In out hammer curls are a little-known exercise for working the biceps muscles. You might have come across the movement before if you've done P90X, which a classic program for building muscle and getting in shape. But other than that, hardly anyone is aware of this exercise's benefits.
This guide shows you how to do a hammer curl in and out style so that you can recruit more muscle fibers in your biceps, brachialis, and brachioradialis—and ultimately sculpt more aesthetic arms.
Related Exercise: Hammer kettlebell curls
In out hammer curls exercise details
- Main Muscles: Biceps brachii, brachialis, brachioradialis
- Secondary Muscles: Forearm extensors, forearm flexors
- Exercise Type: Strength
- Exercise Mechanics: Isolation
- Difficulty Level: Intermediate
- Equipment Needed: Dumbbells
How to do in and out hammer curls
- Hold a pair of dumbbells by your sides with a neutral grip.
- Ensure that your thumbs are pointing straight ahead.
- Curl the weights toward your shoulders until your biceps and forearms make forceful contact.
- Lower the weights under control until your elbows are close to a full lockout.
- Immediately externally rotate your shoulders slightly so that your thumbs are now pointing out to the sides.
- With your elbows pressed into your sides and your forearms flared out, curl the weights toward your shoulders and squeeze your biceps as before.
- Lower the dumbbells under control. This is one rep.
- Repeat for 3-4 sets of 6-8 reps in total.
In and out hammer curl pros and cons
Like standard in and out curls, in and out hammer curls have been around for a long time (more than 10 years!). Yet, they don't seem to have caught on among the gym-going population. Here are the pros and cons that you'll need to weigh up before deciding whether or not this little-known bicep-builder is right for you.
Pro: Better bicep stretch
When you perform the "out" part of in out hammer curls, you get a really good stretch in your biceps, even more so than in dumbbell spider hammer curls. Why is stretch this important?
Because hammer curls are generally considered an all-rounder for the arms. In other words, the traditional version works your biceps, brachioradialis, and brachialis fairly equally.
And you still get that muscle-building equality with the "in" portion of in out hammer curls. However, when you do the outer part, your biceps, in particular, get put under an incredibly deep weighted stretch.
This extra stimulation helps to break down more muscle fibers because it overloads your biceps with heavy mechanical tension, which is actually one of the most important prerequisites for muscle growth. 
Con: More complicated form
Exercises such as machine hammer curls are ideal for beginners because the technique is remarkably simple.
However, this isn't the case with in-out hammer curls.
Sure, there are certainly more complicated movements out there, but if you've been doing regular hammer curls for years like I have, then it can be challenging to remember to do the "out" part of the rep. This is because the traditional hammer curl technique has already been ingrained into my brain from years of training.
Also, if you're new to lifting, then there's a chance that you'll unintentionally sacrifice the quality of your form in order to remember the correct movement pattern. Specifically, you might get tempted to shortcut the range of motion if you know that each rep of in-and-out hammer curls is essentially like two reps of traditional hammer curls.
Pro: Proven to work
P90X is a proven workout program with numerous testimonials from people who've gained muscle, supercharged their strength, all while getting in great shape in the process. As such, you're not taking a gamble by performing the in-out hammer curl because you already know that it reliably builds better biceps.
Many people like to invent their own exercise variations. And while these exercises can indeed work if you have plenty of training experience or biomechanical knowledge, sometimes sticking to proven muscle-building movements is the way to go. In out hammer curls are a great example of such an exercise.
Con: More fatiguing
One rep of in and out hammer curls is equal to two reps of regular hammer curls. This means that your muscles are getting twice as much time under tension and, as such, are getting a more potent growth stimulus.
Of course, doing twice the work is undoubtedly more tiring than just doing half of the job. However, since you're not doing any extra sets, the additional reps that you're performing don't actually feel as fatiguing as you might think.
In fact, training this way is psychologically powerful because it results in shorter, more intense workouts, which most people tend to prefer in comparison to training sessions that take hours to complete.
Conclusion: Are in out hammer curls worth doing?
Usually, if you wanted to work your arms from different angles—to simulate the various muscle fibers—then you'd need to perform two separate exercises. But with in out hammer curls, you get this muscle-building benefit in one movement.
Not only does this make your workouts shorter, but it also overloads your biceps with more tension, which is to say, with a more potent growth stimulus.
Not many people know this, but in and out hammer curls are also a fantastic exercise for the brachialis, which is the small muscle that lies beneath the biceps and which can make your arms look thicker when well-developed.
The reason for this is that the brachialis is quite a slow-twitch muscle.  This means that it responds best to high amounts of training volume and plenty of time under tension. Well, that's precisely what you're getting with in-out hammer curls because every rep is essentially like two reps of the regular version!
So enjoy those gains. They will certainly come if your recovery is good and if your diet is on point.
- Krzysztofik, Wilk, Wojdała, & Gołaś. (2019). Maximizing Muscle Hypertrophy: A Systematic Review of Advanced Resistance Training Techniques and Methods. International Journal of Environmental Research and Public Health, 16(24), 4897. https://doi.org/10.3390/ijerph16244897
- Hyght Dc, N. C. T. (2015, January 21). The Complete Guide to Biceps Training | T Nation. T NATION. https://www.t-nation.com/training/complete-guide-to-biceps-training