Full-body strength training is a quick and efficient way to hit all the major muscle groups, but there are benefits to taking at least one day a week to zero in on one or two key areas. That's why Le Sweat [1] founder Charlee Atkins, CSCS, created this toning twofer for POPSUGAR'S 4-Week Workout Challenge. "Most total-body workouts include lots of squats and lunges, which require both a lot of energy and movement," Atkins said. "This upper body and core workout is a nice active recovery. It won't be as intense as a complete strength training day."

Your core is also essential to all of your workouts. If your middle isn't strong enough, you'll find that not only will your form suffer, but it will also be harder to get in the amount of reps that are required for a circuit. Keep that in mind as you work through these exercises, making sure to keep your abs engaged throughout the workout.

20-Minute, No-Equipment Upper Body and Core Workout

Directions: Complete each exercise for 40 seconds, followed by a 10-second recovery. You'll do three rounds total with a one-minute rest in between rounds. Don't forget to spend at least five minutes warming up (these low-impact exercises are a good start), and take time to cool down after your final circuit with these full-body stretches [2].

Exercises:

• Shoulder taps
• Leg lowers
• Butterfly sit-ups
• Eccentric push-ups
• Glutes march
• Triple-triple

Shoulder Taps

• Begin in a high plank position with your feet slightly wider than your hips for added stability.
• Keeping your torso stable, bring your right hand to your left shoulder, then return your right hand back to the mat.
• Bring your left hand to your right shoulder and return it to the mat.
• Continue alternating hands for 40 seconds.

Leg Lowers

• Lie on your back with your legs straight up, perpendicular to the floor.
• Bring your elbows out to your sides and your arms to your chest, making a fist with each hand.
• Keeping your foot flat and your core engaged, slowly lower your left leg toward the mat, pausing a few inches before your heel would touch the floor. Make sure your lower back stays anchored to the mat.
• Press your elbows into the mat and squeeze your core as you raise the leg back to starting position.
• Repeat with your right leg, and continue alternating sides for 40 seconds.

Butterfly Sit-Ups

• Lie on your back, and open your legs into a diamond shape with the soles of your feet pressed together and your knees wide. Extend your arms overhead.
• Inhale and squeeze your core as you roll all the way up to a seated position, touching your hands to your toes.
• Exhale as you lower back to the starting position. Keep your core engaged and move with control, instead of flopping down on the mat.
• Continue this movement for 40 seconds.

Eccentric Push-Ups

• Start in a plank position with your arms and legs straight, shoulders above your wrists, core engaged.
• As you inhale, slowly bend your elbows, keeping them close to your sides as you lower your body toward the floor.
• When your shoulders are in line with your elbows, drop to your knees.
• As you exhale, press your palms into the floor and straighten your elbows, keeping your core tight.
• Get back into a plank position, with your knees off the floor, and continue this movement for 40 seconds.

Glutes March

• Lie on your back with your feet about 12 inches from your butt. Place your hands slightly out to the side with your palms facing up.
• Press your heels into the ground and lift your pelvis up until your knees, pelvis, and shoulders form a straight line.
• Hold this position as you lift your right knee toward your chest, until your hip is at a 90-degree angle. Return your heel to the floor and repeat with the left leg.
• Continue alternating legs for 40 seconds. Make sure to keep your core engaged to prevent your pelvis from sagging down.

Triple-Triple

• Lie on your back with your hands behind your head. Raise your legs so your knees are bent at a 90-degree angle; keep your feet flat and toes pointed toward the ceiling.
• Perform three crunches, making sure not to pull on your neck as you lift your back off the floor.
• With your legs still bent at a 90-degree angle, drop your arms down to your sides, palms touching the ground.
• Perform three reverse crunches, using your lower abs to slowly curl your hips off the floor and into your chest, then back down to the mat.
• Continue this sequence for 40 seconds.