Ground Reaction Force (GRF)

Helping Students Jump Higher

Hello, this is Young-in teacher! :)

In PE class, you’ve probably seen this many times:
you ask everyone to jump, but one student springs up like a coil,
while another barely leaves the ground.

That difference is not just about “leg strength”. Very often, it comes from how well they push against the ground — Ground Reaction Force (GRF).

Today, let’s study the key GRF concepts PE teachers should know
to help students jump, run, and land more safely and effectively!

1️⃣ What is Ground Reaction Force (GRF)?

When we stomp or push the ground with our feet,
the ground pushes back with the same amount of force in the opposite direction.
(That’s Newton’s Third Law: action–reaction.)

The force that the ground sends back up into our body is called
� Ground Reaction Force (GRF).

If you want to jump up higher,
→ you must push down on the ground more strongly with your feet.
→ Then the ground returns a larger upward reaction force, lifting your body.

So every jump and every stride in running is based on this interaction:

“I push the ground downward,
the ground pushes me upward.”

That’s the basic principle behind all jumping and running movements.

2️⃣ GRF in Jumping and Running

Once you understand this, you can give students much more precise feedback.

� Standing long jump / vertical jump

When students bend their knees deeply and prepare well,

then push the ground strongly in a short moment,
→ the action force increases,
→ the ground reaction force increases as well,
→ and they fly farther and higher.

�‍♀️ Sprinting and fast running

The foot only stays on the ground for a very short time,
but if, in that instant, they
→ push the ground forcefully backward,
then the ground reaction force
→ drives the body forward, increasing speed.

For students, you can say:

“The harder you push the ground,
the more the ground pushes you —
it’s like the ground is helping you jump higher and run faster!”

3️⃣ Safe Landing and Injury Prevention (Shock Absorption)

GRF isn’t only important while taking off. It can be even more critical when landing.

If you watch taekwondo or martial arts classes, you’ll often see students practicing how to fall and land safely (낙법). In PE, we don’t have to teach full martial arts techniques,

but we should teach simple, safe landing techniques at least once.

❌ Poor landing

Landing with knees straight and body stiff
→ A large ground reaction force is transmitted all at once in a short time.
→ Huge stress goes into the knees and ankles.

✅ Good landing

Landing with ankles, knees, and hips bending in sequence
→ The same total GRF is spread out over a longer time
→ The impact is distributed and absorbed, protecting the joints.

That’s why when we teach “how to jump higher,”
we must always pair it with
“how to land softly with bent knees and quiet feet.”

4️⃣ Easy Ways to Explain GRF to Kids (Analogies & Feel)

Instead of formulas, try using sensations and images.

� Balloon or sponge analogy

“When you press a balloon or sponge down with your hand,
it pushes back against your hand, right? In the same way, when we press the ground with our feet, the ground pushes us back up.”

� Simple on-the-spot experiment

Have students alternate between:

“Soft jump” – small, light jump

“Power jump” – bend knees and jump as powerfully as they can

Then ask:

“When did your body feel like it floated up more easily?”

“Which landing felt like more impact on your legs?”

After sharing answers, say:

“That difference you just felt—that’s you using Ground Reaction Force.”

When students can feel the concept, the term GRF settles naturally in their minds.

� Classroom Application – Young-in Teacher’s TIP! (Use it right away)

✅ Tip ① “Ninja Jump vs Boom Jump”

Ask students to:

First, jump and land making a big “BOOM!” sound.

Then, jump again but land like a ninja — as quietly as possible.

Afterward, explain:

“The smaller the sound, the better you spread out the impact
and control the ground reaction force safely!”

This helps them learn safe GRF control with their bodies, not just their heads.

✅ Tip ② Landing photo/video feedback

Record students’ jump and landing in slow motion.

Watch together and look for the sequence: Foot → knee → hip → upper body absorbing the shock like a spring.

Then have each student choose:

“One thing in my landing posture I want to improve.”

This turns GRF into a great self-reflection and movement analysis activity.

� In Closing

“To jump high, you must first sit low and push the ground firmly.”

This is a principle of biomechanics, but it also resembles a principle of life.

In PE class today, why not help students feel both:

the sensation of powerfully pushing the ground, and

the sensation of meeting the ground softly and safely on landing?

Those experiences become the foundationfor stronger bodies and smarter, safer movers.