This program is based on the use of three exercises (leg extensions, leg presses and hack squats) that directly target the quadriceps with little stress placed on the hamstrings and calves. If your quads are a weakness, you should prioritize them by training them all-out after a rest day, when they are freshest.
You can substitute front squats for leg presses, but that may fatigue your hamstrings, glutes and lower back before your quads get enough work. Since any type of squat requires balancing the weight under control and consideration of protecting the back, leg presses can be more effective in keeping full attention on the quads.
EXERCISE NO. 1: Leg Extenions
Done twice in the workout, this will serve as both a warm-up, utilizing the Weider pre-exhaust principle, and as a finishing movement. At the beginning, perform three sets of 20 reps with a moderate weight to drive blood into the muscles. Raise and lower the weight at a moderate pace and under control, working the muscles through a full range of motion.
EXERCISE NO. 2: Leg Presses
Start with a moderate weight that you can handle for at least 15 reps. Varying your foot position will allow you to hit your quads from a variety of angles. Keep your back against the pad during the full movement; don't risk injury to your lower back by rolling your tailbone under as you lower the weight. Lower the weight slowly, then push through your heels on the upward portion of the movement. The pace should be slow and controlled on the way down and explosive on the way up. Consider pyramiding up in weight for each set, but make sure to get at least 10 reps.
EXERCISE NO. 3: Hack Squats
Hack squats will give you the feeling of squats, but they keep the stress focused on your quads and not your back. Begin with a stance slightly closer than shoulder width. You can change foot placement here as well to utilize different angles. Rest your back against the pad with your shoulders underneath the shoulder pads. Raise the weight, and then lower until your butt almost touches your calves. This allows a very deep stretch, but you should maintain contact between your lower back and the sliding pad at all times. This will use some glute strength, but it's great for your quads. Using your quads, explode up and drive the weight until your legs are almost completely
EXERCISE NO. 4: Leg Extensions
The workout ends with four more sets of leg extensions to burn detail into the quads. Perform these final sets like you did at the beginning, with a moderate pace and a squeeze at the top. Unlike the first group, they can be pyramided up in weight.
QUAD THRUST ROUTINE Exercise Sets Reps Leg extensions 3 20 Leg presses 4-5 10-20 Hack squats 4-5 10-20 Leg extensions 4 10-15