Hi I've been working on my leg press lately and I'm reaching an obstacle with what I think is my joint strength. I can manage 3 x 12 90kg sets but going up to 100 kg causes me to experience pain in my knees.

At the moment, 90kg isn't really that difficult, and while 100kg does feel more challenging, the knee pain makes me feel like it's a bad idea. On the machine, 10kg is the lowest amount of weight I can so how do I progress up to this safely? Do I do more sets/repetitions at 90kg first? Is it that the joint itself is weak?

I've never injured my knees or anything, they're a little stiff because I'm out of shape and not that flexible but I'm only 25 and I think they're otherwise healthy.