Bars: peak pull on the patellar tendon, in body weights.
The bump below your kneecap is where the patellar tendon attaches. Whenever your quads straighten the knee or brake it, they pull on that spot.
This page works out that pull from the body's position, seen from the side, and colors each muscle by what it's doing. Tendons get stronger from loads they can handle, repeated. The routine builds up toward the loads that flare it, slowly and under control.
A teaching model, not a diagnosis. Compare exercises with each other, not with lab numbers.