Hi,
Biological PWM? 
A non robotics page where you have to sign up to read it?
Our muscles don't use PWM as we know it, but the electro-chemical feedback system will give correction signals whose magnitude is determined by the amount of correction needed, so in most instances may look that way - just standing upright, your brain, muscles and nerve system work really hard to keep your balance - might be why it's more tiring going shopping than a brisk walk (where much less brain and nerves are used to keep balance - walking is a constantly evaded fall).
An important part of keeping balance - your artificial horizon/reference actually - is the Eustachian tube in your middle ear. Flush your ears with first hot (not scolding!) and then cold water a few times and you'll feel like Tyson just punched you (less the pain), as you'll probably not be able to stand on your own two feet without falling (it's a test used to test for some kind of ear disorders).
The human body is a masterpiece and we're not going to be able to copy (or fully explain) it anytime soon.
