I did 100 plank jacks every day for a week — here’s what happened to my body

a photo of a woman doing a plank
(Image credit: Shutterstock)

What do you get if you cross a jumping jack with a plank? A plank jack. This killer dynamic core move is as intense as it sounds, as you engage your stabilizer muscles to secure your body as you jump your legs in and out. But what are the benefits of a plank jack, and what would happen if I added 100 plank jacks to my exercise routine for a week? 

If you’re looking to get more bang for your buck during your ab workout, the plank jack is a great move to try, as it works your abs and shoulders at the same time. As you jump your legs in and out, your shoulders will be holding your body in a plank, meaning you’ll be working on your shoulder strength, as well as challenging your abdominal muscles in this move. 

As a reminder, 100 reps of anything is a lot, and what works for me might not be right for you and your body. If you’re new to plank jacks, or you’re returning to exercise following an injury, it’s a good idea to get a personal trainer to check your form before adding reps, to ensure you’re moving with the correct form. 

How to do a plank jack 

To do a plank jack, start in a plank position, with your arms extended, your hands under your shoulders, your feet together, and so your body is forming a straight line from the crown of your head to the heels of your feet.

Keep your core engaged by thinking about sucking your belly button into your spine. Jump both feet out to the side, as you would in a jumping jack. If you’re using a yoga mat, aim to land your feet at the edges of the mat. Quickly jump your feet back in, and continue jumping your feet in and out, keeping your core engaged.

Read more on how to do a plank jack, the benefits of plank jacks, and the modifications to try here.

how to do plank jacks illo

(Image credit: Shutterstock)

I did plank jacks every day for a week — here’s what happened 

Are plank jacks the secret to a stronger core? I set out to find out more. Here's what I learned: 

My core had to work super hard during this move

There’s no doubt about it — 100 reps of plank jacks was a lot. On day one I decided the best way to make this move more bearable was to break up the reps, so I did four sets of 25 plank jacks. This helped me focus on my form during the move — it’s important to keep your core muscles engaged throughout to avoid putting any strain on your lower back. 

Planks, and plank variations, including the plank jack, active all of the core muscles. Including the rectus abdominis (the outer ‘six-pack’ muscles), the transverse abdominis (the deeper core muscles), and the obliques. They also work into the hips and back. Far from being just an aesthetic goal, a strong core can help improve your balance and posture, as well as relieve lower back pain. 

I felt it in my shoulders

I wasn’t expecting to feel this exercise in my shoulders as much as I did, but by day three’s reps, I had to lower down to my elbows for the plank jacks. As a runner, I don’t spend an awful lot of time working my upper body, and this challenge reminded me I definitely had some work to do. 

I also found this exercise to be more of a cardio burst than regular planks or side planks. Similar to mountain climbers, the plank jacks raised my heart rate as I jumped my legs in and out. They are also much lower impact than say burpees or high knees, so this is definitely a move I’ll be adding to my warm-ups in the future. 

I swapped in some modifications

I added this challenge to my routine on week four of marathon training and after one particularly grueling 13-mile run, doing 100 plank jacks felt like a step too far for my tired muscles. Instead, I swapped the jacks out for plank jack toe taps, where I tapped one foot out to the side of my exercise mat, then the other, rather than jumping. I actually found the toe taps took me longer to complete, which worked my core as hard. This was a great reminder that rest days are important, but also that you should always listen to your body, and modify exercises if need be.

On another day, I decided to up the ante and put one of the best resistance bands around my thighs to add some resistance to my plank jacks. This made me feel the exercise more in my legs, and forced me to work harder to keep my pelvis stable. 

I’ll be doing this one again in the future

Of course, a week of plank jacks didn’t visibly change my core — visible ab muscles are the result of a low body fat percentage, not endless plank jacks or sit-ups (here’s how to calculate your body fat percentage, and why it matters). That said, I really enjoyed this challenge, and will definitely be adding plank jacks to my warm-ups in the future, just not 700 of them. 

Looking for more workout inspiration? Here's what happened when I added a plank to my morning routine for a week, and when I did walking planks every day. Also, check out this calisthenics workout, that only uses four exercises to build full-body strength.

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.