04/07/2026
"Hop hop, quick like a bunny." — My Nana
She had no idea she was onto something... Ground reaction forces.
What the heck is a ground reaction force, and what meaningful role does it play in my life? Great question. Let's talk about it.
A ground reaction force (GRF) is the force exerted by the ground on a body in contact with it — equal and opposite to the force that body exerts on the ground. Physics, baby! It becomes very important when we start talking about bone density. In order for bone cells, called osteoblasts, to respond effectively to the force placed on them, that FORCE needs to be approximately 3- 4 times your body weight, not the actual load you're using. An important distinction. Let's split the difference and call it 3.5X.
There are many activities that create increased forces over our stationary ground reaction force, which we'll call 1X. Walking is approximately 1.2 to 1.5X. Running is approximately 2.5X — and by running, I mean jogging, which for me feels exceedingly strenuous, but for many, apparently, it is not.
Neither one gets us to 3.5X.
So what does? Jumping. Hopping. Things many of us have not done since we were children.
And here's the other thing about bunnies — they're twitchy. Very twitchy. Turns out, so are the muscle fibers we recruit when we hop and jump. Fast-twitch muscle fibers are the ones responsible for quick, reactive movement — the kind that catches you when you stumble. They decline faster than any other muscle fiber type as we age, and they are directly linked to balance and fall prevention. Hopping and jumping are one of the few ways to actually wake them back up.
Bones AND balance. Nana was truly ahead her time!
In order for us to achieve a sufficient stimulus to protect our bone density — maintain or even build — we need to shoot for that 3.5X mark. How the heck do you do this, especially when many haven't hopped, skipped, or jumped in years and or decades?
Follow along this week and I will show you exactly where to start and how to progress to create enough force to keep those bones nice and strong and dense well into our older years.
Let's do this. 🐇