Athletes who are competing strenuously in an endurance event have been know to experience something referred to as the second wind. For example, a runner is participating in a 10 kilometer road race. After thirty or forty minutes they seem to have come to the end of their endurance. Their legs are wobbly and there is a feeling of being out of gas. If the runner persists in their effort they may notice, after ten minutes of agony, a resurrgance of energy.
I will not go into all the theories or explanations for the second-wind phenomena, except to explain on my own terms what is the cause, how it can be overcome, and how it has nothing to do with breathing.
The energy your body expends in physical effort comes from glycogen stored in your muscles and liver. At least this is the energy source at the outset – enough for thirty to sixty minutes of strenuous effort and readily available. When an inexperienced or out-of-shape runner goes past this point for the first time the effects will seem strange and awful.
When your glycogen stores run low, your body will switch to an alternate energy source – burning fat. This energy source is not so readily available, and if your body is not used to this switching-over process, it may not happen very quickly either . Hence, the runner experiences a lack of energy for a few, very long minutes, then slowly it comes back. They may have had to walk for a little while, but that second wind eventually did kick in.
As the athlete gets in better shape, and repeats this process of switching from glycogen to fat-burning, the switch becomes less noticeable and more efficient. You no longer need that second-wind and you now know it has nothing to do with breathing or lung capacity.