Grizzly's nightly run doesn't just involve fast laps across the cage and around the pigloos. He also runs in and out of each pigloo super fast. So fast I don't know how he has time to turn around inside. He's like a ball bouncing off the back wall.
I can also hear him bouncing off the back wall, so he runs into it at least a little. Doesn't seem to bother him, though.