Just yesterday I started getting serious knee pain doing my first warm up set. I couldn't even finish the set at 145lbs. I had to go over to the leg press. I had no problem putting up 4 pain free sets up to 630lbs. I don't know if this is an injury or just old age. Maybe I haven't been stretching enough. I'll give it another try in a couple days after a really good stretch. I know squats are usually a must for growth but what if I can't do them anymore? I've really been trying to work my legs hard but with this knee pain I might be stuck with presses.