How to Know If You're Missing a Muscle in Your Arm

Going through life absent an entire forearm muscle feels like something you’d probably have noticed by now, but as Jonathan Bennion of the Institute of Human Anatomy explains in a recent YouTube video, most people have no clue that they’re missing their palmaris longus.

Located in the anterior forearm, and stretching from the elbow joint to the root of the palm, the slender tendon is one of 20 muscles in the antebrachium. There’s a reason that region has so many muscles — the forearm and hand (which has another 11 muscles itself) are responsible for a wide variety of movements, including flexion, extension, abduction and adduction).

Because there are so many muscles for these tasks, though, some are a bit unnecessary. The palmaris longus is only really responsible for a weak flex of the wrist. Which is why, according to one study, it’s now absent in about 14% of the population. Over many, many years of evolution, certain groups of homo sapiens have evolved to phase the palmaris longus out of their arms, recognizing that it plainly has little to offer.

Wondering if you’re part of that 14%? (Which could be larger, by the way. More recent, racially-inclusive studies suggest its absence soars up to 64% in some nations around the globe.) Fortunately, there’s a dead-simple test you can administer to figure out whether you’re missing the tendon.

Turn your arm over and stretch it out, with your palm facing the sky. Gently flex your wrist back towards you. Now touch the fingertip of your thumb to your pinkie. If you have a palmaris longus, it will appear raised and taut in the middle of your wrist. You can trace its line down your wrist, from where it meets the hand to where it disappears into the forearm. Take note: There’s another tendon in that area, too. The palmaris longus is larger and concentrated closer to your pinkie-finger side.

If none of this is making any sense because there doesn’t seem to be anything popping out of your wrist … well, you don’t have the muscle. It’s possible to have the palmaris longus in both wrists, neither or just one. Is not having one something to be concerned about? No, not at all. There are situations in which the tendon is used as graft for a torn tendon elsewhere (think Tommy John surgery in baseball), but other than that it’s a pretty useless little section of the body.

Its best use, in fact, is probably as a party trick. Next time you’re around friends or family, have them test the palmaris longus for themselves. If they’re miffed about missing it, let them know, as Bennion says in the video, that they’re just “more evolved” than everyone else.