The 21s exercise blasts the biceps muscles with a 7-7-7 sequence of bottom half, top half, and then full rep curls. It's an intense training drill that can help you to smash through plateaus because it provides your biceps with more time under tension than other exercises.
This guide shows you how to do biceps 21s and then discusses whether doing half curls for the biceps like this is good for muscle growth.
Barbell 21s exercise details
- Also Known As: Twenty ones exercise
- Main Muscles: Biceps brachii
- Secondary Muscles: Brachioradialis, brachialis, forearm flexors
- Exercise Type: Strength
- Exercise Mechanics: Isolation
- Difficulty Level: Intermediate
- Equipment Needed: Barbell, weights
How to do 21s for your biceps
- Load an appropriate weight onto a barbell. For more convenience, use a fixed bar or dumbbells.
- With the bar resting on your thighs, perform 7 partial curls in the bottom half of the range of motion (stop just before your elbows reach 90 degrees).
- Then do 7 curls in the top half of the range of motion (start at 90 degrees and curl up).
- Finish the set by performing 7 full range of motion curls.
- Do 2-4 sets in total (one set is equal to 3 x 7 reps).
Bicep 21s benefits
Doing 21 biceps curls in a row is tough because it generates a lot of lactic acid, which can really make your biceps burn. However, if you perform the twenty one exercise consistently, then you can reap some muscle-building rewards from your hard work as well.
Incredible arm pump
Performing 21 curl repetitions in quick succession creates an intense pump in your biceps, especially because you're not locking out your elbows and giving the blood a chance to escape from your muscles.
As such, barbell curl 21s make an excellent finishing movement for your biceps at the end of a hard and heavy workout. While they can certainly build mass as well, you're best off doing 21s for your biceps at the end of your session so that they don't interfere with the exercises that you're trying to gain strength on regularly.
Your arms will look more vascular after doing 21 curls because of the sheer volume of lactic acid in your biceps. This visible change in upper arm aesthetics lets you know that the exercise is working and can help to keep you motivated during training.
Improved volume tolerance
Total training volume is the primary determinant of muscle growth.  So if you can increase your volume tolerance, then you'll naturally be able to perform more reps and sets during a given session, week, or training cycle.
Ultimately, as long as you can recover from it, performing higher volume for a given body part usually leads to more short-term muscle growth.
So if you want to get ahead and speed up your bicep growth, then it's a wise idea to do exercises that improve your strength-endurance, which is essentially your ability to handle resistance training volume.
Performing 21 bicep curls in a row will get your biceps accustomed to handling lactic acid and repeated muscle contractions, which is to say training volume. So by doing bicep 21s, you'll also improve your ability to perform extra reps and sets on other exercises too because your biceps will have better endurance for resistance training.
More bicep size
Doing 21s on bicep curls means doubling the number of reps that many lifters typically perform. So on a per-set basis, your biceps are getting more tension from barbell 21s than traditional curls.
This helps to recruit extra muscle fibers and creates more robust signaling for hypertrophy, which, over time, will result in better bicep development if you pair your training with a good diet and sufficient recovery.
The arms, in general, also respond excellently to high-rep training, which may simply be because you're performing more volume with high reps. 
Variation: Dumbbell 21s
Doing dumbbell 21s is a great way to ensure that both of your biceps receive equal work because you have to lift each dumbbell independently. Therefore, you should do them instead of barbell curl 21s if you have muscular imbalances that you need to correct or if you want to limit their chance of occurring in the future.
The only downside of using dumbbells for 21s is that you can't overload your biceps with as much resistance as you can with the barbell version. The trade-off, however, is that dumbbells put less pressure on your wrists than the straight bar, so they might well be a better long-term exercise.
You can also do twisting curls when you use dumbbells by forcefully supinating your forearms. So even though you might be lifting less weight, you may actually be putting more tension and torque through your biceps, which is what counts for muscle growth.
21s workout routine
This 21s arm workout will beef up your biceps and enhance your aesthetics. As you can see from our recommendations, when you're doing so much volume per set, there's no need to risk overtraining by annihilating your arms with an excessive amount of sets. Doing 2-4 sets per exercise is ideal for a 21 bicep workout.
1: Bicep 21s — 2-4 sets
2: Tricep Extension 21s — 2-4 sets
3: Forearm 21s — 2-3 sets
See Also: Power curl
Conclusion: Should you do 21s for bicep curls?
Performing barbell 21s and dumbbell 21s is one of the most effective methods for pumping up your biceps. Due to the sheer amount of per set volume, 21s are also beneficial for building new muscle because you're overloading your biceps with so much tension.
Similarly, because you're performing high reps during 21s, they also have a joint-sparing effect because they don't put as much tension on your connective tissue as traditional, lower rep bicep curls do. 
- Coratella, G., & Schena, F. (2016). Eccentric resistance training increases and retains maximal strength, muscle endurance, and hypertrophy in trained men. Applied Physiology, Nutrition, and Metabolism, 41(11), 1184–1189. https://doi.org/10.1139/apnm-2016-0321
- Israetel, M. (2020, October 29). Bicep Growth Training Tips. Renaissance Periodization. https://renaissanceperiodization.com/bicep-training-tips-hypertrophy/
- Stoppani, J. (2018, February 2). Build Muscle Mass While Preventing Joint Damage. Muscle & Fitness. https://www.muscleandfitness.com/flexonline/training/build-muscle-mass-while-preventing-joint-damage/