The sissy squat isn't an exercise that you find within the average person's Smith machine leg workout.
But, then again, who wants average quads?
To build attention-demanding quadriceps that cause people to ask if you're a competitive bodybuilder, you need to think outside the box. And that means doing more than just back squats.
Though not generally thought of as a "mass builder", the Smith machine sissy squat is practically unrivalled when it comes to adding separation and details to your quads.
So let's get into it...
Here's how to do sissy squats on Smith machine stations:
The Smith bar acts as a stabiliser that enables you to squat while maintaining an upright torso and hence, keep the majority of the tension on your quads. But to maximise the effects of this setup, I want you to actively think about pushing your lower legs into the bar because doing so naturally engages your quads.
Also, think about moving your back towards the wall (or whatever's behind you) rather than merely squatting. This way, you get a deep stretch in the hard-to-develop rectus femoris that will make you sore for days.
Essentially, you're trying to maximise knee extension while minimising hip flexion. Why? Because the rectus femoris crosses both the hip joint and the knee joint and, therefore, can't receive full stimulation during a standard squat because it's impossible to fully flex your hips and extend your knees at the same time. So I like to think of this exercise as a sissy stretch, rather than a sissy squat .
It's also why traditional sissy squats done on a sissy squat machine don't look like squats at all—you have to bend your knees why keeping your hips totally straight.
You know what? Even bodyweight Smith machine sissy squats are enough to set your quads on fire and make you puke, especially after other leg work.
Although doing sissy squats Smith machine style provides a great pump, they're absolutely brutal. You could be a 300kg squatter and still struggle with these bad boys. So I recommend starting off with bodyweight only before adding external resistance.
But once you master bodyweight sissy squats...
Wearing a comfortable weighted vest is the most effective way to add resistance to your sissy squat (and thereby mass to your quads) because you can lift hard without having to focus on holding the weight. Obviously, this loading method helps you to develop a stronger-mind muscle connection, which is absolutely pivotal with an isolation exercise like the sissy squat.
You can also elevate your feet and perform a Smith machine deficit sissy squat, to make the exercise harder. However, you could easily lose your balance by doing this, so I'd stick to wearing the vest—just to be safe.
Admittedly, sissy squats aren't the most comfortable exercise for those with knee problems. And since my knees aren't exactly like new, I don't do sissy squats—or any quad exercise, for that matter—without my Iron Bull Knee Sleeves.
These knee sleeves reduce the inevitable knee pressure that comes with hard leg training by providing your patella with rock-solid stability. Similarly, these contoured knee sleeves speed up your recovery by increasing the blood flow to your damaged muscle tissue.
Oh, and it's just hit me that—rather ironically—my Beast Gear Knee Sleeves provide more recovery benefits than so-called "recovery supplements". And you don't even need to replenish your supplies each month, because Beast Gear guarantees that your new knee sleeves will last forever!
While loading your sissy squats with dumbbells or a weight plate does work, it's sub-optimal for bodybuilding because holding the weight detracts from your mind-muscle connection.
A weighted vest, on the other hand, enables you to add external resistance, and therefore, develop larger quads, without having to waste mental (and physical) energy on holding the weight. I personally wear the Runfast Weight Vest.
It comes in 5 different sizes and is fully adjustable to fit any body size, meaning that me and my wife can both use it comfortably, which saves a nice chunk of change for me to spend on other fitness goodies!
I wasn't a big believer in foam rolling until my physio friend convinced me to give it a try. For some reason, I thought that it was a practice reserved for old ladies after their yoga class. But after foam rolling consistently after every leg workout for the past 6 months, I can proudly admit my ignorance.
Foam rolling decreases my stress levels and speeds up my recovery faster than any supplement I've tried (and I've tried a lot of supplements). Plus, my Foam Roller 4 Piece Set comes with a full mobility kit, so I can easily work on my other tight muscles and improve my flexibility.
My wife uses the Trigger Point Foam Roller after her workouts. She thought that the ring-bound exercise guide, which comes free with the foam roller, was very helpful for learning the ropes. And, more importantly, she really enjoys the process of smoothing out any tightness in her muscles, which is what counts.
Leg extensions are a brilliant exercise for isolating your quads. And with the addition of proper form, they can build a surprising amount of mass. However, unless you do them laying all the way down (which is impossible on most machines), they're incredibly ineffective for targeting the often-neglected rectus femoris muscle  .
Sissy squats, on the other hand, are one of only a handful of quad exercises that hammer your rectus femoris. And by my reckoning, the Smith machine is the second-best way to perform them.
The best way, of course, is to use a dedicated sissy squat machine. Unfortunately, however, most modern gyms (who cater to the do-you-even-lift general population) don't have such equipment. And I doubt that most gym owners are even aware that these quad-blasting machines exist.
Anyway, since I have a home gym, and since sissy squat machines cost less than a bodybuilder's monthly supplement stack, I decided to invest in my leg development and buy this quality Sissy Squat Bench.
If only I'd invested in this bench when I first started training, I'd have much better quad development by now. But better late than never, I suppose.
Doing Sissy squats on Smith machine systems tends to activate more core muscles than doing regular squats using the Smith machine because most people's bodies simply aren't used to the exercise. As such, your core has to work harder to compensate for your underdeveloped muscles. In this case, your rectus femoris.
The glutes play a stabilising role during sissy squats, but that's about it. You're essentially getting pure quad isolation, which is why the exercise is so challenging.
As with the glutes, your erectors play only a minor stabilising role during sissy squats, making the exercise ideal if you have lower back issues, but still want to build your quads.
Muscle isolation is one of the reasons why bodybuilders are much bigger than powerlifters and Olympic weightlifters. To develop a muscle to its fullest potential, it has to be the limiting factor in at least one exercise.
Besides leg extensions, no other exercise provides more effective, intense quad isolation than sissy squats. By taking your glutes and hamstrings out of the equation, your quads are forced to bear the brunt of the load. And with a good enough diet, they practically have no choice but to grow! 
Since virtually everyone (except pro bodybuilders) has weak rectus femoris muscles, the sissy squat isn't suited to heavy loading. And for me, the beauty of the exercise is in the pump. And to get the blood pumping, you need to perform high rep sets.
As brutal as high rep sissy squats are, I can't think of another exercise that's improved my quads by such a noticeable amount in recent years.
If for whatever reason, you can't or don't want to perform leg extensions, then don't worry about missing out on any quad gains, because sissy squats can completely replace them (and dare I say, provide even better results).
If you want to learn the proper squat form, then doing a Zercher squat on the Smith machine is one of the most effective methods for doing so, because it allows you to go deeper than virtually any other squat variation.
Also, because the bar is positioned in the crux of your elbows—and not directly over your spine—it's incredibly easy on your lower back compared to traditional back squats.
While the Smith machine sissy squat is definitely the best exercise for hitting your rectus femoris, Smith machine front squats are, by my reckoning, the most effective movement for working the other 3 quad muscles: the vastus lateralis, vastus medialis and the vastus intermedius.
Smith machine sumo squats are the polar opposite of front squats. Rather than bulking up your outer sweep, they emphasise the inner quad, glutes and adductors, all of which are important muscles in their own right.