Science isn't everything. There is much to be gained, for example, from great art, though there is also the peril of those exhausted feet that always come with gallery-viewing.
Why is it so tiring to just stand around in front of paintings, anyway? It isn't like you are getting great amounts of exercise, you're just...standing around.
There is an answer to his question, which must have been asked by every gallery viewer at some point. It lies exactly in the fact that while you ARE standing, you AREN'T moving your legs. Although this seems like an easy thing to do, it isn't at all the same thing as resting.
Think of it this way: when we walk at a steady pace, we are constantly changing the pressure on each foot. At no point do you have both feet flat on the ground.
That alternation of stress and relaxation has two effects. First, it gives your foot a little rest before having to exert a force again, each time you step. Add all those little rests together and you'll see you spend roughly half your walking time off your feet. Not bad.
The other effect is that walking helps circulate the blood. It's the increased difficulty in circulation when you are standing still that causes blood to pool a little in your feet. When not enough fresh blood reaches those muscles, they tend to become uncomfortable.