Crunches
PRIMARY MUSCLE
Core
EQUIPMENT
Bodyweight
OVERVIEW
Crunches
It's 2026 and online you still find two diametrically opposed schools of thought: those who say the crunch is the only exercise you need for the abs, and those who say it's an obsolete and dangerous exercise for the back that should be wiped off the face of the earth. The truth, as often happens, is in the middle, but closer to the first camp than you might think.
The floor crunch is the most basic, most accessible, and most studied exercise to train the rectus abdominis. It requires no equipment, no experience, and, when done with correct technique, isolates trunk flexion with an effectiveness few other bodyweight movements can match. EMG studies confirm that the classic crunch significantly activates the rectus abdominis, and its simplicity makes it the ideal starting point for anyone who wants to build a strong, visible core.
The problem isn't the exercise itself, it's how it's performed. Pulling the neck with the hands, bouncing off the floor, doing 100 reps at the speed of light hoping to "burn belly fat": this is the recipe for wasting time and getting hurt. In this guide we cover how to perform the crunch in the way that matters, with the right biomechanics, the correct time under tension, and a clear progression that turns a "beginner" exercise into a real muscle-building tool. Because, as Jeff Nippard explains, abs respond to progressive overload exactly like any other muscle, and treating them differently is a mistake.
Is the crunch the best exercise for the abs?
The short answer: it's one of the best, not the only one, and not for all the reasons you think.
The classic floor crunch is excellent for isolating the rectus abdominis in its primary function, trunk flexion. It's the bodyweight exercise with the lowest learning curve and the highest portability: you do it anywhere, with nothing. For those starting to train the core directly, it's the natural starting point.
But "best exercise" depends on the goal. If you want abs hypertrophy (making the six-pack blocks pop), the crunch has a structural limit: resistance is fixed (the weight of your torso) and doesn't scale easily. As Mike Israetel emphasizes, growing abs like any other muscle requires progressive overload, and here the cable crunch, the machine crunch, and the crunch with a plate on the chest become superior because you can add weight over time.
If you want core stability for squat, deadlift, and athletic movements, the crunch is not the best choice: anti-flexion (plank), anti-rotation (Pallof press), and anti-extension (ab rollout) exercises train the core in the function it serves in compounds, stabilizing, not moving.
If you want visible abs, the crunch doesn't burn belly fat (no exercise does, in a localized way), you need a calorie deficit. But building mass in the rectus abdominis with direct exercises makes the abs thicker and more visible at higher body fat percentages.
In summary: the bodyweight crunch is the best starting point for those who don't train the abs directly. To progress over time, you have to move to variants that let you add external resistance. A smart core program includes both the crunch (or its weighted variant) and stability exercises, not one or the other.
MUSCLES INVOLVED
Muscles involved
The crunch works primarily on the rectus abdominis, the long flat muscle that runs from the sternum and costal cartilages to the pubis. Its mechanical action is trunk flexion, bringing the ribs closer to the pelvis, and it's exactly the movement of the crunch. The rectus abdominis is a single muscle: there's no "upper" and "lower" portion that activate separately. EMG studies show that during the classic crunch activation is more pronounced in the upper portion simply because the trunk flexes from top to bottom, but the entire muscle works.
The external and internal obliques activate as synergists during the classic crunch, even without rotation. Their primary function is rotation and lateral flexion of the trunk, but they also contribute to frontal flexion. If you want to emphasize the obliques, the crunch with rotation (bringing the shoulder toward the opposite knee) increases their activation, but the classic straight crunch still stimulates them.
The transverse abdominis, the deepest muscle of the abdominal wall, works as a stabilizer. It doesn't produce visible movement, but it contracts to increase intra-abdominal pressure and stabilize the spine during flexion. Activating it consciously ("pulling the navel toward the spine" before starting the flexion) improves the quality of the crunch.
The hip flexors (iliopsoas, rectus femoris) are the "enemy" of the crunch: if the flexion starts from the hip instead of the trunk, the work shifts from the rectus abdominis to the hip flexors. That's why the crunch is a short trunk flexion (shoulder blades coming off the floor), not a full sit-up (torso reaching the knees).
EXECUTION
How to perform Crunches
TIPS
Execution tips
The most important tip on the crunch is the quality of the contraction, not the number of reps. Doing 50 fast crunches without control stimulates the abs much less than 15 slow crunches with peak contraction and controlled eccentric. Studies on time under tension confirm that the rectus abdominis responds better to sets with 30-60 seconds of continuous tension than to fast reps.
On tempo: concentric in 2 seconds (trunk flexion), peak pause of 1-2 seconds, eccentric in 2-3 seconds (controlled descent). This brings a set of 12-15 reps to about 60-75 seconds, the ideal range for hypertrophic stimulus on the abs.
On progression: the bodyweight crunch has a resistance ceiling. When 3-4 sets of 15-20 reps with controlled tempo become easy, you have three options to progress: (1) add a plate on the chest holding it with arms crossed, (2) move to the cable crunch where you can increase the load continuously, (3) move to the machine crunch. As Jeff Nippard emphasizes, treating abs like any other muscle, with progressive overload, is the key to growing them. Log reps and load (if you use added weight) every session: you'll see the progress.
On breathing: exhale during flexion (concentric), inhale during descent (eccentric). Forced exhalation during flexion activates the transverse abdominis and improves the quality of the rectus contraction.
On frequency: abs recover quickly and tolerate high frequency. 6-12 weekly sets of direct work on the rectus abdominis, distributed across 2-3 sessions, is a productive range for hypertrophy. If you train the core only with 2 sets of crunches at the end of the workout, you're under-dosing the volume, like doing 2 sets of bench and expecting the chest to grow.
On the belly fat myth: the crunch doesn't burn belly fat. No exercise burns fat in a localized way. Abs become visible when body fat percentage drops below a certain threshold, and that depends on diet, not on the number of crunches. But training the abs with volume and progressive overload makes them thicker and more visible at slightly higher body fat percentages.
COMMON MISTAKES
Common mistakes
Pulling the neck with the hands
The most iconic crunch mistake: hands clasped behind the neck pulling the head forward. Result: the neck flexes, the cervical spine gets stressed, and the rectus abdominis works less because you're using your arms to "help" the movement. Fix: keep the fingers at the sides of the head (behind the ears, not behind the neck) or cross them on the chest. The hands must never pull, the head follows the trunk passively.
Turning the crunch into a sit-up
The crunch is a short trunk flexion, the shoulder blades come off the floor, the lower back stays in contact with the ground. If you reach a fully seated position (vertical torso, bent knees), you've done a sit-up: the hip flexors (iliopsoas) take over and the rectus abdominis loses tension in the second half of the movement. Fix: stop when the shoulder blades have fully come off and abdominal contraction is at its peak. You don't need elbows on the knees.
Excessive speed and momentum
Crunches done at maximum speed, bouncing off the floor, using the momentum of the descent to come back up. Time under tension collapses, the rectus abdominis works for a fraction of a second per rep, and effective work volume is minimal despite the high number of reps. Fix: use a controlled tempo (2 seconds up, 1-2 seconds peak pause, 2-3 seconds down). 15 slow crunches are worth more than 50 fast crunches.
Pelvis lifting off the floor
If the pelvis comes off the floor during the crunch, you're using the hip flexors to generate the movement. The crunch becomes a hip flexion, not a trunk flexion. The rectus abdominis loses isolation. Fix: focus on "bringing the ribs closer to the pelvis," not on "bringing the chest toward the knees." The pelvis and lower back stay glued to the floor throughout the set.
Fully relaxing the abs between reps
Going back down, fully resting the shoulders, relaxing the entire core, and then starting again. Continuous tension breaks, and every rep becomes a mini-exercise on its own instead of part of a fluid set. Fix: lower slowly and stop just before the shoulder blades fully touch the ground. Maintain slight tension in the rectus abdominis between reps.
Frequently asked questions
Track your core progress
Download IRON for free and log every crunches set to really train.
Download on Google Play