- vestibular (equilibrium)
- proprioception (body position awareness), and
These elements work together: sometimes all 3 contributing equally, sometimes we rely more on one system or two, to keep us stable. A couple of years back I read a fascinating book on the topic: Balance: In Search of the Lost Sense: highly recommended.
Anyway, SueC has had a couple of good posts recently which are worth reading on the subject of balance:
Martial arts - a balancing act - I needed to understand a little more about what balance actually is and how the body controls it
Balance exercises - Intrinsic balance can be improved by doing exercises that focus on improving the senses important in detecting a loss of balance - Sue describes some good exercises to play with