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Old 08-27-2016, 12:06 PM
Mangetout Mangetout is offline
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Join Date: May 2001
Location: England
Posts: 57,056
It's a good point that forward speed isn't the same as vertical lift, but I think it's implied that a super-human able to run at 70mph would be doing so precisely because they were able to exert more force using their legs.
More force imparted on the water provides more reaction.

There's another thought experiment that I think works here:
When I was younger and fitter, I could tread water and exert enough force doing so that I could raise my upper body above the water surface, for a short (but sustained) time - to a level where the water line was somewhere below my nipples, but above my navel.
If I was endowed with greater strength, I would have been able to rise and remain further out of the water.
If I had superhuman strength, I would have been able to go further, and so on - like a dolphin tail-swimming. It is a game of diminishing returns, as the more of your body is out of the water, the less is able to act upon the water.

Now, I know humans are not dolphins - and the area of a dolphin's tail fluke is greater than the surface area of human feet, but these are all just scale factors - if you have smaller area, you need more force (or if you have more force available, you need less area).