Hi Brad,
Thanks for the plug.
A couple of quick points about blackouts and the applicability of exhales for spearing:
Firstly, BOs can still occur in the pool. Part of the problem is that there is insufficient pressure, achieved through depth, to protect the diver. Certainly, hyperventilating before submerging on exhale will precipitate this effect. In this respect, there is no difference between 'inhale' and 'exhale'. It should be obvious to everyone that diving efficiency increases with depth so that long breath-holds in the shallows were access to air is readily available are irrational; at depth access is restricted and so it stands to reason that there will be greater protection. In fact the more pronounced the dive response (DR), achieved through hydrostatic pressure and relative cold of depth compared to the surface, the lower the risk of a BO. A stronger DR is associated with a reduced rate of oxygen desaturation. Trained divers generally have are more prompt and accentuated DR and are therefore at reduced risk of succumbing to a BO, all other things being equal. Furthermore, exercise, e.g., swimming, during the descent will oppose the effects of the DR, which is why we aim to descend passively to depth, that is, at least until the DR is well established and not likely to be undermined by an exercise response.
Exhale diving for spearfishing does not automatically result in a switch to anaerobic metabolism and concomitant production of an excess amount of metabolic waste products and, hence, undue fatigue. A moderate dive response, one that wont completely shut-off circulation and hence oxygen to the working msucles, can still be achieved if the diver warms-up before diving. The purpose of the dive response under these conditions is not to cease the flow of oxygen to the exercising muscles but rather to render, other,temporarily not essentail organs, e.g., non-exercising muscles, skin, gut, etc., hypometabolic. If blood flow to these areas ceases or is greatly impeded then these organs/tissues will cool very quickly to ambient (water) temeprature. This is advantageous because it slows metabolic reactions in those tissues. Under these circumstances and for the duration of the dive these tissues/organs do not necessarily have to switch over to anaerobic metabilsm for lack of oxygen, they can simply use less oxygen.
For extreme dives on the other hand, there must be a switch to anaerobic metabolism even in those working muscles. This is better achieved by doing exhale dive, not warmin-up and not swimming down. Of course, we don't want to render the muscles to 'lactic', too early in the dive. We can achieve this by swimming as little as possible (=> sinking instead of swimming), shuting off blood supply, everywhere except the most vital of organs, e.g., heart and CNS. All other organs can have their circulation clamped down, cooled and slowed down. This effect, if frequently repeated in training, will result in enhanced MbO2 stores. This in itself will delay the build-up of waste products and premature fatigue in non-vital organs by maintianing them aerobic.
Lastly, its true that my exhale breath-hold duration is about half of what I can achieve on inhale. There is however one very important difference: exhale statics in themselves are a pointless unless they simulate descents and are followed by dynamics, which simulate ascents. In this respect, my exhale simulated descent/ascent are now longer than my constant-weight dives.
About BOs being degrading: although I don't encourage BOs they are sometimes a good educational tool in bringing awareness to those who may have thought themselves immune and demonstrating, in parctice, what not to do. In this respect, they have the desired result of scaring the 'bejesus' out of those who may think they are beyond them.
Cheers,
Seb
