. . . \%?¹

See section 3.2 Muscles and Tendons in the textbook.

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. . . times,²

You should choose a weight heavy enough that "as fast as you can" does not have you actually pulling the weight back down on each swing, otherwise you will be doing extra work that this calculation does not account for. (Imagine doing this exercise in free fall and you will understand.) But don't lift such a heavy weight that it causes injury!

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. . . humans.³

See section 3.2.4 Muscle Efficiency.

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. . . )?⁴

Here $Re = u R/\nu$, where $u$ = flow velocity, $R$ = typical radius of flow channel and $\nu$ = kinematic viscosity of air - see Eq. (4.12) and the text between Figs. 4.8 and 4.10.

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