NFL broadcasts of Philadelphia Eagles games are full of shots of the opposing team's defense struggling to catch its breath. The announcers often discuss how Chip Kelly's fluid, no-huddle offense wears out the other team's defense due to its pace--even though the Eagle's time of possession tends to be lower than that of offenses of otherwise similar quality.
I don't understand why a no-huddle offense would wear out the defense more than the offense. If the defense can't make substitutions, then neither can the offense (right?). Announcers will sometimes mention the Eagle's training and conditioning, but I assume that all NFL teams have rigorous training and conditioning.
So is there something structural going on here: is defense inherently more tiring than offense? Is this especially true for fast-paced offenses?