Dr. Laskowski: The knee extension is an exercise you can do with a weight machine to work the muscle on the front of the thigh. Specifically, the knee extension targets the quadriceps muscle. Strong quadriceps muscles make it easier to walk, run, jump and squat.
Nicole Krupa: To do a knee extension with a weight machine, start by sitting on the weight bench. Position the pad on the tops of your ankles. Make sure there's about a 90-degree angle between your thighs and your lower legs. Slowly move your lower legs upwards until your knees are nearly straight. Then slowly return to the starting position. You'll feel tension in the muscles on the front of your thighs.
To minimize stress on your knees, do not lock your knees when they are extended. Also be careful to return all the way to the starting position. Shorter movements do not target the quadriceps effectively and can place too much stress on your knees. To reduce stress on your knees, move the pad upward, onto your lower legs.
You can also try knee extensions with ankle weights. Sit on a weight bench and position an ankle weight around your ankle. Slowly straighten the knee, and then return to the starting position.
For most people, one set of 12 to 15 repetitions is adequate.
Remember, for best results, do not lock your knees when they're extended, and return all of the way to the starting position. Also keep your movements smooth and controlled.