Adjust the machine seat and choose an appropriate weight taking account of your fitness level. Sit with your feet firmly anchored under the machine rest. Keep your feet at shoulder width.
From the starting position raise your legs to a complete stretch out (moving forward). Your legs are at a complete extension in the knee joint in the upper position.Use a controlled motion to go down almost to the starting position, meaning that the muscle tension is not completely relaxed in the lower position. Do the required number of repetitions depending on your type of training. Exhale while your legs move upwards and inhale while they move downwards.
Make sure the machine is adjusted correctly for your height and choose an appropriate weight. Keep your torso fixed and do not bend at the waist, especially when overcoming heavier weight resistance. Your hands are to remain firmly anchored to the machine grips. Return to the starting position with a slow, controlled motion rather than quickly relaxing the tension in your muscles. Do the motion by taking account of your breathing and type of training.
Start with lighter resistance to master the technique well.
Leg extensions are a very popular exercise among novice gym-goers. They are simple, pretty safe and as it seems, some people are just happy that they can sit while doing them (and during the breaks between sets).
The reasons why leg extensions are not equally popular among advanced bodybuilders are several: first, this is a pretty isolated exercise – while you work whole lower body by squats you only work your quadriceps on leg extension machine. Second – the heads of quadriceps muscle are not involved equally. The mid-front portion (rectus femoris) does most work while other important muscle heads (the drop-shaped vastus medialis and vastus lateralis) are not as active.
There is a very widespread myth that leg extensions are THE exercise to form a perfect vastus medialis but this is not based on serious scientific research.