Since loss of muscle is such a big problem in extended space travel I wondered (and practicalty is a different issue) if astronauts could maintain muscle mass by donning oxygen masks and playing a game for a couple hours a day in a clear liquid with a high viscosity? It would seem this would work every muscle in the body.
Could astronauts maintain muscle mass by playing a game in a viscous fluid - probably but it would depend on the game.
The loss of muscle mass is due to the muscles not being used as frequently and/or not being used for as high a mechanical load as they were on Earth. In space no one can hear you scream, but more importantly there's almost no perceived gravity either - everything is (for the purposes of this answer at least) weightless. Big and massive things are actually harder to get moving than less massive things due to inertia but once they are moving the only thing you need to worry about is air resistance (assuming they are not in contact with another surface). On Earth this isn't the case because whatever you do, a dense object will quickly become in contact with another surface due to gravity - so you constantly have to hold off the acceleration due to gravity and move the object.
So the viscous fluid would increase the 'air resistance' (no longer air of course) significantly and require more force to move and keep things moving. This would not necessarily work every muscle in the body though.
I swim a lot, and you hear people say swimming works every muscle in the body (Apollo Creed even implies it to Rocky in Rocky III). It doesn't, there are many muscles that you don't naturally use when swimming, so there might need to be a stricter training schedule than simply playing a game. That being said some games would certainly be better than others. Which leads me to my final thought on the type of game;
When trying to maintain muscle mass by playing in a viscous fluid in micro gravity it's better to play something like tag than something like GTA V .