Exercise or Flash Mob?

Something I have seen in China a few times now, that I never see in America is people exercising together out in the streets. When I first saw people dancing in the streets, and I mean many people, all with such precision, I thought at first that I had walked into a flash mob. Although I was pretty excited about seeing a flash mob in China, after seeing the same dance over and over, I had to think it was likely not a coincidence. I asked my host sister in Harbin about it, and her response was that they were exercising. She left it at that, as if it were the most common thing in the world, which I suppose to her it was.  Here is a short video of a group in Harbin exercising.