A leg extension machine is a piece of exercise equipment that isolates the quadriceps muscles. When using a leg extension machine, an athlete sits in the machine with the knee bent behind a padded bar, then straightens the leg to workout the muscles in the thigh. Some leg extension machines work each leg individually, and others are designed to work both legs at the same time.
A leg extension machine consists of a padded seat, a weight rack of some kind, some kind of handles and a padded bar. Some leg extension machines use pulleys so that the athlete can change the weight using a standard pin system, and other machines allow the athlete to load weight plates directly onto the extension mechanism. It also is common for a leg extension machine to be part of a larger piece of exercise equipment such as a bench or leg extension/leg curl combo machine. When this is the case, the seat usually does not have a back, whereas most standalone leg extension machines provide some kind of back support. Sometimes these machines will provide a belt so that athletes lifting large amounts of weight will have the proper leverage.
To use a leg extension machine, the athlete first loads the weight and sits on the seat. If the machine has a belt, the athlete should buckle it around his or her waist. The athlete then bends his or her knees, usually slightly past a 90-degree angle, and places the feet behind the padded bar. The bar should rest in the curve of the leg where the foot meets the shin. The athlete should then take hold of the handles or the sides of the seat for added stability.
From this starting position the athlete then starts to straighten the legs completely by raising the lower leg. The resistance that the padded bar provides should put a strain on the quadriceps during this motion. The athlete must be careful to maintain a steady rate of motion so that the legs are working the entire time. If the athlete needs to generate momentum in order to fully extend the legs then he or she should reduce the amount of weight. Once the legs are straight, the athlete should bend the knees again and return to the starting position.
The quadriceps are a relatively strong muscle group, so leg extension machines need to be able to support a large amount of weight. For this reason, it is important to ensure that a leg extension machine is stable and the that the construction is of high quality. The joint that attaches the padded bar to the seat can sometimes become loose, so it is important for an athlete to ensure that the joint is stable and that the bar doesn't wobble when it is in motion.