The deep posterior compartment sounds as if it ought to be some hidden region in your bum... in fact it's not, it's a small relatively inaccessible area of your inside leg just above the ankle.
It's called a "compartment" because it's an area of space inside the soft tissue of the leg formed within, and surrounded by, fascia. This space allows muscles to run down the inside of the leg, buried under the calf muscle.
The only place where you can actually feel the muscles of the deep posterior compartment is for a couple of inches just below the fleshy part of the inside of your calf (feel where the calf muscle meets the bone, low down on your leg). If you turn your foot inwards, and feel movement under your fingers, you're in the right place. They're not "exposed" for long though, because those muscles then disappear again behind the Achilles (or Calcaneal tendon).
The deep posterior compartment contains some pretty important muscles:
which, though they have separate actions, combined together have the effect of pulling your foot inwards in a movement called supination (think of turning the palm of your hand uppermost as if you were going to carry a soup plate - that's the movement of supination... soup-sup, get it?). Supination of the ankle is also called inversion.
I have no scientific experience to back this up, only my own meandering experience, but a sizeable majority of the people who come into my treatment room are pretty good at the movement of supination.
They're pretty lousy at the opposite movement, called pronation or eversion - try swivelling your foot outwards from the ankle (keeping your knee still). It's a posture and lifestyle thing more than anything else, associated with spending a lot of time sitting down.
There will be a whole bunch of runners who will read this and think that it doesn't apply to them because they've been told that they overpronate by someone in a running shoe shop. That's a whole other story. Most of the people who can supinate their feet, but can't pronate fully are actually runners.
What happens is that with over-use, those muscles of the deep posterior compartment begin to pull full time on the foot at the inside. This constant pulling on the inside of the foot causes it to become "set" in an inverted position. Those muscles are so tight that they become hard to stretch out, which is what's necessary for the opposite ankle movement of eversion.
When those muscles become really tight, problems can set in - especially related to plantar fasciitis which I've blogged about earlier.
If you can swivel your ankle outwards in eversion, then there's no problem. If you can't then you need to take some action.
If you've got a little bit of movement, then simply introducing some proprioception exercises will, over time, help: try sitting so that you're foot is off the ground (on a stool, or with one knee crossed over the other) and practise the movement of supination (eversion). It'll be hard, but keep it up and don't get discouraged. The more you do it, the more effect it will have - over time!
If you really can't do the movement, don't panic - go see a good sports massage therapist, sports therapist, or physiotherapist and they will help you regain movement.
As ever, if you think I can help, give me a call.