Extensor Carpi Ulnaris

The Swiss Army Knife of the Forearm

Sometimes referred to as the Ulnaris Lateralis, the Extensor Carpi Ulnaris is the most caudal muscle in the craniolateral antebrachial group.

It originates from the lateral epicondyle of the humerus, caudal to the lateral collateral ligament of the elbow. Starting from a long tendon, it transitions into a relatively small muscle belly that travels down the caudolateral ulna. This is a superficial muscle that lies under the skin and fascia.

About two thirds of the way down the ulna it transitions into a tendon. As it passes over the distal ulna and lateral carpus the tendon is held in place by connective tissue. Unlike most of the other muscles in this group it does not have a bony groove to help contain it. 

The tendon inserts into the lateral base of metacarpal 5 (MC5) on the lateral carpus. Two fascial connections arising from the Accessory Carpal Bone (ACB) will also blend into the ECU tendon of insertion. This is why some sources will say that the ECU inserts into the MC5, and other sources will say MC5 and the ACB. 

Prior to its insertion the tendon of the ECU is very palpable on the distal lateral antebrachium. When standing under load this tendon will feel much like a bony structure on a dog. Note this tendon forms the cranial border of the depression in which you will find the acupuncture point Heart 7. 

This muscle performs quite a few functions. It abducts the manus or paw. Moving the paw laterally away from the body. This makes sense. It attaches on the lateral elbow and paw. If this muscle contracts, it will pull the paw into abduction. The second function is that it flexes the carpus. This is interesting because that means it is the only carpal flexor that originates from the lateral elbow and forearm. It is also the only carpal flexor innervated by the radial nerve. It has been shown to be active when the flexor carpi ulnaris is active (late swing phase and first third to half of the stance phase), reinforcing its role as a carpal flexor. Third, it stabilizes the carpus when the carpus is in extension, supporting the wrist against gravity. This muscle is predominantly Type 1 muscle fibers which is often seen with stabilizer muscles. Finally it laterally rotates the carpus. 

So this is a multifunction muscle that I often like to refer to as the Swiss Army Knife of the Antebrachium.

Extensor Carpi Ulnaris tendinitis, while not well covered in the available literature, is reported by vets and is specifically mentioned to affect sporting breeds and to lead to lameness and decreased performance. Due to its insertion on the lateral carpus, injury to this muscle will likely lead to lateral laxity of the carpus.