Practice... Makes... Perfect.
Seriously, play with some friends, it helps if your paying attention to what they are playing, not what you are playing.
In order to build the muscles for double bass, you need to play double bass, just like everything else.
In order to build muscles to skate, you need to skate.
There are other excersizes you could do, but whats the point?
You could practice drums, and work those muscles at the same time.