There are numerous posts about Power Lung and Expand-a-Lung but they are largely focused on breath hold time and increasing lung capacity. In large part the consensus is that a more flexible ribcage and upper body achieves better results. After reading an article on the matter https://www.active.com/cycling/articles/how-to-train-your-respiratory-muscles there is a focus on Rating of Perceived Exertion (RPE). With the benefit of stronger respiratory muscles and potentially a reduced oxygen consumption during breath up has anyone felt more relaxed and less fatigued during a long session of diving after incorporating such dedicated strength training. I see a lot of posts about people having trouble with breath holds and the perception that they are overtraining for days after diving or even static practices. It makes me ask if they are not used to the CO2 yet or if their lungs are just fatigued.