[Help] Why does my dog poop a lot when active and why is it runny?

Seems like the more active my dog is, his poop becomes "liquid". The first couple poops are usually solid, however, whenever he is playing, going on long walks, or is just physically active, he tends to poop A LOT. Is that normal to begin with? For example, on today's long walk (est. 4-5 miles) he went poop about 5 times. As I've mentioned, the first two times were solid. Then, after each successive poop, it became runnier. I just assume it's because there's really nothing left to expel and that I have no reason to worry. I notice this at the dog parks too where he tends to run wild. He poops very frequently and the runny ones happen after the first couple times. Any ideas? via /r/dogs http://ift.tt/2rGkNPl