It occurred to me the other day (probably should have thought of it years earlier), balance exists mostly in the mind. Sure, there's some muscular tone involved, and you can train your nervous system to fine tune your balance some, but consider this...
If you find a 4-inch wide curb or a 4-inch wide stripe on the road, and you try to walk along it, it's not too difficult, but for most folks it is at least a little bit of a challenge. Everyone has to exert at least a little bit of attention to this task. Most everyone that walks down an eight-foot 2x4 a few times will fall off the side occasionally.
But think about this for a minute (we learnt this in college in a biomechanics class)... Healthy adults walking on level ground, not thinking about their balance at all, their average gait width (horizontal distance heel-to-heel) is about 4 inches. This means that if they were to take a walk and you were to follow them drawing a line through all their right footprints and another line through all their left footprints, those lines would average about 4 inches apart.
Everyone walks 4" wide paths all the time every day, and these people almost never fall over sideways or even stumble!
But as soon as you tell them to walk on a 4" wide line and they think there are consequences to failure (embarrassment about poor balance, threat of being arrested for DUI, perhaps fear of falling...) then suddenly walking a 4" wide path is a challenge.
What is the difference? Not the path - the state of mind with which we undertake the path.
Balance is mostly a function of the mind.
That is why I could teach an ukemi (falling) seminar to a bunch of yoga experts a few weeks ago, and immediately after the class, several of them told me that they felt more stable and centered and poised in their challenging balance poses - because I was able to remove or reduce a bit of the fear of falling out of those balance poses. We didn't do enough actual exercise of sufficient repetition or intensity to create a neuromuscular training effect - I just changed their minds about how they approached their balance poses.
Balance is mostly in the mind.