'Body By Jake' -- Prez Obama Can Pump Some Iron!

President Obama wasn't going soft with those puny dumbbells, he was going smart -- at least according to the fitness guru behind "Body By Jake."

Jake Steinfeld weighed in on Obama's now famous workout in Poland -- and while a lot of people are giving the Prez crap ... Jake's giving him props for hitting the gym at all.

When we spotted Jake in Bev Hills ... he was quick to defend B.O. pumping iron with light weights ... because he says sometimes it's better to go for higher reps when you're squeezing in a quick workout on the road.

As for the President's chosen exercises and form -- you gotta hear why Jake was actually impressed.

He also reveals who he considers our "most fit" Commander-in-Chief ever.

Hint: It's NOT William H. Taft.