This thread got a mention on the podcast! I feel famous 
Chad - you asked if I could read the study you cited in the podcast - Powers et al., 1989. Below are my thoughts. I’m open to further discussion - finding this pretty interesting. I should mention that while I am in the fortunate position of being in academia, and hence being able to access the full texts of most papers, I am (no longer) in exercise physiology, this is just something I’m interested in and a nice excuse to learn some more interesting exercise physiology.
Thoughts on Powers et al.:
- Looks like a pretty valid study to me! Although it’s just one study, and has very few participants and is from >30 years ago (see below for more recent papers)
- This study also cites a number of (even!) earlier studies which show similar findings - increase lung pp oxygen increases work capacity significantly (but by a fairly small amount)
Given the above limitations, I used the Web of Science database (www.webofscience.com, annoyingly not a free tool but perhaps one that would be helpful for @chad in planning podcast deep dives?) to find more recent papers which cite the above Powers et al. paper, in the hope of finding evidence which strengthens or undermines Powers et al.'s findings. I found the recent review by Cardinale and Ekblom (2017) - “Hyperoxia for performance and training” - informative and interesting. A few relevant points stuck out:
- The condition that Powers et al. decribe where highly trained athletes show decreased blood oxygen saturation at high work loads is called exercise induced arterial hypoxemia (EIAH).
- EIAH appears to be caused by reduced blood-air contact time at the alveoli (the blood is being pumped too fast through the lungs for complete Oxygen transfer)
- EIAH is more pronounced in female than male athletes.
- As would be expected, the higher the effect of EIAH the greater the effect of increased inspired partial pressure of oxygen increases performance
- Hyperoxia may increase performance by decreasing central fatigue. There is some evidence for this, but it appears more work is needed to elucidate a mechanism.
- As theorised above by @BT-7274 and @angusr " a competition for blood distribution as maximal exercise effort is approached not only between different working muscle groups (i.e. arms vs. legs) but also between locomotor muscles and respiratory muscles (Dominelli et al., 2017).", therefore supplemental oxygen can reduce breathing muscle blood flow requirements, thereby allowing more blood to be sent to the legs.
- As discussed in the podcast, the potential for hyperoxia to enhance training was discussed in this article.
- The authors open this section with “While much is known about the acute effects of hyperoxia during acute exercise, the mechanisms underlying adaptation to hyperoxic-supplemented endurance training is not very well known.”. As with much of exercise/training science, it is much more difficult to study prolonged effects than acute effects, especially over the course of a whole (or multiple) seasons.
- The overall literature reviewed on effect of hyperoxic training training appears to be inconclusive/in need of more work, but the authors (somewhat tentatively) conlcude that “hyperoxic supplemented training has unclear effects on V̇O2max and a likely positive effect on performance compared to normoxic training.”
- The authors conclude on training in hyperoxic conditions: “before any specific recommendation can be made on the use of hyperoxic-supplemented training more studies are needed for elucidating optimal training intensities and frequencies and safety issues.”
- “Further research is needed before hyperoxic-supplemented endurance training can be recommended for health purposes for the general population. For elite athletes seeking marginal gains in performance, the use of hyperoxia in regular endurance training may result in enhanced performance. Untrained to well-trained individuals should first focus on maximizing the training load and then optimizing the basics of training adaptation such as taking care of periodization, training methodology, nutrition, and recovery before considering the use of hyperoxia supplementation during training.”
I hope this is interesting/helpful to someone. If nothing else, I enjoyed doing some reading up on this 