Forget planks — this 15-minute crunches workout sculpts your abs

a photo of a woman doing abdominal crunches on her back
(Image credit: Getty Images/Ciricvelibor)

Love them or loathe them, there’s a reason why crunches feature heavily in ab workouts — compared to sit-ups, they really isolate the abdominal muscles, helping you work your core hard. There’s a reason why Lily James did 600 reps of a day when getting in shape to play Pamela Anderson — when done correctly, crunches and sit-ups can really torch your core, when done with proper form. 

If you’re new to crunches, it’s important to nail your technique, before adding reps and trying different variations. Start on your back with both hands resting on the back of your head, bend your knees, and place both feet on your mat, hip-width apart. Engage your stomach muscles, then lift your upper back off the mat while keeping the lower back pressed into the mat. Pause, then slowly lower back to your starting position. Remember to move slowly, and with control — the movement should be coming from your abdominal muscles, and you shouldn’t be pulling on your neck to raise your torso up off the floor. Here's more on how to do crunches.

Like many of the best ab exercises, including sit-ups, crunches develop a stronger core, which improves posture, mobility, and flexibility and helps you function better day-to-day. For many people, crunches are accessible, but if you’re working with a back injury, we recommend checking in with a personal trainer or doctor first. 

What is the 15-minute crunches workout? 

The workout, created by YouTube trainer Caroline Girvan is a 15-minute bodyweight workout, primarily targeting the upper abs. You do each exercise for 45 seconds, followed by a 15-second rest. You won’t need any equipment other than one of the best yoga mats or an exercise mat, so you can put your best adjustable dumbbells and the best kettlebells to one side. 

You can follow along with Girvan in real-time, but here are some of the exercises you can expect: 

X over knee crunch: For this variation, start in a crunch position, with your feet pressed into the floor and your hands resting behind your head. Cross one leg over the other, so your foot is resting on the opposite knee, as you crunch up, reach the opposite elbow to your knee, it doesn’t matter if they don’t actually touch. Pause at the top, then lower back to your starting position. In the workout, Girvan does 45 seconds on one side, before swapping to the other. 

Reverse crunch: To do a reverse crunch, start by lying on your back, with your lower back, head, and neck pressed into the floor. Bend your legs at the knee, press your feet into the mat, and engage your core, thinking about sucking your belly button into your spine. Keep your knees bent, inhale, and raise your legs, hips, and bottom off the mat. Pause at the top of the movement, using your core to keep your hips raised off the mat. Your head and neck should stay imprinted on the mat. As you exhale, lower your hips back to the mat. That’s one rep. Here’s more on how to do a reverse crunch, the benefits, and the modifications to try

Toe reach crunch: Also known as vertical leg crunches, start by lying on your back with your arms outstretched to the ceiling. Raise your legs together, lifting them to the ceiling, with a slight bend in your knee. Ensure your lower back is pressed into the floor, and that there isn’t any space between your back and the mat. Squeeze your core, thinking about sucking your belly button into your spine. Curl your head, neck, and shoulder blades up off the mat, keeping your legs straight, and reach your arms towards your toes. Pause and hold the movement when your shoulders are lifted off the floor. Lower your upper body back to the mat, moving slowly and with control. Keep your legs lifted, don’t let them crash back to the mat. That’s one rep. 

What are the benefits? 

There are a number of benefits to doing crunches to work on your core. Some crunch variations target the oblique muscles through lateral flexion and rotation, while others hit the rectus abdominal muscles (the six-pack muscles) with flexion and extension of the spine. You’ll undoubtedly hit various other major muscles too (depending on the crunch), such as the shoulders, arms, lower back, glutes and hip flexor muscles. 

As this ab workout doesn’t repeat exercises, you won’t get bored, and it’s a good one to add onto the end of a strength session, a run, or to try on a rest day to work on the muscles in your core. 

That said, hundreds of crunches won’t guarantee a svelte midsection. Remember that stubborn genetics and other lifestyle factors like body fat percentage and diet play a part. Here are 5 reasons you can’t see your abs yet despite working out.  

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Jane McGuire
Fitness editor

Jane McGuire is Tom's Guide's Fitness editor, which means she looks after everything fitness related - from running gear to yoga mats. An avid runner, Jane has tested and reviewed fitness products for the past five years, so knows what to look for when finding a good running watch or a pair of shorts with pockets big enough for your smartphone. When she's not pounding the pavements, you'll find Jane striding round the Surrey Hills, taking far too many photos of her puppy.