Ground reaction force is the force the ground pushes back with every time your body pushes into it. Newton's third law, applied to your feet. Walking generates about 1 to 1.5 times your body weight per step. Running lands at 2 to 3 times. Jumping and hopping can hit 4 to 5. That push-back is the mechanical signal your bones and tendons read and adapt to.
Why it matters
Bone doesn't get stronger from wishing. It responds to brief, high-magnitude, novel loading, which is exactly what impact provides. This is why runners and jumpers tend to have denser leg bones than cyclists and swimmers, even when the cyclists and swimmers train more hours. The forces just aren't there in low-impact sports. Same logic for tendons: they stiffen and toughen when they're asked to absorb and return force fast. Avoid impact for years and the whole structure quietly de-tunes.
How to use it in training
Add small doses of impact, then build. Start with low hops in place, skipping, or hopping down from a low step. 10 to 20 quality contacts, two or three days a week. That's genuinely enough to start signaling bone.
Progress slowly toward higher forces: faster hops, single-leg hops, small jumps with a stuck landing. Land soft, quiet, knees tracking over toes. New to impact, heavier, or coming back from injury? Stretch the ramp over months, not weeks. Connective tissue adapts slower than muscle.
Related terms
Go deeper
Want the studies behind jump training? Read our full breakdown: Plyometric training research.