There is only one weight machine I warn my patients to stay away from at the gym. I cringe when I see people using the knee extension machine. This is the piece of equipment you sit on, hook your feet under a t shaped bar and straighten your knees. The problem with this machine is that your knee is most unstable and vulnerable when it is straight. Your knee is subjected to way too much pressure and stain when you add resistance to the equation. Strengthening your quadriceps with bent knees is a better way to go. Try the shuttle machine or the leg press machine instead.