WHY BEGINNERS AND INTERMEDIATE FREEDIVERS ARE AT HIGHER RISK
I think the reason so many freedivers put themselves at risk of squeeze is because lung packing and a rapid descent seem like solutions to simple problems: equalizing and fear.
Most new freedivers start to run out of equalizing air at around 25-30m. If they switch to Frenzel they might make it to 35m -40m. Doing Frenzel deeper than 40m requires either the mouthfill technique or excellent chest flexibility, which in turn requires much more diving experience than a novice would have.
Packing an extra litre or two immediately provides more air for all equalizing techniques. Suddenly, 45m-50m is not impossible at all, even with a crappy equalizing technique. It is common for freedivers to progress from 30-45m very quickly. Ten metres jumps in depth in a weekend are not unheard of. But often squeeze happens as well.
Excessive descent speed reflects the fear a freediver has with a new depth or pb. If she were completely comfortable, Freediver A would realize that there's no rush and that a slower, relaxed descent is more efficient. But often the impulse is to make the depth as fast as possible and get the hell back to the surface! At my first nationals in 2001, I sprinted to 46m and suffered a moderate squeeze (coughing blood, wheezing). The dive, if I remember correctly was probably around 1'20" or less. Now it takes me over 1'30" to reach 60m on the descent alone and no squeeze.
Diving deep without packing is possible. All it takes is more time to adapt and equalizing skill. (Umberto Pelizzari dove over 80m without packing.)
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Okay, Sebastian, I'll try to address how negative pressure dives and FRC dives are not the same for training chest flexibility.
1. The goal in my mind for helping your body adapt is to simulate in training as close as possible what actually happens to you on a dive. If my goal is to dive in a way where I allow enough time and create a gradual and adequate stimulus for blood shunt and peripheral vasoconstriction, then it is obvious to me that a negative pressure dive (full exhale), is not appropriate. I am also talking about negatives in the way that most people do them: between 5-10 metres, free immersion or sink down, hang for some time, then come up. Depth of course depends on your interpretation of a "full exhale!"
So, if I do a negative pressure dive, I go from 1 atm of simulated pressure to over 8-9 atm in only 10-20 seconds. This is not what happens in an inhale dive, not even at any point in the dive. And furthermore, I don't think it promotes blood shift in the right way. Think about it: in a negative pressure dive, you reach the limit depth and wait until your body reacts instead of having your body react as you go down, thus preventing injury. For me, the final proof that negatives are not the best technique for everyone is that I know of several people who have used them (including myself) and have done them quite deep, and yet still get squeezed.
Not only that, but I've always felt that negative pressure dives are not as good for stimulating an increase in water pressure on the whole body's core. Sure the lung volume is reduced considerably but the rest of the body is still only at 1.5-2.0 atm. Plus, if a freediver starts of with 2-3 negatives while he's still hot and dry in his suit, I doubt there would be much stimulus for vasoconstriction (constrast this with a no-suit dive). Again, specificity is key.
2. On a properly done FRC dive with extrememly slow ascent (sink from the surface) and single mouthfill equalizing, I believe the phase between 20-25m and 40-45m is easily and gently simulated up to 20m of depth. I have done FRC dives much deeper but have found repeated dives to 20-25m to be enough. The key is time and stimulus for blood shunt. When I say sink from the surface, I mean that literally. You should sink like a dead body, you might even sink ass first, and it could take 30 seconds just to reach 10m of depth. But this is extremely important for giving your body adequate time for full blood shunt and vasoconstriction. Repeated 6 - 7 times in a session (warmth allowing) and a little as twice a week makes an amazing difference in chest flexibility. But, the effects are far more USEFUL if you also modify you inhale diving style to match your training. So don't do this exercise and then pack 3 litres and sprint to depth. You might still get squeezed.
As I said above, my average speed for a descent is approximately 0.65 m/s but in the first 20m it is more like 0.50 m/s.
Am I the riskier diver of us two? Well, neither of us know about the risks we currently take in freediving, except I know that you jumped from 50m - 65m in seven days, something that I would never be able to do without injury after several months off. That's too risky for me!
My squeezes were in 2001 and 2002. I have not suffered one since that time. Please keep in mind that when I had my squeezes in 2001, no one knew how to avoid it, and none of the local freedivers, not even the world record trainers in town had any useful advice for avoiding it or reducing the risk. I contacted the diving experts like Dr. Claes Lundgren and he didn't have any advice about prevention. So while the squeezes were still only moderate in severity, perhaps like you've experienced, I tried everything, including negatives and a progressive and tiring warm-up sequence, but it kept happening from time to time.
Then came my wake-up call, a dive to 42m without fins in 2002 that resulted in a squeeze that profoundly scared me. 42m was shallow for me at the time. The squeeze shouldn't have happened, but it did and it was horrible. Is it being risky to make a dive when you don't even know what the consequences are? If I had know that diving while congested and cold with a rapid descent would almost certainly result in a lung injury I would not have done it.
This is my motivation for sharing and discussing this information. If someone has a squeeze and doesn't know why, I feel that perhaps they can benefit from my unpleasant experiences and avoid them in the future, and hopefully avoid something more serious like I had. Luckily, I can still enjoy freediving. But I wonder how many others cannot.
Hope these explanations help.
Pete
