The quad has a simple job - extend, or straighten the knee. Two ways you see this from an exercise perspective below, knee extension and a squat. There’s more than this (lunge, step up, SL squat, sissy squat, reverse nordic, etc, you get the point) but the point is foot on the ground, foot not on the ground.
Let’s talk real life, functional, aka “FuNksHunAlL”
Who are the players here?
The surface
Gravity (consecutive world champ)
You
Extreme example to illustrate the point. A gymnast doing some depth jumps to vertical jump. She lands, the knees bend/load, and she immediately jumps back up.
Gravity and the ground produce an External Knee Flexion Moment. Forces outside the knee (external) are trying to make the knee bend (flexion).
For her to land, load, then jump, she needs to create an Internal Knee Extension Moment - her quad strength (internal) has to straighten (extend) the knee.
So the INTERNAL (quad) moment has to be GREATER than the EXTERNAL (gravity) moment to reverse direction.
If this doesn’t happen, we can speculate what will happen.
You can also the math on how much GREATER the INTERNAL forces need to be in you changed any conditions, like:
increased height
heavier athlete
landing on 1 leg
moving laterally at high speed (like Jimmy G-forces up there)
Am I in trouble?
A quick screen.
This gets you in the ballpark for gross function. There’s a series of other tests people like, and it seems like every year they invent new ones.
Oh snap (pun intended), what do I do?
You have to train it.
make the quad stronger (peak torque)
make the quad stronger, faster (rate of force development)
land and jump in multiple directions
Are you talking to me?
Have you had a knee injury in the last 1/5/10 years? Did you do stuff like this in rehab?
If not, probably so. Reach out if you’re ready to jump to the next level.