Short answer:  The chin-up is a pull-up variation done with a supinated/underhand grip (palms facing towards you). Subsequently, the biggest arm flexors i.e. the biceps receive a significant market share in the exercise.

The contribution of the biceps combined with the ability to scale the chin-up forever by adding weight, turn the movement into a highly potent biceps builder.

Nonetheless, chin-ups do not always equal big biceps. The multitude of muscle groups involved in the lift has the potential to make the biceps less dominant. The structural characteristics of the lifter also influence the extent of bicep activation.

Not everyone can develop big biceps from chin-ups. In some situations, supplementary direct arm work is necessary to maximize arm growth.

The Problem with Compound Exercises

Not all team members contribute equally to the end goal. Some do more than others and compensate for the underperforming units. The same mechanism is observed in compound exercises – the naturally strong muscle groups do extra work and cover the weakness of their underdeveloped brothers.

Besides the biceps, the chin-up engages the latissimus dorsi, the traps, the rhomboids, the brachialis, the forearms, the long head of the triceps…etc. As a result, torso-dominant people may fail to get the same arm engagement in the movement as limb-dominant brahs.

Your personal anthropometry matters too. In the search for efficiency, the body activates the muscles in the most mechanically advantageous position to complete a lift.

When You Have Good Biceps Genetics Almost Everything Works

If you have long biceps with very short, almost invisible, tendons inserting straight into the elbow, you have a very high chance of developing them with almost any exercise that hits the area.

I’m on the opposite end of the spectrum – I have very short biceps and long tendons. My biceps are lacking. Conversely, I have low lats going down to the lower back. My lats have always been somewhat developed – even when I wasn’t training. Hence why long muscle bellies are a very desirable genetic trait in bodybuilding.

The calisthenics athlete Zef Zakaveli has top tier biceps development and is a perfect example of this dependence. I can’t know for sure, but the video material showing him perform endless sets of pull-ups, dips and push-ups strongly suggests that those are his main exercises.

I wish I could tell you that pull-ups and chin-ups have the secret ability to catapult your biceps into an equally impressive muscular prosperity, but that would be a lie. If you look closely at Zakaveli’s arms, you will see that he has some Sergio Oliva type of insertions.

Long muscle bellies have a higher potential for growth because there is more muscle to hypertrophy in the first place.

For the same reason, bodybuilders who tear their biceps or triceps (e.g., Dorian Yates) often come back with weak arms. The muscle is shorter and even anabolics cannot boost it into majesty.

Another example would be King Kamali – people have always criticized him for his underdeveloped arms. Realistically, however, he can’t do much to bring them up as their size and shape are the product of his long tendons and short muscle bellies.

Weighted Chins-ups vs. Curls: What will build bigger biceps?

According to the 5×5 wizards, weighted chin-ups are the ultimate biceps exercise because they expose the area to brutally heavy weights. You’re lifting yourself plus a ton of iron hanging off of you.

Meanwhile, curls are for losers because you are moving weights a little heavier than your school bag. Your arms can never grow from that…Right?

Logic disagrees. You can pull a lot of weight during chin-ups thanks to the back which is many times bigger than your biceps.

The 5×5 zealots are manipulatively comparing multi-joint exercises to isolation movements mainly to overhype their “Reg Park approved routines”. They want you to think that you will launch your natural potential into the orbit if you become a real man doing compound exercises. It cannot happen.

So, what’s better for biceps – curls or weighted chin-ups?

The answer is clear – curls. They directly hit the biceps and do not leave room for other muscle groups to take over when the exercise is done with proper technique. If you have weak biceps, it’s very logical to attack them directly.

There’s a reason why arm wrestlers do a lot of curls – the exercise is scalable and can produce seriously strong arm flexors.

Another bonus of curls is their ability to hit the biceps hard regardless of your anthropometry. The biceps cannot hide from curls even when you’re a torso dominant brah.

Having said that, it’s highly advisable not to become a curl monkey. Select a curl variation that agrees with your joints, stick to it and don’t overdo it.

The Reality Pill

Neither curls nor weighted chin-ups can compensate for poverty arm genetics as much we would like. If you have poor insertions, all the curls and chin-ups in the world won’t give you the full arms of Zef Zakaveli, for example.

How Do Gymnasts Develop Such Massive Arms?

Gymnasts are the poster boys for bodyweight-induced hypertrophy. Unsurprisingly, the promoters of gymnastic training sometimes “forget” crucial details that instantly change the nuance of the presented image.

The professors are quick to show you pictures of gymnasts with bodybuilding arms, but rarely tell you the following:

1. Gymnasts do dozens of bodyweight exercises from a very young age.

Pull-ups, chin-ups dips and push-ups are warm-up to elite athletes. You will need 3-5+ years to get to the truly advanced gymnastic stunts. If you are not built for the sport, you’ll never reach the top.

2. Gymnasts often have short limbs.

Every sport has a favorable body type. During the Soviet era, gym coaches and sports scouts were categorizing the potential athletes by their structure rather than the parents’ wishes. Short people were often selected as Olympic weightlifters and gymnasts whereas tall people were sent to the track or the volleyball field.

If you are not built for a sport, you’re swimming against the current and sentencing yourself to mediocrity at best. Doesn’t sound very sweet, but it’s the case nonetheless.

Most pro gymnasts are short because that’s an advantage in their craft. It’s easier to perform an iron cross when your arms are not very long. Good luck doing a maltese as a basketball player.

Guess, what? The advanced elements and the preparatory movements leading to them are the actual growth stimulators in gymnastics. Trying to replicate those stunts solely for their hypertrophy value is not practical and potentially dangerous, especially if you have a structure (long limbs) multiplying the torque on the joints during straight arm work.

3. Not all gymnasts have enormous arms.

Many gymnasts are significantly smaller than you think.

4. Many gymnasts do curls.

A surprising number of gymnasts do curls as an assistance exercise to condition the muscles and tendons of the arm.

Summary: Weighted chin-ups are an effective biceps exercise, but they don’t possess the mythical power attributed to them by 5×5 marketers and other prophets. People with good biceps genetics may do just fine without performing a curl in their life, but torso-dominant individuals could benefit from direct biceps work i.e. curls. If you don’t have long muscle bellies, your biceps will probably remain smaller than you want them.