Leg extension machine

Not mentioned above, but I would double-check your knee placement in relation to the pivot point of the machine. Just an inch or two forwards or back from the actual pivot point can have a world of impact on stress placed upon the knee. Simply placing the end of the seat at the back of your leg is not necessarily a perfect placement for everybody.

Just to add a bit of sauce, I'll suggest maybe Job 1 is NOT to build quads while knees just keep on a-hurtin', but (very dependent on medical diagnosis) to fix up those knees. Ordinary knee pain (e.g. patellar tendonitis) can be effectively addressed with targeted and usually overlooked hip and knee exercises and, while isometric holds (or your pulses) are not the best muscle builders they can be better and less painful than dynamic movement for building joint tissue, that typically requires significantly more time. Kneecap problems that plagued me just walking around ten years ago have been addressed to pain-free workout status after some months focusing on pilates-type mobility and isometric holds using straps, and good nutrition. I'd suggest reconsidering personal priorities and research into joint repair.

I endorse checking that out. It's individual.

Bystanders and a lot of articles talk about increase quad strength to reduce knee pain. They told me that, too, but I was skeptical. (In my case, I'd been doing things for a long time that tend to give good quad strength.) Sure enough, when I got referred for physical therapy, my therapist assessed me and said quad strength was fine - the problem was elsewhere.

That may apply to you, it may not apply to you . . . and if your quad strength is OK, there are diverse other variables that can be triggering knee pain, which is one reason I'm not saying what my issues turned out to be. Yours would almost certainly be different. Quad strength is a common issue, but far from the only possibility.