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If you have to use both arms for the lifting phase, then only use one for the lowering phase, you may cut the time in half, but you're also cutting in half how many halves of your body you can exercise simultaneously. Is there a net benefit somewhere?

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This is what I was wondering as well. I suppose it cuts down on rest time between sets, because you can rest one arm while lifting with another.

Isn't the burden on that working body half 2x then?

It would be the same burden. You're still using the same dumbbell, you're just working one arm at a time instead of two.

While it's the same amount of mass, the distribution of force on your body will change as will the muscles engaged etc. no?

The mass would halve, because you are now only lifting one dumbbell with one arm instead of two dumbbells with two arms.

The issue at hand I believe is how the descent of your lift (eccentric muscle contraction) is more beneficial than the opposite, so to optimise for this you'd lift one dumbbell with two arms and then bring it down with one (i.e. minimise the effort required to lift, maximise effort required to descend).

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