But Campbell insists he believes strongly in the fitness methods he learned from Arsene Wenger and his staff at Arsenal, and thinks he can play on for several more seasons at the top level after recently signing a new two-year contract at Fratton Park.
He said: "It's not rocket science but I would definitely say I learned from that.
"There comes a time in your career when you have got to make sure you maintain your condition with the right exercises, the right rest and so on.
"You can muck about a bit in life when you are 18 and still run around all over the place, but since about the age of 27 or 28 I've been more careful to keep doing the right things, to maintain my all-round fitness.
"It paid off last season and it is paying off now apart from the last couple of weeks."