Hakeem Olajuwon is now renowned around the NBA for his premium big-man workouts. Although it may look like it, mountain biking isn't a part of them.
This photo from 1995 of The Dream and Shaq riding bikes was for a Taco Bell ad campaign featuring the legendary centers for their Double Decker tacos. Also of note is how absolutely massive those bikes must be to look normal on those two.
Below is one of the Taco Bell ads from nearly 20 years ago.
Thanks to Andy Gray (@SI_Vault) for the find.