Single-leg strength is important. Unfortunately, when your goals are more about aesthetics than athleticism, single-leg work doesn't get much love.
But split squats are one of the all-time greats. Many would even argue their superiority over heavy back squats for athleticism. Here are two ways that'll help you improve single-leg strength, while also packing on some muscle.
Split Squats for Quads
During standard split squats, at least 70 percent of your weight should be through your front leg. While it's true that the quads of the back leg would get some loading through knee extension (the back leg somewhat resembles a sissy squat), for most, purposefully loading the back leg isn't a good idea.
Rear-foot elevated split squats take the back leg out of the equation while also shifting weight forward – more weight over the lead leg. Here you can safely and effectively load your quads.
Using a squat rack for these is perfectly acceptable (cue the gym police). The rack setup is the most efficient way to do these. Alternatively, a split squat stand, or even some leg extension machines, will allow you to hook your back foot on the support.
The advantage of the rack, though, is the additional support it provides. By placing one hand on the dumbbell and the other on the rack you'll be more stable. More stability means more output, and more load you can put through your quads without worrying about toppling over. Stability is something many meatheads struggle with when attempting split squats.
You could also set up behind a bench set on an incline, using the back rest for support. I've tried a broomstick with this one too, albeit this has a little less support. All you need is something to hold on to.
In addition, having something to grab will allow you to get in a few extra reps. As you fatigue you can grind out more reps with help from the supported arm on both the lifting and lowering phase. You can also use assistance on the lifting portion only (lift with help, lower without), meaning more of an eccentric overload. If you try this approach, good luck standing the next day!
The height of the back foot is up to you. Begin with a very low elevation and work up. Going too high can cause knee pain in some, as well as poor pelvic alignment.
Keep your abs tight, ribcage down, and try to contract your glute on the backside leg. Start and finish at the bottom of each rep, where you can either drop the dumbbell and switch sides, or place the dumbbell in the other hand and get going again.
Split Squats for Glutes
Deficit split squats work because they take you further into hip flexion. While it's true they're also a horrendous quad-burner, going further into hip flexion will take your glutes into a deeper stretch position.
When you stand on the box, either you or the weight should be going below the height of the step. That's why it's called a deficit!
Specifically, it'll be the lower fibers of your glute max that'll get hit hardest – yes, they're a thing. A study by McAndrew et al. showed that the glutes are comprised of upper (cranial), middle, and lower (caudal) sections, each of which activate differently.
While hip thrusts, glute bridges, and cable kickbacks tend to bias more of the upper fibers, "stretcher" type exercises such as split squats, high step-ups, and forward lunges tend to target more of the lower portion of your glutes. If you're capable of going deep, you'll stretch and load these fibers even more.
You don't need to have a massive deficit either. Just work within your own range of motion and progress to larger deficits over time.
To achieve a slight deficit in split squats, a few heavy plates on the floor work just fine. A foam pad in the middle helps with comfort, while also adding a depth target. Just a gentle tap-and-go on a pad will limit cheating and help you hit the deficit every time.
Start and finish in the bottom position where you can get set and grab your dumbbells. Feel free to add some pauses at the bottom portion of each rep too. That'll really emphasize the stretched position. Your ass will be on fire for days!