Some recent knockout losses suffered by former MMA greats have many fans wondering if enough is enough. We all know the potential dangers of fighters hanging on too long, especially when concussions are involved. So in our latest "Ask the Fight Doc" installment, MMAjunkie.com medical consultant and columnist Dr. Johnny Benjamin discusses how exactly MMA fighters are medically cleared to fight and some of the pitfalls that are part of the process, especially with aging competitors.