If you have thumb pain or rhizarthrosis, which is arthritis at the base of the thumb, understanding the muscles in your forearm might help you make sense of your symptoms. Researchers recently studied a special muscle called the accessory abductor pollicis longus muscle. This muscle does not exist in everyone. Scientists wanted to know how common it is and what it looks like.
Here are three important things they found:
- This extra muscle appears in half of all people.
- The muscle has tendons that could be used for surgical repairs.
- The muscle might play a role in certain thumb pain conditions.
Key Findings at a Glance
- According to this study, the accessory abductor pollicis longus muscle was found in 50% of the forearms examined.
- Research shows that most cases were one sided, with only one person having the muscle on both arms.
- The distal tendon, which is the part that attaches further down the arm, measured 3.91 cm on average and may be useful for reconstructive surgery.
- Studies indicate that this muscle variant might contribute to De Quervain’s tenosynovitis, a painful condition of the thumb tendons.
- The proximal tendon, which is the part that attaches higher up, was too short for grafting at just 1.20 cm on average.
In this article:
- What Did the Researchers Study?
- Who Has This Extra Muscle?
- What Did the Measurements Show?
- Why Does This Matter for Your Thumb Pain?
- Frequently Asked Questions
- What Does This Mean for You?
- Important Information
- Research Source
What Did the Researchers Study?
Scientists in Bolivia looked at the forearms of eight adults who had donated their bodies to science. They studied 16 forearms in total. Six of the people were men and two were women. The researchers carefully opened the skin and tissue to find and measure a special muscle.
Some people have an extra version of this muscle. It is called the accessory abductor pollicis longus. The word accessory means extra or additional. Think of it like having a helper muscle.
You may be wondering why this matters.
This extra muscle connects to your thumb with rope like structures called tendons. Tendons are what attach muscles to bones. When doctors need to fix damaged tendons in the hand, they sometimes borrow tendons from other places. Knowing where these extra tendons are can help surgeons plan better operations.
Who Has This Extra Muscle?
Research shows that the accessory muscle was present in 50% of the forearms studied. This means half of the arms had it and half did not. That is a lot more common than you might think.
Here is where it gets interesting.
Most people who had this muscle only had it on one arm. Just one person in the study had the muscle on both the right and left sides. This tells us that even though the muscle is fairly common, it usually shows up differently on each side of your body.
The muscle had a spindle shape. Imagine a football or an almond. The fibers inside the muscle all ran in the same direction, like strings on a guitar. This shape and structure tell doctors how the muscle works and how strong it might be.
What Did the Measurements Show?
According to this study, the scientists measured four different parts of each muscle they found. They used a special digital tool called a caliper that measures very precisely.
Let us look closer at what they found.
| Part of the Muscle | Average Size | What It Means |
|---|---|---|
| Proximal tendon length | 1.20 cm | The part attached higher up in the arm |
| Distal tendon length | 3.91 cm | The part attached lower down near the thumb |
| Muscle length | 5.30 cm | The fleshy middle part |
| Transverse muscle length | 0.55 cm | How wide the muscle is across |
The proximal tendon is the part that attaches higher up in your forearm. Proximal means closer to the center of your body. This tendon was quite short at about 1.2 centimeters. That is less than half an inch.
The distal tendon is the part that attaches lower down, closer to your thumb. Distal means farther from the center of your body. This tendon was much longer at almost 4 centimeters. That is a little over an inch and a half.
One person in the study had something very unusual. They had an extra tendon branching off the main muscle. This extra tendon measured 6.23 centimeters. This shows that even within this already unusual muscle, there can be even more variation.
Why Does This Matter for Your Thumb Pain?
If you have thumb pain, you might have heard of a condition called De Quervain’s tenosynovitis. This is a painful swelling of the tendons on the thumb side of your wrist. It hurts when you make a fist or turn your wrist.
Studies indicate that having extra muscles or extra tendons in this area might make this condition more likely. When you have more structures trying to fit through the same narrow tunnel, there is more crowding and more friction. This can lead to inflammation and pain.
For people who need surgery, this research is also important. If a tendon in your thumb or hand is badly damaged, a surgeon might need to borrow a tendon from somewhere else to fix it. This is called a tendon transfer or tendon graft.
Research shows that the distal tendon of this accessory muscle is long enough to be used for grafts. At almost 4 centimeters, it gives surgeons enough length to work with. The proximal tendon is too short. But knowing that the distal tendon is there gives doctors another option when planning reconstructive surgery.
The muscle itself is small and thin. At only about half a centimeter wide, it is not very bulky. This makes it easier to move without leaving a big gap where it used to be.
What About Rhizarthrosis?
Rhizarthrosis is another name for arthritis at the base of your thumb. The joint where your thumb connects to your wrist is called the carpometacarpal joint. When the cartilage in this joint wears away, the bones rub together. This causes pain, stiffness, and weakness.
Understanding the muscles and tendons around this joint helps doctors plan treatments. Some surgeries for rhizarthrosis involve removing part of a bone and using a tendon to cushion the space. Knowing that an accessory muscle might be present gives surgeons more choices.
Even if you are not having surgery, knowing about these variations can help explain why your symptoms might be different from someone else with the same diagnosis. Your anatomy is unique.
Frequently Asked Questions
Does everyone have this accessory muscle?
No. According to this study, only 50% of people have this extra muscle. That means half of people have it and half do not. It is also usually only on one side of the body.
Can this muscle cause thumb pain?
It might. Research shows that extra muscles and tendons in the thumb area can contribute to conditions like De Quervain’s tenosynovitis. The extra structures can create crowding and friction. However, many people have this muscle and never have any pain at all.
How would I know if I have this muscle?
You cannot tell just by looking or feeling your arm. The muscle is deep under the skin. A doctor would need to use imaging like an ultrasound or MRI to see it. Sometimes it is discovered during surgery. Most of the time, it does not matter whether you have it or not unless you are having symptoms or need surgery.
Can this muscle be used to fix other problems?
Yes. Studies indicate that the distal tendon of this muscle is long enough to be used in reconstructive surgery. If you need a tendon graft to repair damage elsewhere in your hand, this could be a source. The proximal tendon is too short, but the distal one works well.
What Does This Mean for You?
If you are living with thumb pain or rhizarthrosis, this research adds to what we know about the complex anatomy of your hand and forearm. Everyone’s body is a little different. Some people have extra muscles or tendons that most people do not have.
Knowing about these variations helps your doctor understand your symptoms better. It also gives surgeons more options if you ever need an operation. The accessory abductor pollicis longus muscle is common enough that doctors should always keep it in mind.
If you have persistent thumb pain, swelling on the thumb side of your wrist, or difficulty gripping, talk to your doctor. They can examine you and help you figure out what is causing your symptoms. There are many treatment options available, from splints and therapy to injections and surgery.
You are not alone in dealing with thumb pain. Many people experience these issues, and research like this helps improve the care you receive.
About This Site
This website is dedicated to helping people like you understand the science behind thumb pain and rhizarthrosis. We read published medical research articles and translate them into plain language. Everything we share comes from real scientific studies. We do not invent facts or give personal medical advice. Our goal is to give you knowledge so you can have better conversations with your healthcare team. Feel free to explore more articles on this site to learn about your condition.
Important Information
This article is for educational purposes only. It is not medical advice. It does not replace the care and guidance of your doctor or healthcare provider. Every person is different. What works for one person may not work for another. If you have thumb pain, swelling, or difficulty using your hand, please see a qualified healthcare professional. They can examine you, order tests if needed, and create a treatment plan that is right for you.
Research Source
This article is based on research published by Duque-Colorado, Jhonatan; Rodriguez-Torrez, Victor Hugo; García-Orozco, Laura; Algieri, Rubén Daniel; Ottone, Nicolás E. in the journal Diagnostics in 2025.
Read the full study here: https://doi.org/10.3390/diagnostics15192455
Now that you understand the anatomy, learn how rhizarthrosis is diagnosed and what the Eaton stages mean.
Diagnosis Guide
