The leg press is an excellent full-body workout, activating every major muscle from your hips down. Burning thigh muscles and fatigued glutes come standard on the leg press machine, but knee pain shouldn't. If your knees hurt while on the leg press, one of three things is happening: You have an injury that's causing the pain, your technique is off, or the leg press machine isn't properly adjusted to fit your body.
The Easy Fixes
If you're injured, it's time to see a doctor. Technique and machine-adjustment issues, however, can be solved relatively easily. First, double-check your positioning inside the machine: Your hips and back should rest against the back of the seat. Adjust the weight sled -- and your foot position -- so that when your feet are flat on the sled and your knees bent at about 90 degrees, your knees don't completely obscure your view of your big toes. Double-check that your toes and knees are both pointing in the same direction. If your toes go one way and your knees the other, your knees have good reason to hurt. Finally, don't lock your knees when your legs are at full extension.