Objects on ground obstruct Whirling Blades

I've come across several environment items that obstruct whirling blades, but not regular movement. These are items I can walk through with normal movement, but if I try to whirl through them, nothing happens.

Example:

http://imgur.com/18q55Lx

That bone is not solid. I can walk right through it. But from that position I can't whirl to the other side of it. There's another object in the same area (another bone) where my feet are hidden inside the object and I can't whirl in any direction while standing in it. But I can walk in any direction.