This exercise is used to isolate the quadriceps and shape the bottom portion of the quads. This exercise will give a great pump in the quads, which can be tough to achieve with squats.
First, you will need to adjust the weight on the leg extension machine before sitting on the machine. Next, sit on the machine and adjust the lower leg pads so they are above your feet. Also, adjust the back support so that your upper leg is fully supported. Begin the exercise by squeezing your quadriceps muscles and extend your legs until your legs are almost locked out. Pause at the top, and then return your legs to the starting position. Repeat this movement until you finish the set.
Do not lock your knees out at the top of this exercise, it will put a lot of stress on your knees. Also, control the weight throughout the movement; many people will swing the weight up, but this will not properly work your quad muscles. Finally, keep the rest of your body relaxed and stationary, focus on contracting and squeezing your quadriceps muscles.