Back squat

The back squat really stimulates the quads and should be performed with the barbell held just across your upper back. Be sure that when you're doing this you adjust your back position slightly more forward so the weight doesn't throw you off-balance (so you don't fall backward).

Hack squat

The hack squat is a slightly more advanced type of squat variation and will have you holding the barbell just behind your legs. Again, with this squat variation, as you move down into the squat movement, your upper body will tend to lean forward slightly, toward the knees, to compensate for the barbell. While you need to move forward slightly in order to move closer to the back of your heel area, you also want to be sure that when you're doing this squat variation to keep your chin up, facing forward in order to effectively remain balanced.

Sissy squat

The next squat variation that you should try is the sissy squat -- this will really test your balance. And because of the position this squat variation calls for, it will also call the abdominal muscles into play, thus giving you a great core workout at the same time.
In order to do this specific squat variation, start by holding a dumbbell behind your legs or holding a weighted plate in front of your chest. Once you’ve found your balance you are to begin the squat motion, leaning backward with the body while simultaneously raising your heels off the ground.

Note that you will perform this exercise with a great deal less weight than you would for a normal squat because of the bending-backward positioning. You must also be very careful to pay full attention to how your back feels while you execute this squat variation, being sure to stop if you feel pain. Trying to keep your back in a good, straight alignment (although tilted diagonally) will also help to keep you pain free.
After you’ve reached the bottom of the squat (going as far down as you feel comfortable), return to the upright position once again before completing another rep.
If you're having trouble maintaining your balance throughout the sissy squat, which is a very common issue, use one arm to hold onto a chair or a beam for support until you get used to the movement.


Sumo squat

If you’re looking to really hit those inner thighs, the sumo squat should be your movement of choice. You’ll execute this lift by taking a stance that is slightly wider than usual, which then correspondingly places more weight on the inner portion of your leg muscles.

When doing so, think of also turning your feet out slightly, but only as much as is comfortable for you since this will further enhance the effects of the lift.

You can still perform this squat with a barbell across the back as usual or, if you prefer, you can hold a dumbbell vertically between your legs as you squat down. Many individuals prefer the dumbbell method since this causes the weight force to be concentrated right between your legs, allowing you to feel it even more in your inner thighs and glutes, as you press up to complete the rep.

Jump squat

For those who wish to work on explosive power and fast-twitch muscles, the jump squat is a terrific choice.

To perform this squat, crouch down in the squatting position, with your fingertips placed on the ground just in front of you, then you’ll rebound off the ground, straightening up, while jumping as high into the air as possible.

Upon landing, move right back into the full squat position with fingers in front of you to complete the rep.

Note that most people doing this exercise will tend to keep their knees pointing forward and fairly close together when they are in the squat position on the ground.

Bulgarian split squat

Finally, the last squat variation to include in your squat repertoire is the Bulgarian split squat.

This movement is performed by placing one leg up on a bench behind you with either a barbell rested across your back or with you holding dumbbells down by your sides in front of you.

Once you've found your balance, lower yourself into the squatting position with your weight focused solely on the supporting leg that’s underneath you.

Note that the further away from the bench you are, the greater this movement will work the hamstrings and glutes, while the closer you are the more it will work the quads.

Just watch that you don't place the standing foot too close to the bench, because this could cause pain in your knee.