaction force feel like at a squat sticking point?

The sticking point is where the force you can produce comes closest to the force required to keep moving the bar. Your ground reaction force is barely above what you and the bar weigh, so the bar slows almost to a stop before you grind through.

How is ground reaction force measured?

With a force plate, a rigid platform containing sensors that read the force applied to it in newtons and in three directions. Sports scientists use force plates to measure jump, sprint, and lifting forces directly rather than estimating them.

References

1. Weyand PG, et al. Faster top running speeds are achieved with greater ground forces not more rapid leg movements. J Appl Physiol, 2000. PubMed 11053354
2. Weyand PG, et al. Are running speeds maximized with simple-spring stance mechanics? J Appl Physiol, 2014. PubMed 25080925
3. Escamilla RF. Knee biomechanics of the dynamic squat exercise. Med Sci Sports Exerc, 2001. PubMed 11194098
4. Fry AC, Smith JC, Schilling BK. Effect of knee position on hip and knee torques during the barbell squat. J Strength Cond Res, 2003. PubMed 14636100
5. Physiology, Skeletal Muscle. StatPearls, NCBI Bookshelf
6. Sprint runners have longer Achilles tendon moment arm than distance runners. J Biomech, 2025. PubMed 39919622