They say it is common for a humans
![Rolling Eyes :roll:](./images/smilies/icon_rolleyes.gif)
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3210598/
"Exercise addiction cannot simply be reduced to a compulsion or impulse control disorder either. Rather, like other high frequency behaviors that create adverse effects and are maintained by both positive and negatively reinforcement, certain patterns of exercise are best described as an addiction".