Have you ever slouched in an ergonomically correct chair? Taken a shortcut across a lawn? People don't always products in the way we might expect.
When we design for the optimal experience, we sometimes forget that users typically look to create their own shortcuts and paths of least resistance which may not be overtly manifest.
If we model our UI for actual behavior and use, we will find a new paradigm of the optimum - one based just as much on real interaction as upon convention.
The rapid emergence of gestural and other "natural" user interfaces that incorporate the physical dynamics of the will, depends on a solid understanding of ergonomics and physical human factors.
