During muscular exercise, blood vessels in muscles dilate and blood flow is increased in order to increase the available oxygen supply. Up to a point, the available oxygen is sufficient to meet the energy needs of the body. However, when muscular exertion is very great, oxygen cannot be supplied to muscle fibers fast enough, and the aerobic breakdown of pyruvic acid cannot produce all the ATP required for further muscle contraction.
If you are going to be exercising for more than a couple of minutes, your body needs to get oxygen to the muscles or the muscles will stop working. Just how much oxygen your muscles will use depends on two processes: getting blood to the muscles and extracting oxygen from the blood into the muscle tissue.
Each time we breathe in, we take about a pint of air. On average, we take about twenty-four thousand breaths a day. The majority of adults develop a faulty pattern of breathing, that is, we tend to suppress diaphragm breathing which results in under-utilization of the bottom section of the lungs. What makes the problem even more acute is that the bottom section, compared to the upper and the middle section of the lungs, is most efficient for intake of oxygen and the release of carbon dioxide.
Without being aware of it, those that advocate shooting only one arrow and then pulling during practice are allowing their oxygen deprived muscles to recover between shots. Most self taught uninstructed archers when shooting a series of arrows, without the aid of a clicker to insure full draw each time, will produce shorter and shorter draws as the oxygen starved muscles cry out in pain and the arrow group widens. It used to be thought that this was due to the build up of lactic acid, but that old theory is no longer held to be viable and instead the emphasis is now put on strenuous anaerobic training to develop mitochondria in order for the muscles to convert the lactic acid into energy. Another consequence of oxygen deficient muscles is TP and snap shooting. Simply put, the inefficient untrained muscles are oxygen starved. That's why you can't hold a hunting weight bow for an interminable period at the shot before release, because you haven't conditioned your body properly. It has nothing to do with arm strength, it's muscle fatigue.