Hi,
You can do this in one of two ways, depending on the desired look.
1. You can use the sliced sprite, and then animate the dimensions parameter. This means the middle bit is more or less one solid color or streteched in one axis.
2. Use the clipped sprite sample which is available in the private support area, and use it to reveal part of the full sprite. This lets you have an image as the bar, which may suit your needs better.
unikron