Muscles worked
A sit-up starts as a crunch and finishes as a hip bend. First, the rectus abdominis curls your spine and lifts your shoulder blades off the floor. Then the hip flexors take over, mainly the deep iliopsoas and the rectus femoris, pulling your torso the rest of the way to your thighs. The obliques help throughout. Arm position sets the difficulty: reaching forward is easiest, crossed over your chest sits in between, and stretched overhead is hardest, as the weight of your arms ends up far from your hips.
Common mistakes
- Your back stays flat all the way up. Hinging up stiffly like a board lets your hip flexors do nearly everything. Roll up one vertebra at a time: head first, then shoulder blades, then lower back.
- You drop back to the floor. The way down is half the rep. Roll down in reverse order over two to three seconds, lower back first and head last.
- Your head hauls you up. Jutting your chin forward makes your neck work for nothing. Keep roughly a fist's width between chin and chest.
Variations and alternatives
- Crunches have a shorter range and less hip flexor work, a good choice if sit-ups bother your lower back.
- The V-up is a harder step in which your legs lift at the same time as your torso.
- The cable crunch loads the same spinal curl with weights at the gym, once bodyweight is no longer enough.
Sets and reps
Two to four sets of 10 to 20 reps suit most people. A set is over when you can no longer roll up without jerking, not when you hit a certain number. How rep ranges match your goal is covered in the guide to sets and reps.
Once 20 reps feel easy, make each rep harder rather than adding more: stretch your arms overhead, hold a weight plate against your chest or pause for a second halfway down. That is progressive overload for your abs. A mat stops your tailbone rubbing on the floor. If your lower back hurts, beyond hard work in your abs, stop the exercise and have a doctor or physiotherapist check it.