1. Squats
If you're seriously weight training, you should already be doing squats religiously. It is one of the best quad exercises, as well as the best overall muscle mass developer. Gaining strength on heavy squats is the only thing that can take your leg development to its maximum potential.
Though you should already be doing squats for heavy sets of 4-6 reps, try doing them for much higher reps to build your quads. If you're really ready for a challenge, try 20-rep squats. To do these, use a weight with which you can get a smooth set of about 10-12 reps.
After about 10 reps, start pausing at the top for 3-4 breaths and grinding out rep after rep until you get to 20. This is one of the most brutally hard things you can do in the weight room, but it will pay enormous dividends towards your muscularity.
2. Hack Squats
If your knees can handle them, hack squats are possibly the best heavy exercise you can do to specifically target your quads. Though it takes you through a full squat range of motion, while hitting your quads preferentially over the hamstrings or glutes.
The best way to perform these is to work up to a heavy set of 6-8 reps, and follow it with a higher rep set. You can use the same protocol for the high rep set as you do for 20-rep squats.
3. Leg Presses
The leg press is one of the best leg builders and overall quad exercises. Unlike the hack squat, it allows you to use your hamstrings and glutes to a large degree, as well. Though it as not as targeted for your quads, this provides the potential for using very heavy weights.
Again, you can use the same protocol with the leg press as you can with the squat and hack squat. You can also simply work up to a few hard sets of 10-12 reps. Either way will build your quads very well. Also, you place your feet high and wide on the platform, with your toes hanging off, to preferentially target the hamstrings.
4. Dumbbell Split Squats
To perform the split squat, you simply place one foot on a bench behind you and use the other leg to squat. You can change the angle and position of your foot to target quads more than hamstrings.
To do this movement with dumbbell, simply set the dumbbells at your sides before you put your foot on the bench. Squat down and pick up the dumbbells to initiate the movement. Your grip may give out, so use straps if you need to.
5. Barbell Lunges
The barbell lunge is one of the best overall quad exercises, hamstrings, and glutes. Using a barbell on your back, simply lunge forward. You can do "walking" lunges, where you continue to lunge one foot after the other, or you can simply step forward and back inside the squat rack.
6. Smith Machine Squats
Though many free weight purists decry the smith machine for squats, it can be a great tool to specifically target your quads. Use much the same form as you would with the squat, but focus on simply pushing with your quads as you do the movement. I personally don't do this movement much, but you may findit to be one of your best quad exercises.
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