Core · Spinal flexion
Crunches
The crunch is an abdominal exercise where you lift your head and shoulder blades off the floor by contracting your abs, without raising your lower back. It trains the rectus abdominis through spinal flexion. Unlike a sit-up, the range is short and the hip flexors stay largely uninvolved.
Also known as: Ab crunch, Abdominal crunch, Floor crunch
- Muscle group
- Core
- Movement pattern
- Spinal flexion
- Type
- Isolation
- Equipment
- Bodyweight
- Difficulty
- Beginner
What muscles does the crunches work?
Primary movers
- Rectus abdominis
Assisting muscles
- Obliques
The rectus abdominis flexes the spine — it pulls your ribcage toward your pelvis. The crunch is built entirely around that single action, which is what makes it a genuine isolation exercise for the abs.
The key difference from a sit-up is where the movement stops. A crunch lifts only the head and shoulder blades, keeping the lower back on the floor, so the movement stays in the thoracic and upper lumbar spine. Once you sit up further, the hip flexors take over and the abs' contribution largely stops.
That short range is a feature rather than a limitation. The rectus abdominis is a relatively short muscle and does not need a large range to be trained effectively — what it needs is a hard contraction against resistance.
The obliques assist, particularly if you add any rotation. Because bodyweight crunches are limited in load, adding resistance — a plate on the chest, or a cable crunch — is usually a better progression than doing more reps.
How to do the crunches
Setup
- 1
Lie on your back with your knees bent and feet flat on the floor, hip-width apart.
- 2
Place your hands lightly at the sides of your head, or crossed over your chest.
- 3
If your hands are by your head, keep them there without pulling — fingertips resting, not gripping.
- 4
Press your lower back gently into the floor.
- 5
Tuck your chin very slightly, keeping a fist-width of space between chin and chest.
Execution
- 1
Exhale and curl your ribcage toward your pelvis, lifting your head and shoulder blades off the floor.
- 2
Stop once your shoulder blades clear the floor — your lower back stays down throughout.
- 3
Squeeze your abs hard at the top for a full second.
- 4
Inhale and lower under control over two seconds.
- 5
Stop just short of resting your head on the floor to keep tension on the abs.
- 6
Keep the movement slow — this exercise gains nothing from speed.
Common crunches mistakes
Pulling on the neck
Using the hands behind the head to yank the head forward. It strains the cervical spine and lets the arms do work the abs should be doing.
The fix: Rest your fingertips lightly against your head without gripping, or cross your arms over your chest instead. If your neck is tired before your abs, this is why.
Using momentum
Bouncing up and down quickly, so the movement is driven by rebound rather than muscular contraction.
The fix: Take two seconds up and two seconds down with a hard squeeze at the top. Fewer slow reps produce more than many fast ones.
Lifting the lower back off the floor
Coming up too far, which turns the movement into a sit-up and brings the hip flexors in. The abs stop being the limiting factor.
The fix: Stop as soon as your shoulder blades clear the floor. Your lower back should stay in contact throughout the entire set.
Holding your breath
Bracing by locking the breath, which raises blood pressure and makes it harder to contract the abs fully.
The fix: Exhale on the way up, inhale on the way down. Exhaling actually helps the abs contract harder at the top.
How many sets and reps?
| Goal | Sets | Reps | Load | Rest |
|---|---|---|---|---|
| Hypertrophy | 3 | 12–20 | Weighted, 1–2 RIR | 60 s |
| Bodyweight | 3 | 15–25 | Bodyweight, controlled | 45–60 s |
| Beginner | 2–3 | 10–15 | Bodyweight | 45 s |
The abs are muscles like any other and respond to progressive overload, so once bodyweight crunches reach 25 clean reps, hold a plate across your chest rather than chasing higher rep counts. Two to three sessions a week is plenty. Crunches train spinal flexion only — pair them with anti-extension work like planks and anti-rotation work for complete core training.
Crunches variations and alternatives
Weighted Crunch
HarderHolding a plate on your chest or behind your head adds direct load. The most straightforward way to keep progressing.
Cable Crunch
HarderKneeling and crunching against a cable stack allows precise, progressive loading and keeps tension constant. The best long-term version for building abs.
Bicycle Crunch
AlternativeAdds rotation, bringing the obliques in alongside the rectus abdominis. Slower is better — this one gets rushed more than most.
Sit-Up
AlternativeA longer range that brings the hip flexors in. More total movement, but less isolated to the abs.
Plank
AlternativeTrains the abs to resist spinal movement rather than produce it. Genuinely complementary — most programs should include both.
Frequently asked questions
Do crunches burn belly fat?
No. Spot reduction is not possible — you cannot choose where your body takes fat from, and training a muscle does not preferentially burn the fat sitting on top of it. Crunches build the abdominal muscles; whether those muscles are visible depends on overall body fat, which is determined by your total energy balance.
How many crunches should I do a day?
Think in sets rather than daily totals, and train abs like any other muscle: two or three sessions a week, three sets of 12–25 controlled reps. Doing hundreds of crunches daily builds endurance in a small muscle rather than size or strength. If 25 reps is easy, add weight instead of reps.
Are crunches bad for your back?
For most healthy people, done correctly and in reasonable volume, no. The concern comes from research on repeated spinal flexion under load, and it is most relevant to people with existing disc issues or very high rep volumes. If you have a history of back problems, anti-extension work like planks and dead bugs is the safer default.
What is the difference between a crunch and a sit-up?
A crunch lifts only the head and shoulder blades while the lower back stays on the floor, isolating the abs through a short range. A sit-up brings the whole torso up to a seated position, which recruits the hip flexors heavily. Crunches are more targeted; sit-ups involve more total musculature.
Why does my neck hurt when I do crunches?
You are pulling your head forward with your hands rather than lifting with your abs. Rest your fingertips lightly against the sides of your head without gripping, or cross your arms over your chest. Keep a fist-width of space between your chin and chest throughout the movement.
Hercules tracks every crunches you log, shows you last session's numbers before you lift, and flags when your progress stalls.
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