The leg extension is a resistance weight training exercise that targets the quadriceps muscle in the legs. The exercise is done using a machine called the Leg Extension Machine.

There are various manufacturers of these machines and each one is slightly different. Most gym and weight rooms will have the machine in their facility.

The leg extension is an isolated exercise targeting one specific muscle group, the quadriceps. It should not be considered as a total leg workout, such as the squat or deadlift.

The exercise consists of bending the leg at the knee and extending the legs, then lowering them back to the original position.

Most fitness experts and coaches advise people to not use an isolating Leg Extension Machine due to the unnatural pressure it places on the knees and ankles. Regular use of the machine can lead to permanent knee injuries

For this exercise you will need to use a leg extension machine. First choose your weight and sit on the machine with your legs under the pad (feet pointed forward) and the hands holding the side bars. This will be your starting position.

You will need to adjust the pad so that it falls on top of your lower leg (just above your feet). Also, make sure that your legs form a 90-degree angle between the lower and upper leg. If the angle is less than 90-degrees then that means the knee is over the toes which in turn creates undue stress at the knee joint.

 If the machine is designed that way, either look for another machine or just make sure that when you start executing the exercise you stop going down once you hit the 90-degree angle.

Source: https://en.wikipedia.org/wiki/Leg_extension