Leg extensions refer to a type of exercise that targets the quadriceps muscles in the front of your thighs. This is done by sitting on a leg extension machine and pushing your knee towards your chest while keeping your upper body stationary. The movement is controlled by the contraction and relaxation of the quad muscle group, which works to lift the weight stack. Leg extensions are often used as part of a strength training program for building stronger legs and improving overall lower body strength.