When the Ama, professional female breath hold divers of Japan, begin their work just at the opening of the harvest season or when they recommence their work after a certain period of interruption due to bad weather or illness, they complain the difficulty of breathing and lowering of diving efficiency. However, after several days from the beginning of diving, they say that they can dive much easier. This phenomenon suggests the building up of an adaptation to the breath-hold diving. In order to explain the mechanism of this phenomenon, the following experiments were carried out: 1) The ventilatory responses to CO2 of eight active Ama were measured, (a) just after the closing of the harvest season, (b) three months later, and (c) six months after the closing period, i.e. just before the opening period. As a result, it was found that the repeated breath hold diving would lower the sensitivity of the respiratory center to CO2. 2) To ascertain this phenomenon experimentally, three male subjects without the experience of diving were loaded by bicycle ergometer exercises and a series of repeated breath-holding for one hour per day, 7 days continuously. Before, during, and after this experiment period, sensitivity of the respiratory center to CO2 was measured at each period, and it was found that this sensitivity decreased significantly with lapse of time. Therefore, this phenomenon is an adaptation favorable for the Ama, and to avoid the low diving efficiency at the beginning of the diving season, it would be practical to repeat breath -hold exercise for several days before they recommence their diving work.
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