Muscles worked
The machine crunch mainly trains the rectus abdominis, which curls your spine and draws your ribcage towards your pelvis, with help from the obliques. The machine guides the path, so you can focus fully on curling up, and the weight rises in small steps. How much your hip flexors join in depends on how hard you push with your feet: the more your legs work, the more it becomes a hip bend instead of a crunch.
Common mistakes
- The machine is not set up for you. With the wrong seat or pad height, the load sits off and you feel it more in your shoulders and arms than your abs. Follow the picture or label most machines have, and test a light weight first.
- The weight stack clanks between reps. Letting the plates land gives your abs a rest every rep. Turn round just before they touch, so the tension lasts the whole set.
- Your feet push hard against the supports. Pressing with your legs hands the work to your hip flexors. Rest your feet lightly, just for stability, and think of your abs bringing your chest down.
Variations and alternatives
- The cable crunch is done kneeling at the cable, with a freer path that asks more of your form.
- Crunches train the same muscle on the floor, when you are away from the gym.
- The reverse crunch curls your spine from the bottom up instead and makes a good swap for variety.
Sets and reps
Because the machine can be loaded, your abs can be trained like any other muscle. Three sets of 8 to 15 reps is a good start, and 15 to 20 works with a lighter weight. Pick a weight where the last reps are hard but you still curl up fully. Reps in reserve is a handy way to judge that, and the guide to sets and reps goes into choosing numbers.
When you reach the top of the range in every set, add the next plate and start again lower down. That is the core of progressive overload. Machines differ between gyms, even identical-looking ones, so compare weights on the same machine and note your seat setting.