I did a little bit of an experiment with this today and found the results kind of interesting.
For the first five miles I was wearing my Fivefingers and discovered that there was indeed a more enhanced sense of stability and feeling of traction (couple of inches of fresh snow last night). I noticed that my feet touching the ground had a more definitive feeling of mid foot connection as well. Kind of neat. There was a bit of ‘tension’ (if that’s the right way to describe it) in my lowercalf/Achilles area...it wasn’t painful, just that sense of tension.
Kind of got cocky and decided to run the last 2.5 unshod. I was able to retain that focus the first couple of miles but the last half mile or so involved some numbness (it’s -7C today with a bit of a breeze) so I can’t say with all certainty that I retained that technique for the end.
Curious to see if I experience any unusual sensations in that lower calf/Achilles area.
I'm guessing that what you're feeling are muscles being used anewly, or at least more intensely than before. When I first began using my big toe I was incredibly sore on the inside of the foot, but that passed after about a week. I make sure to use the big toe properly now when walking as well, not just running. Somehow walking this way affects my physical balance but also my emotional one, and my confidence level in general.
Holy crap -7°C?! That is arctic. I'm struggling with 8°C, though my general vitality is still low, it'll take me time to build that I guess. Let me know how things go on with your big toe and lower calf? Thanks for sharing by the way.