Want to work your legs and butt but don't have any weights? Find a step, box, bench, or sturdy chair and learn how to do a step-up. This exercise combines the movement of a lunge with stepping upward, like climbing stairs, to build functional strength.

"A step-up is one of my favorite moves for multiple reasons," says NASM-certified trainer Justin Norris, cofounder of the LIT Method. "First, I am a big fan of stability and strength, and that's exactly what this move focuses on. During a step-up, you are activating your hamstrings, glutes, core, and stabilizers."

All those muscles work synergistically to execute the movement, says Eric O'Connor, certified CrossFit coach, which can even increase your functional range of motion (i.e. mobility) when performed correctly. Not to mention, it's a great way to build unilateral — aka single-leg — strength.

"The unilateral nature of the step-up is a nice complement to the bilateral elements [think: traditional squats, deadlifts, etc.] commonly performed in the gym," he says. "Not only is strength being developed but there are additional stability, coordination, and balance demands required of this movement." That's right: aside from a lower-body burn, step-ups are also a great cardio and core exercise. They get your heart rate up and also require balance, forcing you to engage your core for stability.

Sold? Read on for everything you need to know about how to do step-ups, no matter your fitness level.

We've broken down the step-up exercise into three variations so you can find the version that works best for your ability. If you're new to this exercise, step on something low, then work your way up to something higher. You can also tweak the height of the surface you're using depending on which muscles you want to target most. "For example, a low box height may target the quads to a larger degree while a higher box may target the posterior muscles [the back of your body] to a larger degree," O'Connor says.

To make step-ups harder, you can hold a dumbbell or a kettlebell in each hand and do weighted step-ups. You can also do all your reps on one leg at a time (instead of alternating) to really burn out your glutes, quads, and hamstrings on that side.

"Another simple tweak is to perform the movement with a lateral step, which will challenge your lower body and coordination in a manner that is often underutilized," O'Connor says. To do this, you'll follow all the instructions below, but stand to one side of the surface you're stepping up on, instead of in front of it.

No matter how advanced you are, focus on quality reps and good form first. Then you can level up. "When performing single step-ups, make sure you start slow and focus on form over speed," Norris recommends.

Before you start, find a step, chair, box, or bench high enough that when you place your foot on it, your knee bends to a 90-degree angle. Weight benches or plyo boxes are often the right height, but a dining-room chair can work too for an at-home strength-training session. If this seems too high, start with a lower step or box.

This is a slightly more advanced step-up exercise variation that will challenge your balance and work your glutes even more. This version works best with a shorter bench or step rather than a chair.

You'll need a pair of dumbbells or kettlebells for this variation. Start with something lightweight (three to eight pounds), then increase to heavier weights (10 to 20 pounds). To make it even more difficult, you can try the toe-tap version (detailed above) while holding weights.

—Additional reporting by Lauren Mazzo