just because i have a degree in repeating myself...i just found this today...
oh, and consider a knee brace when actively lifting...
5. The Thing You Kind of Know About Strength
Joint health is extremely important to strength. The body has sensors and proprioceptors throughout its framework to tell it what's going on. Joint stability and joint integrity is a very important concept for the body. If your joint is in pain, the body will turn off (deactivate) parts of the agonist muscles that cross the joint and produce the movement.
The body does this because the lower levels of force represent a reduced chance of injury to the already fragile joint. This is why, in my opinion, it's generally not advisable to train through joint pain. Even if you're tough enough to do it, you're using less of your muscle so you'll get compromised results – this is ignoring the fact that the pain is a warning something is wrong and further work might really mess up the joint.
While many factors affect joint health, a big one is joint stability. This is a reason why lifting aids like a belt or a bench shirt have become popular – the belt adds to the stability of the joint by externally stabilizing it. This allows the muscles that cross the joint to contract more strongly (recruiting more motor units) and thus more weight is lifted or more force generated.
This is also why powerlifters who wear gear (bench shirts, squat suits, etc.) often have a hard time calculating how much their gear helps them. In one sense it's simple – how much can you lift raw versus how much can you lift in gear – but another factor is how much the gear is adding to the stability of the joint and thus allowing the muscles to contract more forcefully.
T NATION | 6 Interesting Things About Strength