Crunch: Muscles Worked and How to Do It
TL;DR
A crunch works your abdominals almost exclusively. Curling your shoulders off the floor while your lower back stays down isolates the rectus abdominis, the long muscle running down the front of your torso, without recruiting your hip flexors the way a full sit-up does.
- Primary muscle:
- Abdominals
Muscles worked
Your abdominals, specifically the rectus abdominis, are the only prime mover in a crunch. Rolling your shoulders a few inches off the floor while keeping your lower back pressed down shortens this muscle through a small range, which keeps the exercise isolated to the abs instead of pulling in the hip flexors.
Keeping your feet flat or resting them on a bench, rather than anchoring them under a pad, also keeps your hip flexors from taking over the movement, so the work stays concentrated on your abdominals.
Log Crunch in PR Streak
PR Streak's library has 873 exercises, each with photos and step-by-step instructions like this one. Log this move and see what you lifted last time right next to every set, with every PR caught automatically and a chart for every exercise.
How to do it
- Lie flat on your back with your feet flat on the ground, or resting on a bench with your knees bent at a 90 degree angle. If you are resting your feet on a bench, place them three to four inches apart and point your toes inward so they touch.
- Now place your hands lightly on either side of your head keeping your elbows in. Tip: Don't lock your fingers behind your head.
- While pushing the small of your back down in the floor to better isolate your abdominal muscles, begin to roll your shoulders off the floor.
- Continue to push down as hard as you can with your lower back as you contract your abdominals and exhale. Your shoulders should come up off the floor only about four inches, and your lower back should remain on the floor. At the top of the movement, contract your abdominals hard and keep the contraction for a second. Tip: Focus on slow, controlled movement - don't cheat yourself by using momentum.
- After the one second contraction, begin to come down slowly again to the starting position as you inhale.
- Repeat for the recommended amount of repetitions.
Form tips
- Keep your lower back pressed into the floor throughout the movement instead of letting your hips tilt.
- Lift your shoulders only a few inches; you do not need to sit all the way up like a sit-up.
- Exhale and hold the contraction at the top for a second before lowering back down.
- Don't pull on your neck with your hands, rest your fingertips lightly beside your head instead.
Exercise details
| Equipment | Bodyweight |
|---|---|
| Level | Beginner |
| Mechanic | Isolation |
Used by lifters following PPL, 5x5, upper/lower, and more.
Frequently Asked Questions
Do crunches work the whole core or just the abs?
Crunches mainly work your abdominals. They do not train your obliques or lower back much, so pair them with rotational moves like the Russian twist for a more complete core routine.
How many crunches should I do per set?
Since crunches are a bodyweight isolation move, most lifters use them for endurance: 12 reps or more per set. Add a slow lowering phase or hold the top position longer to increase the challenge before adding external weight.
More abdominals exercises
Exercise instructions and photos come from the free-exercise-db library, public domain, the same library used inside the PR Streak app.
