What is the sinus tarsi?

I get lots of questions about "sinus tarsi syndrome" in runners.

It seems like most runners who call me don't even really know what it is. 

The sinus tarsi is not a structure, so much as a space.  

That space contains some structures that can cause pain after an ankle sprain. The sinus tarsi may also ache in flatfooted runners as well.

The sinus tarsi confuses many runners because it is kind of in the ankle, but kind of in the foot as well.

What is the sinus tarsi? 

Well, that's what we're talking about today on the Doc On The Run Podcast.