They do not strain one set of muscles by long continued effort, as a strong man does, but call one into play after another.
There is no compulsion on the reader to accept this speculation at any valuation whatever.

There is not even any charge for it.

I will lay down this simple axiom: No strong man, is a healthy man from the athlete in the circus who lifts pieces of artillery and catches cannon balls, to the exhibition swell in a country gymnasium.

If my theory is not a sufficient explanation of this, there is nothing to prevent the reader from building up one to suit him better..