So you want to step up your workouts—which means you should probably join a fancy boutique studio or invest big bucks in a trainer, right? Eh, not necessarily. No matter how much gym cred you have, some of the most effective exercises out there can be done anywhere, for free—using just your body weight.

Many bodyweight moves utilize multiple muscle groups, boosting your calorie burn and shaping your body at the same time, says Michele Mammon, C.P.T., C.E.S. of Body Elite Fitness. And if you think these classic exercises are for beginners only, think again. Make a few tweaks and you’ll feel a whole new burn. These next-level variations on foundational bodyweight moves—straight from seven trainers themselves—will kick your butt and knock your ego down a notch.

Pushups

This traditional exercise may bring back memories of high school P.E., but that doesn’t mean it belongs in your fitness routine’s past. The move recruits your chest, triceps, shoulders, and it also requires a stable core, says Mammon.

“You can make pushups harder by focusing on the lowering part of the movement,” she says. When you lower yourself to the ground, hold yourself there for a moment before pushing back up.

Other ways to make pushups more challenging: Place your hands in the shape of a diamond by connecting your thumbs and pointer fingers, elevate your hands or feet on a bench, or fold one arm behind your back to attempt one-armed pushups, Mammon says.

Lunges

Sometimes taking a step backward when working out can be a good thing—that is, if it means you’re changing the direction of your lunges. Fire up your lower-body in a new way by doing lunges either out to the side (called ‘lateral lunges’) or on an angle (like ‘curtsy lunges’), recommends Chris Heuisler, National Run Concierge of Westin Hotels & Resorts. “The older we get, the more accustomed we get to only walking in straight lines,” Heuisler says. Incorporating different planes of motion into your lunges strengthens your muscles’ movement through multiple angles, he says.

You can also turn up your lunge by reaching your arm (on the side you’re stepping forward with) toward the ceiling as you step forward into your lunge, says Katie Fanok, a personal and group trainer in Westchester, N.Y. As you do this, reach your opposite arm back and try to touch your back heel. You’ll test your balance and coordination—and get a good stretch in.

To get your heart rate pumping, turn your lunges into jump lunges. Start in lunge position and jump up, switching your leg position as you do so, landing in a lunge on the opposite side, Fanok says.

Related: 15 Bodyweight Exercises That Show Major Results

Planks

Tons of trainers will tell you: Your best bet for scoring a six-pack isn’t suffering through crunch after crunch, but staying completely still in a plank position. Problem is, staying completely still can get a little, well, boring.

Put a twist on your planks—and light up your obliques—by trying side planks, recommends Joshua Martin, co-founder of FindYourTrainer. With your left foot stacked on top of your right, plant your right hand on the ground and push up so your body forms a straight line on your side. Your shoulders should be aligned above your right arm. You can keep your left hand on your hip or reach it straight up. Don’t let your hips drop! Martin suggests starting with 15 seconds on each side, and adding increments of 10 to 15 seconds as you get stronger.

Need more movement? Create a little side-to-side plank flow. Start off in a side plank and slowly transition to a regular high plank. Then slowly move into a side plank on the other side, Martin says. “The key is to be as smooth, stable, and graceful in your transition as possible, which is no easy task!” he says. Start out with two or three full flows, and add more as you get better.

Another way to make planks more dynamic: Add alternating shoulder taps, lift one foot off the ground at a time, or move up and down between a high plank and elbow plank, suggests Allison Tibbs, a San Francisco-based personal trainer and healthy lifestyle coach. “That’s what I love about body weight—adding dynamic or plyometric movements can change the intensity,” she says.

Squats

You already know that squats—bodyweight or not—are one of the best moves for your body.

For a heart rate bump and a quick burn, turn your squats into jump squats. “Perform a regular squat, pausing for a brief moment at the bottom of the movement, then extend explosively and jump as high as you can,” says Patrick Schoeneborn, C.P.T. and owner of Body 100. Try doing as many reps as you can in one minute.

Make your usual squat more challenging by adding an element of instability to the move, recommends Schoeneborn. Try performing your squats while standing on a BOSU ball. “Fold your arms across your chest to make it even harder,” he says.

If you’re feeling really ambitious, go for one-legged squats (known as ‘pistol squats’). This squat variation is particularly intense, says Paul Baytan, C.P.T. You’ll extend one leg out in front of you and lower into your squat on the other leg. You working leg has to fire twice as hard as usual to support your weight on its own, while your core works extra hard to keep you balanced, he says.

Hollow Body Holds

The hollow body hold, one of Baytan’s favorite core exercises, is another move in which you’re not moving at all. (It’s also known as the ‘dead bug.’) In the easiest version of the exercise, you lie on your back and extend your arms and legs straight upwards, bending your knees at a 90-degree angle. “Push your palms and knees to the sky while pushing your lower back into the floor and maintain a tight core,” Baytan says.

To level up the intensity, keep both your arms and legs straight, extended out at 45-degree angles from your torso. Still, maintain a tight core and push your lower back into the floor, he says.

Now the hardest version: “Fully extend your arms and legs, and lift them off the ground just about six to 10 inches,” Baytan says. You’ll hear this variation referred to as ‘the banana’ because your body takes on that slightly curved shape. The goal is to stay still and keep your core tense—it’s surprisingly taxing.