The leg extension machine has been demonized in the fitness industry for the last two decades. It's accused of creating chronic knee irritation by creating highly compressive forces at the patella-femoral joint, leading to breakdown of the joint surfaces.
That's a valid point, but the leg extension is also responsible for helping build some of the most impressive quads in bodybuilding. It continues to be a staple in many programs because it produces results for those wanting muscular hypertrophy. So what's the answer if you want tree trunks for legs AND knee health? Replace it with something to protect the knees for the long haul.
A Better Exercise
While fatiguing muscular fibers in the quads is guaranteed on the leg extension machine, that same recruitment can be possible through split stance loaded variations such as the lunge or split squat. If you've proved the ability to dominate a static split squat and earned the right to progress, try the rear foot elevated split squat.
Rear-foot elevated split squat:
This exercise has the ability to create massive amounts of mechanical and metabolic stress in the quads and the posterior chain stabilizers, making it a more highly transferrable exercise without losing any of the quad-targeting the leg extension is known for. If you're searching for a way to elicit as much pain as possible, give this a shot. When your quads lock up after a set you'll have all but forgotten about the leg extension.