The leg press and leg extension machines are used to train your legs. The leg press is a compound exercise using both the knee and hip joints. The leg extension machine is an isolation exercise, using only the knee joint. While neither exercise is a replacement for the barbell squat, they can be used to develop your legs. Consult your physician before doing leg presses or leg extensions.
The majority of the work done by your legs is performed by your hips, quadriceps and hamstrings. Neither the leg extension nor the leg press recruits any of the muscles of your hips very much. Your quadriceps, or the muscles on the front of your thigh, work to extend your knee joint to straighten your leg. Your hamstrings serve two purposes: flexing your knee and extending your torso. Flexion, or straightening of the torso, does not occur during either exercise, but your knee flexes as it bends. Your hamstrings also protect your knee joint when it flexes.
The leg press is a machine that you sit in and extend your legs against the resistance of a weighted sled. When seating yourself, adjust the back rest so that you do not round your back at the bottom of the repetition or allow your hips to come off the seat. For optimal range of motion, place your feet wide to allow your legs to descend without your legs pushing on your ribcage. The wider stance works your hamstrings more, although still less than a squat.
The leg extension is a machine that you sit in to work your quadriceps. It requires you to sit with your ankles braced against a pad. You then straighten your leg against the resistance. This machine is an attempt to specifically recruit the quadriceps, but there is no greater activation of your quadriceps in this exercise when compared to the leg press. While the leg extension has been used in rehab, high tension occurs when the leg is fully extended, creating much stress in the knee's anterior cruciate ligament, or ACL, often causing injury.
Terminal Knee Extensions
An alternative to leg extensions is the terminal knee extension. While this exercise does not recruit your hamstrings to the degree of the leg press, there is more recruitment of the hamstring than in the leg extension. This makes terminal extensions a safer alternative for working the musculature surrounding your knee joint.
Wrap one end of a resistance band around a fixed support at knee height, and the other end around the back of your knee. Back up until there is tension in the band, and bend your knee. Straighten your leg by flexing your quadriceps while keeping your foot firmly on the ground. This exercise can be done for up to 20 repetitions, and should be performed with each leg.