I've always checked for bilateral soleus tightness when I see some low back pain in runners/desk workers. Active, then lots of sitting lifestyle. When the soleus muscles get tight, they tend to put your center of gravity more on your toes which fires the back extensors more.
I was reading through some Travell and Simmons Trigger point book the other day and came across the trigger point referral pattern for the soleus. To my surprise, I saw that it actually referred pain to the TMJ.
I did some more reading on the soleus and found a few massage therapy articles where the soleus is known as the "second heart." It's considered the bottom of the cardiovascular system, pumping blood from the lower extremities to the heart.
The soleus now becomes part of the vascular system. When they are tight, along with the adductors, can put compression on the adductor hiatus and effect the neurovascular structures that run through that.
They relate tight soleus muscles to migraines. Many people suffer from migraine pain. In my experience, it's hit or miss with what I call typical chiropractic adjustments. I find since I've been doing nerve entrapment release that my success has gone up, but not a ton. I still feel like it's 50/50. If working the soleus muscle provides one or two more success cases, that is some very, very valuable information.
So, if you or someone you are treating presents with TMJ or Migraine headaches, it is worth just a quick palpation of the soleus muscle. It's a requirement for low back and SI joint pain.