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January 12, 2021 10 min read

Your biceps could probably use some work. No offense, but if you’re not a professional bodybuilder or the strongest man in the world, then your muscles could always use a little improvement.

Maybe you think you’ve hit your own ceiling. If you feel like your growth has plateaued, and you truly can’t get any more out of your arms, then 21s are probably the solution.

You’ll improve your connection to your body, you’ll improve your range of motion, and most importantly, you’ll get the size and definition you’ve been seeking out this whole time. All of this from a minor tweak in your technique. Sound too good to be true? We’ve got science and Arnold Schwarzenegger to back us up.

What is This?

21s are a seven-rep three-part curl. They’re designed to take advantage of your body’s anatomy, the effects of gravity, and your muscle growth mechanisms.

From an outside perspective, they sound like a magic hyper-advanced technique that should take years to master, but in reality, they can be applied to any exercise you do with any part of your body with a little examination of your natural range of motion and a touch of critical thinking. 

The three parts of these barbell curls are:

  • Seven reps of your lower range of motion
  • Seven reps of your upper range of motion
  • Seven reps of your full range of motion

You’ll notice that you’re not going to need any special equipment or any extra discipline, all you’re doing here is breaking your exercise into two halves and putting it back together to put a bow on things. 


The idea here is to maximize the amount of time you spend putting tension on your muscles. Tension is one of the best ways to wear your muscles down and encourage hypertrophy.

By stopping short of your full range of motion (full ROM) for two-thirds of your sets you’re forcing your body to put a lot of tension on your working muscles. You don’t get to take a break at the end of each rep by reaching a neutral position and resting on your laurels. The three-part curl is all about exposing hiding places and eliminating them. 

Switching out your regular curls for 21s is an excellent way to build muscle for those simple reasons. If you’re taking the time to make it out to the gym, you might as well be making the most of it. It’s also a great way to break out of a rut, if you feel like you’ve plateaued, then this is a great way to put your muscles to the test. You might find that part of your range of motion has been suffering from neglect.

21s force you to evaluate your entire range of motion by focusing on each half. Maybe you’ve been accidentally relying on momentum to get your weights half of the way up. By forcing yourself to methodically perform your curls you’ll find your weak spots immediately and work past them. When you’ve worked your muscles evenly and fully you’ll be lifting heavier weights without a second thought in no time. 

You’re also going to be spending a lot of time increasing your mind-muscle connection. If this is your first time thinking about the mind-muscle connection, it may sound like a lot of pseudoscience, but it’s been proven to increase max reps by a great deal.

The reason for this is simple. You’re not doing anything more than fully engaging your muscles in the exact way they need to be in order to more efficiently lift your weights. We spend so much of our lives moving our bodies without thinking about it, but something like three-part curls forces you to really confront the minute movements of your biceps, and how those movements apply to lifting weights. When you fully understand your body and its mechanisms you can really exploit that knowledge to push yourself further than you ever thought possible.

Mind-biceps Connection

With that in mind, let’s talk about your biceps. Since you’re going to be spending this time getting to know your biceps so well, it’s important to learn how they work, where they really are, and what they do for your body.

The Latin name for your biceps is “musculus biceps brachii.” That translates to "two-headed muscle of the arm.” That fantastic sounding name comes from how your biceps are shaped. The two heads of your biceps start on the scapula and merge together into one larger muscle and attach to your upper forearm.

That single muscle belly where the two heads of your biceps converge is what you’re seeing when you flex your arms. The two-headed shape of your biceps makes them responsible for supination and flexion. Imagine doing a punch in reverse. The twisting motion is the two heads allowing your arm to twist, this is supination, and when you pull your arm back in towards your body this is the flexion.

Burning Fat

21s are a great exercise for burning fat. If you’re looking to drop weight while lifting weights or if you’re looking for increased muscle definition this exercise is great to have under your belt. Burning fat in your body is a matter of burning through all of the energy your body has stored up. Three-part curls are great for this. Maintaining tension in your muscles is one of the best ways to expend energy during your workouts, and that’s what 21s are all about.

The primary adjustment in this exercise is targeted directly at all of the wasted potentials in standard biceps curls. You can supplement this fat-burning effect by watching your diet closely. Keeping an excess amount of fat out of your diet is one of the best ways to clear this obstacle to fat burn. Monitoring the number of carbohydrates and unused calories you add into your system is also going to help in preventing your body from secreting away excess energy into fat cells for use later in the week.

Bicep man eating salad

Building Muscle

Building muscle is a tough thing to pin down. 21s try to take the best from each potential hypertrophy trigger and bundles them all together in a handy package. You’re going to be keeping tension on your muscles throughout the entirety of this exercise, and lifting progressively heavier weights each time you come back to 3-part curls. You’re also doing these three relatively intense exercises back to back, keeping your body in a hypoxic state. All of this together is a surefire way to add mass to your muscles no matter what your body tends to require as a trigger.

You can help this along by adding plenty of protein to your diet and allowing yourself to rest your arms. Building muscle is all about patching together your body with protein and increasing the efficiency of your muscle fibers. When you push your muscles to the brink your body wants to repair that microtrauma, and you need raw material to pull that off. When you’re building muscle you’re replacing and augmenting your torn muscle fibers.

Your body used the protein in your diet to make your individual muscles bigger than they were when you started, you’re making your body more effective at the tasks you’ve been subjecting it to. Your body also makes your muscles more effective at storing glycogen. That means less storage in fat cells, and thicker, stronger muscles. 

You need rest to get those bigger muscles. Traumatizing your biceps daily without a rest day in sight means you’re blunting the effects of your exercise. Always allow your muscles at least one day off, that doesn’t mean you have to completely leave the gym behind. At the very least, switch up the muscles you’re working from day to day if you absolutely can’t bring yourself to take even a single day fully to yourself, and you’ll come out the other side more swole than you can possibly imagine.

How to do 21s

21s are easy to remember. All you have to keep in mind is a standard biceps curl. Break it in half and put it back together, and you’re good to go.

7 reps of the lower range of motion

  • Stand upright with your weights by your side
  • Lift them no further than halfway up through your biceps’s range of motion
  • Slowly lower the weights back to your side
  • Repeat seven times

7 reps of the upper range of motion

  • Stand upright with your weights at a 90-degree angle through your biceps’s range of motion
  • Lift the weight deliberately through the rest of your range of motion
  • Lower them back to the halfway point
  • Repeat seven times

Seven reps of your full range of motion

  • Stand upright with the weights down by your side
  • Lift them up through your biceps’s full range of motion, squeezing at the top
  • Lower them all the way back into the starting position
  • Repeat seven times

Improving on Improvement

21s are a little dated. They’re the exercise that Arnold Schwarzenegger popularized in his Encyclopedia of Modern Bodybuilding, and that still holds a lot of water to this day. He’s one of the most popular bodybuilders to this day. He’s a household name, and he’s been the Chairman of the President's Council on Physical Fitness and Sports. He knows what he’s talking about, and he’s put in the work to prove it works. But over time we’ve figured out how to build on the foundation he laid out for us.

For example, we know now from a study published in the Journal of Strength and Conditioning Research in May of 2019 that one of the more effective ways to induce hypertrophy is through tension on the muscles applied by utilizing your partial range of motion. This means a few things. Arnold in the 80s was right on the money. His 21s were forcing him to apply tension for extended periods of time by using his partial range of motion, but with this new knowledge, we can expand on that, if you’re up for the challenge. 


Welcome to Advanced 21s:

Advanced 21s are the same basic idea as 21s. You’re still breaking down your basic curl, but we’re going to take into account the full range of motion your biceps are capable of. We’re also adding in more isometric movements. The full set of Advanced 21s is:’

7 reps of mid-range partial reps

  • Stand upright with a dumbbell in each hand
  • Bring your arms up to a 45-degree angle
  • Lift until your arms are 45 degrees past the mid-point of your arm’s natural range of motion
  • Lower back to your starting position
  • Repeat seven times

7  seconds of Isometric mid-range holds with wrist twists

  • Hold your arms in a mid-range position
  • Keeping your wrists rigid, twist your forearms through their range of motion
  • Repeat for the full 7 seconds

 7 reps of the full range of motion

  • Stand upright with the weights by your side in a hammer grip
  • Lift all the way through your range of motion 
  • While lifting twist your weights, you should start with a hammer grip and end with the weights parallel to your chest
  • Repeat seven times

Apply Yourself

The most interesting thing about 21s is their flexibility. Now that you know the secret to their success you can apply these to any part of your body that you’re trying to get an edge on. Nearly every exercise you do has some kind of motion to it. Take the time to learn about your body, and you can fully deconstruct every exercise and put it back together into a 21 modified version of it. 

Have you been working on your abs? Do you think they could benefit from something more intense? Try applying this method to your sit-ups. Your abs, or the rectus abdominis muscles, are responsible for flexing the lumbar spine. Experiment with that full range of motion. Your abs bring your ribs down towards your pelvis when you’re doing a crunch, but if you fix your upper body in place your abs also bring your pelvis up to your rib cage. 

Now that you know which movements your abs are specifically responsible for, you can create a 21 style crunch. You could try alternating between fixing your upper and lower body in place.

You could crunch halfway off the ground for one half and lower your body to your halfway point for the other half. Don’t forget that isometric exercises are also important in the three-part curl, so you could keep that spirit alive by holding your upper or lower body off of the ground for an extended period of time. Round out your two pronged approaches by moving through your full range of motion, and you’ll have reinvented your routine.

You’ll notice for that example that you don’t need and free weights to take advantage of 21s. Isometric exercises are just about maintaining tension in your muscles. Lots of isometric exercises can be done with your bodyweight alone.

This is something you can apply to every single one of your muscle groups and every body part, so keep your mind open, watch for areas you may be struggling in, and take the time to break them down. 

Additional Tips

  • Don’t rely on momentum to get yourself through these reps. If you can keep tension on your muscles throughout the entire range of motion, you’ll be reaping greater benefits from the exercise. This is a modification that relies on you being willing to push yourself further than the standard variant of this exercise. Sneaking shortcuts into this exercise will defeat the purpose
  • You’re not limited to dumbbells with this exercise. A barbell or EZ bar will yield similar results. Barbells are great for ensuring you’re moving each of your arms at an equal rate, and they’ll make it easier to pack on more weight in the long run. Dumbbells force you to keep the efforts of each of your arms separate. You won’t be able to compensate with your stronger arm while lifting with a dumbbell in each hand. Dumbbells are also the only way you’ll be able to take on the advanced variant of the 3-part curl. Keeping that in mind, choose what’s right for you. 
  • Breaking this exercise down into its component parts like this will incidentally target other muscles at well. When you’re working on your upper range of motion, don’t be afraid to include your triceps. They’re responsible for the extension of your arms, so they’re naturally going to become involved in lowering your arms if you make sure to keep tension on your arms throughout the entire exercise
  • When lowering your arms, keep tension on them by keeping your arms slightly bent when you make it all the way to the bottom. This tiny adjustment to your form will keep your muscles begging for mercy

3-part curls aren’t a magic heaven-sent workout. They’re a carefully considered technique that takes knowledge of the human and applies it from a different angle. Your workout is all about harnessing the processes of your body and working them to the absolute limit. 21s are a simple change you can make to your routine right now without upsetting the rhythm of your workout routine in the slightest. Being willing to take new knowledge and apply it to your already existing workout will push you further and get you more visible and satisfying results.