Extensor Carpi Radialis

The Extensor Carpi Radialis is the largest of four muscles in the cranial lateral group on the antebrachium. It is primarily a carpal extensor, but it also plays a small role in elbow flexion. It is easily palpable on the cranial forearm. After the free surface of the radius, it is the first muscle that you will feel going from medial to cranial.

Watch my full length video on the Extensor Carpi Radialis on YouTube.

The other three muscles in the cranial lateral antebrachial group originate on the lateral epicondyle on the humerus. The Extensor Carpi Radialis is different. It originates from the lateral epicondylar crest, sometimes also called the lateral supracondylar crest. Both terms refer to 'above' or 'over'. This is located just above the lateral epicondyle, on the humerus. The brachioradialis, when present, also originates from the lateral epicondylar crest, just a little proximal to the Extensor Carpi Radialis.

The Extensor Carpi Radialis is actually two muscle bellies. Later we will see that it has two tendons, and two points of insertion. For the most part the craniolateral antebrachial muscles will have a quantity of muscle bellies equal to (or at least close to) the number of tendons of insertions. These muscle bellies are partially fused and very closely bound together. 

From the epicondylar crest the muscle goes over the joint capsule of the elbow and moves cranially, descending down the dorsal/cranial antebrachium. The joint capsule is acting like a bursa, reducing friction and helping everything to move smoothly. 

Fascia coming from the brachialis and biceps brachii wraps over the proximal Extensor Carpi Radialis. Why? I'm not exactly sure. Stability? Unity between the main elbow flexors? Connection to distribute forces generated by the elbow flexors of the arm? I think that is probably the case. Spreading out the load will put less strain on an individual muscle or tendon, creating a more robust system of connective tissues. We see this occur in human anatomy as well. Proximally it is also briefly connected to the Extensor Digitorum Communis by fascia.

It is described as a long, thick, fleshy muscle. While not a huge muscle, it is long and thick in relation to the other muscles in the craniolateral group. Fleshy? It is a muscle, but couldn't you describe every muscle that way? I suppose some muscles do have greater or lesser amounts of muscle tissue in relation to tendon size. The Extensor Digitorum Communis and the Extensor Digitorum Lateralis for example are tendinous for much of the forearm. 

Anyway, it continues distally, until about ⅔ of the way down the radius it splits into two tendons. The smaller Extensor Carpi Radialis Longus, located more medially, and the larger Extensor Carpi Radialis Brevis, located more laterally. The two tendons stay together down the forearm. They pass over the middle sulcus of the radius on the distal radius. There are three grooves on the distal radius. The most medial contains the Abductor Pollicis Longus tendon which crosses over the Extensor Carpi Radialis tendons. The lateral groove contains the Extensor Digitorum Communis tendons. The middle one, which is the largest, contains the tendons of the Extensor Carpi Radialis. 

At the level of the ulnar notch the tendons can be palpated. The Ulnar notch is just proximal to the distal end of the radius on the lateral side where the radius and ulna meet. The tendons pass under the extensor retinaculum which keeps them in their place as they pass over the dorsal surface of the carpus. Once they pass over the intercarpal joint level they diverge and the Extensor Carpi Radialis Longus inserts into a small tuberosity on the dorsal surface at the base of metacarpal 2 and the Extensor Carpi Radialis Brevis inserts into another small tuberosity on dorsal surface at the base of metacarpal 3. Remember the 'base' of the metacarpals is located proximally and the 'head' is located distally, with the body connecting the ends together.

Dogs will have varying lengths of tendon sheaths. About half of dogs will have a common tendon sheath that wraps about both tendons from their beginning, all the way down to the proximal metacarpals. Others may have a synovial sheath present just over the carpus. Often a synovial bursa will be under the tendons at the proximal carpal bones, helping everything to slide smoothly.

The Extensor Carpi Radialis is innervated by the Radial nerve, as are all of the muscles in the cranial lateral group. If a dog suffers a radial nerve injury you may see loss of carpal extension, leading the dog to put the paw down with the carpus flexed, walking on the dorsal surface of the carpus.

The primary function of the Extensor Carpi Radialis is carpal extension. But when does it extend the carpus? Well, remember it crosses the elbow joint, and plays a small role in elbow flexion. That means the muscle is being elongated when the elbow extends. Think about a dog in the late swing phase. The shoulder and elbow are extended. The arm reaches forward. The carpus flicks forward into extension preparing the paw to meet the ground. The Extensor Carpi Radialis is this extensor. 

In fact, because a muscle and tendon can only be stretched so far, this mechanism happens passively. Meaning even in a cadaver, if you extend the elbow, the carpus extends as well. The Extensor Carpi Radialis is responsible for this action. When one end gets longer, the other end needs to shorten. The elbow extends, the Extensor Carpi Radialis is stretched so the carpus extends.

As I said, the Extensor Carpi Radialis also contributes to elbow flexion to some degree. This isn't surprising since it crosses the elbow joint on the cranial side. But it isn't a significant role. One study said the Extensor Carpi Radialis flexes the elbow during the swing phase of the gait, but only when the dog is walking. When the dog transitions to a trot or gallop this muscle activity goes away.

The Extensor Carpi Radialis also plays a role in stabilization. They did a study on cadavers where they looked at what structures limited elbow flexion and extension. Elbow flexion has a hard bony stop. But elbow extension is more complicated. As you start to cut structures away, elbow extension increases. When the Extensor Carpi Radialis was cut elbow extension increased by 5.5 degrees in greyhounds and 3.9 degrees in other breeds (Chong & Davies 2017). This was one of the largest increases that occurred after removing a structure. So the Extensor Carpi Radialis is one of the key structures stabilizing the elbow and limiting elbow extension. If you were curious, the deep fascia is the number one limiter of elbow extension. It is also said to stabilize the carpal joint during extension as well.

While not commonly reported, myofascial trigger points MFTPs have been reported in this muscle in dogs (Formenton et al, 2025). I'm not surprised by these MFTPs. This is the main carpal extensor, and it has that fascial connection to the brachialis and biceps brachii. So it will be absorbing its own forces and also receiving some from associated muscles.

Chong & Davies, (2017) Investigating canine elbow joint stabilisation through mechanical constraints of the deep fascia and other soft tissues

Formenton et al, (2025), Prevalence and location of myofascial trigger points in dogs with osteoarthritis