I just purchased a Microsoft Natural Ergo Keyboard 4000 from Best Buy, since my wrists were starting to complain. It is a great ergonomic keyboard; however, one thing is exceedingly bothersome... the stiff space bar. If you don't hit it just right there is too much resistance.
I searched the web for solutions. I ran across http://www.paulstravelpictures.com/MS-Natural-Ergo-Keyboard-Sticky-Spacebar-Fix/index.html , which suggested one of the following:
1. Twist the ends of the sway bar under the space bar, or
2. Remove the sway bar altogether
Neither worked well for me. I finally came up with a solution that really solves the problem:
Break off the inner sway bar clips.
If you pull out the space bar and look underneath, the metal sway bar is secured in two places on both sides. I pulled the following picture from the site above and added arrows to show you what to do. What you what to do is break off one side of both inner clips:
Update 2/12/15: I recently bought another one of these keyboards and have found that the space bar is just fine as is and I haven't needed to do this. It seems like Microsoft has improved the keyboard in recent revisions.