'Tis true the tendons seem to get all the media attention for the hard-working muscles they serve. I try to change that here, but it's been mostly a lonely job. I'm sorry for your pain, but glad that at least you suspect something could be wrong with the largest system of organs in your body (muscle). The really good news is they are usually so much easier (and cheaper) to fix!

The Tib Posterior tends to refer pain lower, along the Achilles tendon its symptoms are often blamed on. It is a supinating muscle, so it is easy to envision how it might get suddenly traumatized by slipping on a rock, if it is indeed related to your pain.

Regardless of muscular origin, the pain indicates a problem in a specific spot, if you can feel something there. If you use your thumbs or a tennis ball (one poster mentioned a lacrosse ball), or some of the other massage products out there, you can probably disarm (at least temporarily) any tension that originates there. I'm afraid that in most cases repeat visits will be required, so you may have to chew up your deductible if you get help.

Be aware that pain can refer to that area from as far away as the minor glutes, through the hamstrings, and from a tiny muscle originating in back of the knee called the Plantaris. If you work the area with no relief, it might not be because you aren't talented or effective, but because you are not working on where the pain is coming from.

My money though, is on local pain from the superficial Soleus muscle, which you would be working anyway enroute to the deeper Tib P. Work it higher and to the outside of where you feel the pain, underneath the overlapping Gastroc (Tib P being more in the center). This requires relaxing both muscles by bending the knee and letting (not forcing) the foot to drop. Just placing the lower leg over your opposite knee and letting gravity and your knee do the work can be all you need. Work to move the pressure from below the target spot upward, toward the back of the knee (for better vein health).

If you can work the spasm out of either calf muscle long enough to increase your mileage, you've probably hit pay dirt, and more attention to that area - spread out over a few days or weeks - should finish the job. If you do not get any results at all, you know it's getting complicated.