Originally Posted by Danny
Not sure why this is, and there are a couple of possibilities. It might be that the muscles needed to do this become tired, but it might also be that, as my lats and back muscles fatigue, they lose flexibility so that it becomes more work to stretch them out.
Or they get less and less oxygen and more and more CO2? What happens if you stop and take just a few breaths and then start again? If your first length is brilliant again (for instance you shave 1SPL) in spite of being tired (your muscles are tired and can't recover in a few seconds), it's a breathing issue, even if you don't feel short of breath.