Do the resistors work on the way in and the way out (inhalation and exhalation)? In that case I would disagree with Duck.

I can't get the whole article but from what I can understand it's a randomised controled trial where the time of training per day is controlled. So the argument that just working at it is just as good does not hold. The other groups were also working at it for the same period of time per day and also gained substantial benefit. But the resistance group gained more. So 33% might be out of the question but one might expect additional improvements to what we already accrue given that the experiemntal samples are different people.

As an aside...

Swimming around or standing vertical in the water column like we do only has resistance on the inhale from the water pressure. The exhalation is actually easier for the same reason that external pressure is being placed on the thorax and abdomen. If you want it easier on both the inhale and exhale then you need an iron lung.

50cm H2O is actually a pretty large amount of pressure if you're talking about respiratory resistance (depending on whether this is measured at the mouth or in the lung). We use a device to treat sleep apnea that applies (for standard severe cases) around 15cmh2O at the mouth. That pressure makes it easier to breath in and harder to breath out.