Question in Sliding Filament Theory in mucles?

So, I have this confusion. I was taught that the myosins are fixed and they pull the actin to the center of sarcomere through their heads. But after watching this video, I noticed that also the myosin change position slightly. This is the video: http://www.youtube.com/watch?v=0kFmbrRJq4w. So, do myosins actually change position or are the fixed in their places?