In my opinion, the fowards do not have to track back, however they must always close down the the ball player. Consider the terminology as follow, a player should close down the ball player if the ball player is within a 10 metre radius and the tracking back is running 30 metres or so to get in line with or just in front of the defence. I think strong foward players like Moses should track back, whereas the more nimble players like Mata, Oscar and Hazard should ruthlessly hound the ball players. If Messi can hound players, I believe that Juan, Dos Santos and Eden are more than able. IF that makes any sense.