Remember gym class back at school? First there was stretching, then some basic strength moves like lunges and push-ups followed by some track-running, kickball, or other good old-fashioned cardiovascular activity. Too bad those days of scheduled sweat sessions (with a dedicated coach! for free!) are long gone. Adulthood means never having time to get a full work out, apparently?

No, not true at all, according to certified strength and conditioning specialist and personal trainer Brett Klika, author of the new book 7 Minutes to Fit: 50 Anytime, Anywhere Interval Workouts. "In the 90s, it was all about doing low-intensity exercise for a long amount of time," he explains. "That's not untrue, but what [experts] started to realize is that it doesn't really work on a large scale. That's a lot to ask in a society of people who don't have a lot of time."

RELATED: 8 Workout Moves With Tracy Anderson

Enter the 7-minute workout-craze. By now you've probably heard of the magic of HIIT, or high-intensity interval training, and how even a few minutes of intense activity can lead to metabolic changes that help your body burn fat and use fuel more efficiently throughout the day. "When you're done with high-intensity exercise, your body continues to use oxygen to recover and beneficial hormones stick around for about two days," explains Klika. "After a low-intensity session, when you're done, you're done."

That's why it makes sense to learn a few double-duty moves—meaning ones that get your heart rate up and strengthen large muscle groups at the same time.

RELATED: 10 Minute Workout: Get Toned From Head to Toe

You can combine the following seven moves into one fun circuit, or mix-and-match them into workouts you're already doing (and perhaps growing bored with). "You really can get favorable changes in 7 minutes or less, but for best results with HIIT you should do it for about 20 minutes. So repeat it until you get there," Klika says.

Perform each exercise for 30 seconds, and then rest for 10 seconds. And finally, to make sure you're reaching the right intensity, "you should be very uncomfortable, but able to answer yes or no questions."

Step-up squats

Step-up squats do it all: they get your heart rate up, and they improve lower-body strength and flexibility.

How to do it: Beginning in a push-up position, bend the left knee to bring the left foot as close to the outside of the left hand as possible. Then do the same with the right foot. Once both feet are in place near the hands, with the heels on the ground, reach the left hand straight up as high as possible, followed by the right hand, so that both hands are reaching upward. As you reach, chest and head should remain upright, heels remain on the ground, and the arms remain straight. Maintaining this position with the chest, arms, and head, move to a standing position. Bring arms back down and return to the bottom of the squat position, placing both hands on the ground.
Step the left, then the right foot back to the start position. Then repeat, leading with the other foot.

RELATED: 6 Workout Moves with Jennifer Widerstrom

Reverse lunge and rotate

The Reverse Lunge and Rotate combines core and lower-body strength. "As soon as you go to rotate, your body is going to want to keel over, so your core has to fire up to prevent that," Klika says.

How to do it: Begin by standing upright, feet at shoulder width or slightly wider, arms straight out in front of the body with hands clasped. Step as far back as possible with the right leg and lower the right knee to the ground, keeping the upper body upright and the arms outstretched. As the right knee bends to approach the ground, simultaneously rotate the torso toward the left knee. Return to the start position after the right knee touches the ground and repeat the motion beginning with the left leg.

From 7 Minutes to Fit By Brett Klika (Chronicle Books)

RELATED: Mix and Match Total-Body Workout

Curtsy lunges

This move will guarantee a sore butt the next day, so get ready! What's especially great about the curtsy lunge is that most traditional exercises move you forward or back, but this one moves you laterally, requiring you to work muscles you might not be working during regular squats and lunges, Klika says.

How to do it: Begin standing upright, with legs slightly farther than shoulder-width apart. Keeping the hips forward, step the left leg behind the right leg laterally. The legs will cross. Once the left leg has reached as far as possible and is placed on the ground without the hips twisting, lower the left knee to the ground. This motion should be similar to a curtsy. Return to the start position, alternate legs, and repeat on the other side. (Watch How to Do a Curtsy Lunge.)

From 7 Minutes to Fit By Brett Klika (Chronicle Books)

RELATED: Intense Cardio and Strength Circuit Workout

10s and 2s

This abdominal exercise targets the hard-to-reach lateral area of the abdomen. One note of caution: "You really want to stay within 10 and 2 o'clock because slamming your legs on the ground can injure your back," Klika says.

How to do it: Lie on your back with legs together, extended straight up toward the ceiling, perpendicu­lar to the floor. Arms should be straight out to the sides of the body, forming a T position. Keeping legs together and the arms out to the side, rotate legs side to side from a ten o’clock position to a two o’clock position.

From 7 Minutes to Fit By Brett Klika (Chronicle Books)

RELATED: The Joy Workout

Push-up spiders

Push-up spiders combine upper-body and core strength. "It's everything you want from a push-up, but requires a huge amount of coordination. As you move your hands and feet, your core has to stabilize you and everything has to work together," Klika says.

How to do it: Begin by doing a push-up. At the top of the motion (once the arms are straight), bring the right hand and left foot off the ground. Bend the left leg to bring the knee toward the chest while simultaneously reaching to the left foot with the right hand, under the body. Keep hips parallel to the ground. Once the right hand and left foot touch in a controlled manner, return to the starting position and repeat the movement with your right knee and left hand.

From 7 Minutes to Fit By Brett Klika (Chronicle Books)

RELATED: Your Perfect No-Gym Workout

Plank to push-up

Plank to push-up targets the core and upper-body muscles. You'll feel this move the most in the triceps, Klika says.

How to do it: Set up in a plank position, on your forearms with your hips parallel to the ground. Slightly shift your weight to the left forearm while placing the palm of the right hand on the ground. Push the palm of the right hand into the ground and straighten the right elbow, as if doing a push-up. Once the right elbow begins to straighten, repeat on the left side, ending in a Push-Up position. Once you reach a Push-Up position, return to the start position by bending the right elbow, then the left. Repeat the movement beginning with the weight on the right forearm and straightening the left arm.

From 7 Minutes to Fit By Brett Klika (Chronicle Books)

RELATED: 5 Best Sites to Stream Workouts

Surfers

Surfers improve upper- and lower-body strength and coordination. These really engage the upper back better than any other move, so doing them regularly can improve posture, Klika says.

How to do it: Begin by lying facedown on the ground, with palms on the ground, as if getting ready to do a push-up. Using the upper body, lower body, and trunk, jump to your feet as quickly as possible, land­ing in a squat position, with one leg in front of the other, body slightly sideways, as if riding a surfboard. Return to a facedown position and repeat with the other leg landing forward.

From 7 Minutes to Fit By Brett Klika (Chronicle Books)

RELATED: The Beach Body Workout

Adapted from 7 Minutes to Fit: 50 Anytime, Anywhere Interval Workouts by Brett Klika ($11, amazon.com)