These pictures and video are from physical therapy. What we're attempting to strengthen is Nathan's core through balancing. She is also working on strengthening his muscles through the use of medicine balls during play (this one is about 3 pounds).
What you should notice here is Nathan's tongue ~ see how its protruding? That's because he's working so hard. At this time, he can't get everything organized. So, when he's working on one set of muscles others are less engaged.