This was a term that used to confuse me until I saw a few. I have seen only a handful now even though I have seen hundreds of Achilles tendon injuries over the last few years – needless to say they are not very common.

So what is it? The term is a bit confusing as it suggests an extra muscle, but actually it is just a soleus muscle that is very low, although in severe cases it can be a separate entity. This means the length of the Achilles tendon from the point of the most distal part of soleus to the proximal calcaneal insertion point (or what is known as the free tendon) is very short, sometimes under 1cm. To put it in perspective some people have a free Achilles length of  up to 12+cm, and this is not explained only by height. Length of Achilles is related to efficiency in functional activities such as running, as a longer Achilles is more active and the muscle less active, but this is a story for another time.

So why is a low soleus and short free tendon an issue? It may lead to a compartment type syndrome of the distal leg (important to mention that a low soleus is asymptomatic in most people). Pain is worse and worse with activity, until the athlete is forced to stop, and symptoms settle quickly after stopping activity, very different to an Achilles tendinopathy pain pattern. Another give away is that palpation tenderness is deep rather than on the Achilles, and the Achilles tendon is usually normal on imaging.

A very important differential to identify. Most respond to conservative treatment but I have also seen patients who have done well post surgical removal.