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When it comes to training the quads, you must understand there are 4 muscles that make them up. Your vastus medialis (inner head), vastus lateralis (outer head), vastus intermedius (middle head/hidden), and rectus femoris (middle head/visible).
So in order to build your quads, you must be working all four of these muscles. But the issue is, an exercise as simple as the leg extension, does not work all four due to the rectus femoris requiring hip flexion to be stimulated. This means you need an exercise where your knee lifts up.
One thing to keep in mind is to imagine as if there is a coin under the arch of your foot and you have to keep as much weight off it as possible while making your main points of pressure your heel and big toe. This will enable you to activate your quads optimally.
The first one is a reverse lunge. The most important thing to remember is to act as if for the first half of the exercise, you are trying to get a one-legged squat, then after you are halfway down, put your foot out on the ground behind you for full range of motion then drive back up through your front foot again and squeeze at the top for the peak contraction.
Next is close stance leg press. You want to be just inside of shoulder width with your toes slightly pointed outward. From there, you will bend your knees to about 90 degrees and push up through your toes to engage your quads and make sure you are squeezing out at the top with locking out your knee joints.
Lastly, and my favorite, back foot elevated split squat. This is a great exercise to get a deep stretch on the hardest quad muscle to work, which is the vastus lateralis (outer head). You will put your back foot on something about a foot off the ground and focus on working your back leg’s quad muscle. From there, make sure your shoulders are directly above your waist and sit back on your back leg about half the distance of a regular split squat and stand back up without locking out your knee.
Try these exercises out on your next leg day and let me know what you think!