The mechanics of your legs are being used surfing, but the natural motion of moving on the earth has a physiological benefit to human beings that can not be reproduced.

That's why longevity & quality of life are both excellent in people who keep their mechanics strong in the fulcrums of their bodies:

Ankles, knees, hips, shoulders, neck.

Running, Crawling, Swimming, Walking.

The natural human motions, but which are more important to a land mammal?