The following exercises can all be done together and are great for adding muscle mass to your legs.
Positioning - Place your feet slightly wider than shoulder width. The weights should be resting on the ground with the bar crossing over the balls of your feet. Squat down, keeping your eyes forward and your back straight. Grasp the bar just slightly wider than your feet.
The Lift - Keeping your back rigid and arms straight and unbent, raise the bar by extending the knees and hips. The power from this exercise should come from your legs not from your back. Bending your back can lead to herniated discs. Once you have reached the upright position complete the lift by raising your shoulders.
Down Phase - With the same pacing and control you used to lift the bar, return it to the starting position by lowering your body from the knees and hips.
Positioning - The bar should be should rest on the rack 3 inches lower than the top of the trapezius muscle. You should stand backed up to the bar. Feet should be slightly wider than shoulder width. Hands should be placed evenly on the bar.
Down Phase - Stand up into the bar, holding it steady across the trapezius. Slowly lower your body be bending at the knees and hips. You should always keep your eyes forward and your torso rigid. The tendency to look the direction you are moving can lead to falling, either forward or backward while lifting.
The Press - Once you have reached the bottom of your knee bend, extend your hips and knees and rise to the fully erect position. Exhale throughout this portion of the movement. Make sure that you maintain control of the bar as you reach the end of your motion.
Positioning - Stand with one foot forward and the other back. The distance between the two should be a little longer than your natural stride. Make sure that the position of your front knee is directly above your front foot. In order to get great muscle mass gains you should have a dumbbell in each hand.
Down Phase - Slowly lower your rear knee to the floor by bending your front knee. Keep your Head up and your eyes forward. This will prevent leaning forward.
Recovery - Squeeze you buttocks tightly and rise by straightening both legs. Use your abdominals and lower back to maintain your upper body's position.
Positioning - This is done on a machine. Sit flat against the chair back, placing your feet firmly on the footpad. Feet should be slightly wider than shoulder width. In the starting position your legs should be bent just past 90°.
The Press - Using your heels as the pressure point (not the balls of your feet), extend your legs to the point just before they lock.
Down Phase - Lower the weight at a controlled pace until you are back to your original starting position.
Positioning - Place your feet firmly on either the base of a calf raise machine or place your toes on the edge of your calf raise block. If you are using a machine then place your shoulders against the pads of the machine. If you are using dumbells make sure that you have them secure in each hand.
The Raise - The only motion should be at the ankle. By pressing down on the balls of your feet you can extend your entire body upwards. Do not bend you knee.
Down Phase - Lower yourself at a controlled pace until the heels of your feet are slightly off the ground.
Positioning - This is done on a leg curl machine. Seated on the chair of the machine your legs go over the top padded bar. This bar should rest behind your knees. Your feet then tuck in behind a lower padded bar. This bar will cross in front of your ankles. Make sure that this pad is just above where your shin meets your foot. The higher that this pad is the shorter you are moving the weight, cheating yourself out muscle mass.
The Extension - Raise the bar by extending your leg in a kicking motion The lift should continue until your calves are parallel to the floor but not so far that you lock your knees.
Down Phase - Lower the weight at a controlled pace until your knees are again bent to the original 90 degree position.