They're just stretching before the concert. I wouldn't be suprised if they did yoga but it sounds as though all of them are way too busy to do that and they do other things in their days off. Also, I'd bet all of them are flexible enough for Y-balance except for Aika who is new.
Anyway, I finished watching the concert yesterday and I gotta say, I noticed Sayu a lot more than I often do. Its probably just me but she tried noticably harder than everyone else. There was a lot of edge to her dancing and singing that everyone else didn't have. Man I love watching that girl dance.