Mark Zuckerberg's Workout Partner Is 16 Months Old

Mark Zuckerberg hit the gym hard Friday morning with his personal trainer ... his 16-month-old daughter, Maxima ... and she kicked his ass pretty good!

The Facebook honcho strapped Max (who we're guessing weighs about 25 pounds?) to his back for a set of pull-ups (he got 5) and some push-ups (he got 14).

Zuck commented, "As soon as I finish she yells 'More! More!' Easily the most demanding trainer I've ever had."