Frog Crunch
Also known as: frog crunches, butterfly crunch, butterfly crunches, frog sit up, butterfly sit up
- Primary muscleAbs
- EquipmentBodyweight
- DifficultyBeginner
- TypeCore · Core flexion
- Secondary muscles
- Hip Flexors
- Body part
- Core
- Position
- Lying
- Laterality
- Bilateral (both sides together)
- Category
- Strength
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Dropping your knees out with the soles of your feet together takes the hip flexors mostly out of the curl, so the short range you do get comes almost entirely from the abs. It shares that logic with the Butterfly Reverse Crunch, only here the torso moves and the legs stay parked.
How to do the Frog Crunch
- Lie flat on your back with your knees bent and feet flat on the ground.
- Place your hands behind your head with your elbows pointing outwards.
- Engaging your abs, lift your upper body off the ground, curling forward until your torso is at a 45-degree angle.
- Pause for a moment at the top, then slowly lower your upper body back down to the starting position.
- Repeat for the desired number of repetitions.
Tip: Breathe out as you crunch and let the last bit of air leave the lungs at the top of each rep.
Muscles worked
Primary
Abs · Core
The rectus abdominis shortens to bend the spine forward, and each rep should end with the upper abs fully contracted and the ribs pulled down.
Secondary
- Hip Flexors — The hip flexors take over if your feet are anchored and you come up too high; keep the range short to leave them out.
Common mistakes
- Pulling on the neck — hands behind the head should support, not yank; keep the elbows wide and the chin about a fist away from the chest.
- Sitting all the way up — a crunch is a curl of the ribs toward the pelvis, not a sit-up; the lower back should stay on the floor the whole time.
- Cranking out fast reps — momentum does the work if you bounce; lift to a squeeze, hold for a count and lower slowly.
- Never adding load — once bodyweight reps get easy, hold a plate on the chest or use a cable rather than doing sets of 50.
Where it fits in a workout
Crunches close out a session after the heavy work, or they can fill the rest periods between sets of a lift that does not need much bracing. Pair a cable crunch with a cable side bend on the same stack.
How many sets, how heavy and how hard: see what the ACSM position stands support.
Variations and alternatives
Similar exercises
Without bodyweight
When to talk to someone
- Chest discomfort, feeling faint or lightheaded, or trouble breathing during or after exercise can be signs of a heart attack. In the US, call 911 right away; elsewhere, call your local emergency number.
- Heart, metabolic or kidney disease, or symptoms like these when you are active: talk to your doctor before you start or step up training.
- Pregnant: ask your obstetric care provider which exercise suits you.
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More exercises
Sources
- ACSM position stand on exercise quantity and quality (Garber et al., 2011)
- ACSM preparticipation health screening (Riebe et al., 2015)
- ACOG Committee Opinion No. 804 on exercise in pregnancy (2020)
- CDC: About Heart Attack Symptoms, Risk, and Recovery (2024)
Sources checked on October 17, 2026. Full citations are in the exercise library.
Animation and exercise data: ExerciseDB, licensed to SetGrip. General information, not medical or coaching advice.