Alternating Leg Extensions
The primary function of the quadricep, named for the four distinct muscles that make it up, is to extend the leg. Therefore, the exercise that best works these four muscles (other than long rides on a bicycle) is the leg extension. Every gym has at least one of these machines, and some have a variety that work slightly differently.
An alternative to the vanilla leg extension is the alternating leg extension. Give it a try when you get bored with your leg workouts.
Adjust the leg pad on a leg extension machine to just above the foot, and adjust the back support to allow the front of the seat to hit just above the underside of the knee. While keeping the toe pointed up towards you, slowly lift the weight with one leg until you have completely contracted the quads. Hold for a second or two, then slowly return to the starting position.
Now, perform the same movement using the other leg, again keeping the toe pointed up. Alternate back and forth for 10 - 12 repetitions on each leg. Increase the intensity by supersetting with a two-leg extension: After a brief (10 second) rest, add about 50 pounds, keep the toes pointed up, and get in another 6-10 repetitions using both legs.
