Leg curl and leg extension machines both exercise your legs, but each works muscles on different sides. Using a leg curl machine involves loading the muscles in the back or posterior of the leg. You either sit or lie down for this exercise, depending on the machine you use to bend your knee, or knee flexion. The leg extension machine involves straightening your leg against resistance and working the muscles on the front or anterior of the leg.
When using either the seated or lying leg curl machine, the muscles on the back of the thigh will contract to bend your knee. The hamstrings are made of three muscles -- the biceps femoris, the semimembranosus and the semitendinosus. The largest of the three is the biceps femoris.
The quadriceps muscles perform knee extensions. These muscles consist of the rectus femoris, the vastus lateralis, the vastus medialis and the vastus intermedius. During this exercise, the rectus femoris is the most-worked muscle. At the end of the motion, when your knee is almost straight, the vastus medialis then contracts.
The calf muscles are made of two muscles, the gastrocnemius and the soleus. But the one that is used the most is the gastrocnemius, which is closer to the skin. It helps the hamstrings to bend the knee when using either of the leg curl machines because it crosses the back of the knee.
When using any machine, it is important to keep proper form. Avoid straightening your knee completely when using the leg extension machine because it can create a shearing force on the knee. This can damage the tissue surrounding the knee.