Want to up your pull-up game with one simple fix? Wrap your thumb around the bar. That’s right, wrapping your thumb helps to create a stable shoulder position to pull from, it’s a stronger grip and it has loads more transferability. Of course it’s going to feel “weird” in the beginning, so did the overhead squat, front squat, kip, false-grip, hook-grip…
Hanging from the bar with the thumb wrapped keeps the shoulders slightly externally rotated and more engaged, thus a stronger pulling platform. This effect is enhanced the further we can get the grip around the bar, similar to a false-grip. Carl Paoli describes it as getting your pinky knuckle on top of the bar. This gives a better movement pattern allowing more lat engagement. You can see this when the athletes body leans back slightly at the beginning of the pull. Without the thumb wrapped this position requires much more effort and focus and is often forgotten leading to the athlete trying to pull with the biceps. This only gets worse as the athlete becomes tired and begins slipping further and further to the fingertips.

Thumb wrapped.

Thumb over.

It’s a stronger grip. This can easily be seen when considering trying to climb a rope with a similar grip. It just isn’t going to work. Having the thumb wrapped gives access to the clamping muscles of the thumb that otherwise would lie dormant and add to the overall strength of the grip. This is especially a big deal when trying to put together bigger sets and is often all that needs to be worked if you have some pull-ups but struggle with linking. Having your knuckles over the bar also provides a more stable and engaged wrist ideal for supporting kipping.

Knuckles around the bar gives a more stable and engaged wrist.

Thumb over leaves a limp wrist that does not help with the kip.

There are no other movements that we do without it. The only exception to that is the bar muscle-up and if you’re not wrapping it in pull-ups I wouldn’t expect it in the bar muscle-up either. All the barbell movements, rope climbs, anything on the rings, all use a wrapped thumb so it has tremendous transferability to everything else we do. If you start using it you’ll find all of those things get better too.
There are situations where a thumb over grip may be better, but for the general masses and those looking to get the most bang for their hour in the shop, a thumb around style gives the most return. Master the thumb around grip and then you can add other styles back in for variation and with other goals in mind. Here’s an easy way to start. Hang. Hang from the pull-up bar with your thumb wrapped for as long as you can. Your hand IS big enough. Do that every time you come in to the shop accumulating a few minutes total and it will begin to feel normal. Start incorporating it into your kipping work, and at the very least in warmups during classes. Remember, as it does become easier to continue working the knuckles further and further around the bar.
More Resources:
Improving Grip
Grip Matters