20-minute workout: core efficiency

No-fuss abs moves that’ll leave your core shaky and your lungs heaving, from Rachel Mariotti.

While you can carve a tantalizingly toned body in less than a half hour, we’re here to tell you what you already know: To reap the rewards—increased calorie burn, strength, power, blissful endorphin high—truncated workouts must be insanely intense. So we asked a team of Equinox trainers and group fitness instructors what they would do if they were given just 20 minutes to have at it. Here is the latest in the series.

The Pro:Rachel Mariotti, Tier 3 trainer, group fitness instructor, former competitive gymnast and Division I collegiate hurdler

The Club:Trainer at 50th Street in New York City and fitness instructor at multiple Manhattan locations

The Workout:Here’s the thing with core-centric workouts: If it feels easy, you’re not doing it right. You’ll spend lots of time in push-up position. Ask yourself: Are you squeezing your abs and glutes? Are your hands directly below shoulders? “These exercises rely on control and will challenge your balance, which keeps the intensity and focus at a high level,” Mariotti says. “Core muscles are not just abs, but all the muscles that support your spine from your hips to your shoulders—having a strong core means you have a strong structure around your vertebrae, which can help injury-prevention and allow you to move more efficiently.” There's a scale-up option included below and if you’re still itching for more, perform 5 burpees between moves.

Perform each move for 2 minutes, repeating once. If fatigue sets in and form starts to suffer, perform each move for 90 seconds and rest 30 seconds, repeating once.

Beast hold
Start in tabletop. Keeping shoulders down and back, chest open, and abs and glutes engaged, lift knees off floor 1 inch and hold.
Make it harder:Slowly lift right hand and right shoulder a few inches off the ground and return to start; repeat on left side, alternating between sides.

Squat punch
Keeping shoulders retracted, abs engaged and chest tall, squat (or wall-sit) and slowly punch, alternating arms.
Make it harder:Punch holding 5 to 10-pound dumbbells in each hand.

Dumbbell slides
Begin in push-up position with a 5 to 10-pound dumbbell on the floor under chest. With right hand, slide dumbbell beneath right shoulder. Return right hand to start and grab dumbbell with left hand and slide under left shoulder; repeat, alternating hands.
Make it harder:After two sets of dumbbell slides, perform four knee-to-elbows, alternating right knee to left elbow, and left knee to right elbow.

Russian twists
Sit on floor, knees pressed together, back straight, heels on the floor. Lift feet a few inches and balance on tailbone. Make a fist with one hand and grasp with the other hand. Rotate torso and head to the left and drop elbows and fist to the left side; return to start and rotate and twist to the right. Continue, alternating sides.
Make it harder:After 20 twists, straighten legs and arms with hands over head; maintain hollow hold for 15 seconds, then resume twists.

Standing tall with soft knees, bend at the waist and drop hands to the floor. Engaging shoulders, abs and glutes, and keeping elbows locked, walk hands forward until they are beneath shoulders. Keeping legs straight, knees soft, walk feet forward and return to standing position. Repeat.
Make it harder:During each rep, while in push-up position, perform 4 mountain climbers, alternately drawing knees toward chest and returning to start.