Doug Miller writes:

Not likely, I admit -- but such machines *do* exist, and it's perhaps
a bit presumptive to suppose that we've already explored and exploited
the limits of such technology. It's still less than a century since
heavier-than-air flight was "known" to be impossible. :-)

This is the bubble-headedness I speak of, that muscle power can do
anything, if we just have the right machines.