Paul Pierce has never really been considered un-athletic per se. But was never revered with the Vince Carters and LeBron's of the world as an "athletic" player. And by some measures (end-to-end speed, leaping ability) he's still not. However, there has to be some reason why he is 20th on the all time scoring list that goes beyond longevity and "savvy." Turns out, by some measures, he's really very athletic.
But according to two very different quantitative measures of athleticism, Pierce bafflingly rates as one of the more athletic players in the league.
Check the link for the specifics, but the general idea is that he changes speeds as well as anyone in the world. In the past I've compared this to a Smashing Pumpkins song. The build up is slow and methodical, but at the right moment, there's a burst of sound and energy that overwhelms you (in a good way).
The ease with which Pierce changes directions and speeds is in itself a physical ability, albeit one maximized by his guile. There’s no question that Pierce understands the value of varied movement, perhaps to a deeper extent than any player in the league. But he is only so successful as a stutter-step player because he’s able to shift gears constantly while maintaining perfect balance. That’s an incredible physical talent, albeit one not captured by a traditional conception of athleticism that favors sprinting speed and aerial acrobatics above the more restrained flares of a player like Pierce.
So this isn't the kind of thing that can be measured in a combine like setting. It isn't going to be featured on posters or even many highlight reels. But it does result in Pierce being what Tommy Heinsohn calls the best offensive player in the history of the Celtics.