Fitness

Running and exercise in general can make you more fit! (go figure)
So the word FITNESS is in the first few letters, bolded or "weighty". But as the word progresses the letters become more and more "toned" until the S at the end is taking off. Getting fit means being active.

I added a Gestalt continuation item at the end for good measure. The lines don't actually mean anything but for some reason as our eyes follow them they seem to give objects motion, like running.