How to sissy squat (progressions). One of the hardest but also one of the most effective quad exercises you can do with just your bodyweight!
Sissy squat is one of the rare bodyweight exercises that will very effectively isolate the quadriceps muscles of the legs.
"Bad for knees!" is the first impression many people have when they see a sissy squat, mostly because the knees go so much past the toes (this is a myth by the way).
A lot of people also DO experience knee pain when they attempt sissy squats. Knee pain in sissy squats is the result of weakness in 1 or more of the 4 quadricep muscles (rectus femoris or vastus lateralis/medialis/intermedius).
The reality is that sissy squat is a safe exercise to do, but it's an advanced exercise that requires strong and structurally balanced legs. Your quads already need to be in a great shape.
Once the legs are relatively strong and the form of your sissy squats is slow, controlled and clean, the sissy squat becomes a phenomenal way to train your quads.
Sissy squats when done right will add strength, size and mobility to your quads better than almost any other exercise out there.
Sissy squat also demands and develops mobility in the ankles, lower back and many of the hip muscles (stabilization). If you do sissy squats without support, you will also develop coordination and balance.
In case you feel knee pain, you need to first master the basic leg exercises such as different types of squats, lunges and knee extensions. Movement 20XX program is also specifically designed to produce structurally balanced legs.
Sissy Squat Progressions
Assisted Sissy Squat

Holding onto something is a perfect way to start practicing the sissy squat. It pretty much eliminates the balance aspect and allows you to assist the movement.
In the beginning you can use lots of assistance but eventually you should only aim to use the arm for balance. You also don't have to go all the way down, instead you can gradually increase the range of motion while you get stronger.
Focus on a slow and controlled execution because that's how you produce the best results.
Sissy Squat

Real sissy squat is a serious strength exercise but there is a difficult balancing and stabilizing aspect as well.
In order to keep your balance, your entire lower body needs to be relatively strong and capable of stabilizing the movement (even your core, lower back and glutes) - otherwise you will mess up the form even if you can go all the way down and up.
The form has to be clean. If your form is shaky and you lose tension in your quads, you are probably doing the sissy squat the wrong way with the wrong muscles. In this case, return back to the assisted sissy squat progression.
Elevated Sissy Squat

Elevated sissy squat is the next step from the standard sissy squat. You will significantly increase the difficulty by adding range of motion.
Squatting lower will increase the working ROM of your knee extension which will increase stretch on your quads and require more strength.
Doing the elevated sissy squat without support is a serious balancing task. There is no problem using assistance for this movement - too much of instability will make the exercise less effective.
Weighted Sissy Squat
In case the regular sissy squat is easy, it's convenient to add weights to make the sissy squat more intense.
Smith machine is an excellent tool because it will eliminate the balancing aspect (you can lean towards the bar) while allowing you to have your both arms free.
There is no problem using free weights though. You can grab a dumbbell with one arm and use the other arm for balance. You can also try freestanding sissy squats without support.
Although weighted sissy squat may sound awesome and effective, you will get much better results by sticking to the normal sissy squat and just cleaning up the form.
Sissy Squat Variations
Aladdin Squat

Aladdin squat is a wide sissy squat where the legs are pointing outwards.
Contrary to the other sissy squat variations, Aladdin squat will also engage the glutes while working the quadriceps (especially the vastus medialis, the teardrop muscle).
Aladdin squat may be easier to balance for a lot of people but you can still use support.
Narrow Sissy Squat

Narrow stance or close-stance sissy squats will maximally target the quadriceps and force them to work the hardest.
It's also possible to point your knees inwards. The range of motion will be a bit smaller but this way you will target more the outer quads.
The inward sissy squat may look rough on the knees, but it all depends on how strong and structurally balanced your legs are.
Similar to all other progressions, focus always on quality over quantity! Clean repetitions always produce better results than the "heavy" ones.
In observing you perform the sissy squat, I noticed you perform it with and without foot wear. Why and are there advantages and or disadvantages.
Barefoot shoes or no shoes at all is good for developing the ankle muscles (ankle stability)