Curl ups do not work your biceps. A curl up is an abdominal exercise, sometimes called a crunch, that targets the muscles of your midsection. Your biceps play essentially no role in the movement. If you’re looking to build your arms, you need a different exercise entirely.
What Curl Ups Actually Target
The name “curl up” can be misleading because “curl” also appears in “bicep curl,” but these are completely different exercises. A curl up involves lying on your back and lifting your head, shoulders, and upper trunk off the floor by flexing your spine. Your arms either cross your chest or rest behind your head. They don’t lift, pull, or push anything.
The muscles doing the work are your rectus abdominis (the “six-pack” muscle), your external and internal obliques (the muscles along your sides), and the deep stabilizing muscles of your core. Electromyography studies measuring electrical activity in these muscles confirm that the rectus abdominis and internal obliques are the most active during a curl up, with activation levels ranging from about 23% to 50% of their maximum capacity depending on speed. The external obliques contribute more as you increase the pace of your repetitions.
Unlike a full sit-up, a curl up deliberately limits the range of motion to keep the work concentrated in your abs rather than recruiting your hip flexors and lower back.
Why Your Biceps Aren’t Involved
Your biceps have three primary jobs: bending your elbow, rotating your forearm so your palm faces up, and assisting with shoulder flexion. All of these actions involve your arm moving against resistance. During a curl up, your arms stay stationary. There’s no elbow bending, no forearm rotation, and no load on the shoulder joint. Without those movements, your biceps have nothing to contract against.
Think of it this way: a muscle only works when it’s asked to shorten or resist lengthening under some kind of load. Resting your hands behind your head during a crunch doesn’t count. Your biceps are along for the ride, not doing any meaningful work.
Curl Up vs. Bicep Curl
The confusion likely comes from the shared word “curl,” but the two exercises couldn’t be more different. A curl up is a core exercise performed on the floor using spinal flexion to work your abs. A bicep curl is an arm exercise where you hold a weight and bend your elbow to lift it, directly targeting the biceps brachii, the brachialis (the muscle underneath your biceps that adds thickness), and the brachioradialis in your forearm.
Bicep curls are an isolation exercise, meaning they focus almost entirely on one muscle group with minimal involvement from anything else. Curl ups are also fairly isolated, just to a completely different part of the body. There is no overlap in the muscles they train.
Exercises That Actually Build Biceps
If your goal is bigger or stronger arms, you need exercises that involve bending your elbow against resistance. Research sponsored by the American Council on Exercise tested eight common bicep exercises using muscle activation data and found that the concentration curl produced significantly higher biceps activation than any other movement tested. A concentration curl is performed seated, with your elbow braced against your inner thigh, curling a single dumbbell. The bracing prevents your shoulder and back from helping, which forces the biceps to do nearly all the work.
Other effective options from that same research include:
- Barbell curls: a classic standing curl using a straight bar, allowing you to load heavier weight
- Cable curls: provide constant tension throughout the range of motion
- Preacher curls: performed on an angled pad that locks your upper arm in place
- Chin-ups: a compound movement that trains both your back and biceps simultaneously, making it efficient for overall upper body development
Traditional dumbbell curls also remain highly effective. EMG data shows they produce greater biceps activation than cable variations like the Bayesian curl, likely because the free weight demands more from the biceps as the primary mover without the stabilizing assistance of a cable system.
Can You Train Both in One Workout?
Absolutely, and it’s a reasonable approach. Curl ups and bicep curls target completely separate muscle groups, so doing both in the same session creates no conflict. You could pair a few sets of curl ups for your core with concentration curls or barbell curls for your arms without one exercise fatiguing the muscles needed for the other. Many training programs already combine core work with upper body exercises in exactly this way.
The key takeaway is simple: if you want stronger abs, do curl ups. If you want bigger biceps, pick up a weight and curl it with your arms. The word “curl” is the only thing these two exercises have in common.