I usually don't have any contraction for depth disciplines. But, on this dive I had one single weird contraction at ~35-37m (I was sinking and relaxed). It was not a contraction from apnea or CO2. I remember that it broke my concentration, I focused my mind on my chest and stomach and the contraction disappeared. No contraction for the rest of the dive. It was a strange feeling.
I've had a similar thing happen a couple of times (but no squeeze so far). I think it has to do with the negative pressure pushing on your tratchea/epiglottis/whatever. Actually, I get similar "pressure contractions" with just negative packing dry apneas (contractions starting very early, way before the Co2 or o2 limits)
It's a moment when, if doing the mouthfill, the air usually escapes from my mouth and I suddenly feel the pressure on the throat-area and get a sort of panicy feeling. For me this is a definite sign to abort the dive, but I know people sometimes try to go past that (it's sort of hard to identify) and it's my gut feeling that in a lot of squeeze cases I've seen, it's been the case. I find that doing negative packing apneas up to really uncomfortable really helps getting used to this and maintaining control when the point approaches. Just speculation, but I believe this is the point when one, with the mouthfill, override the RV and "normal equalization" would start to fail. Ie you go deeper than you maybe should be going relative to your experience (with the mouthfill it's very easy to make huge leaps in depth, but it's not always just about equalization and hypoxia...).
For me, "normal contractions" at any depth I've been to seem to cause no problems, I've done some pretty long hangs (taking dozens of contractions) at depths close to or past my RV, and I'm pretty convinced that it's the kind of contractions described above that are the dangerous ones. Then again, never suffered a squeeze, so maybe I'm just one of the lucky ones and not that prone to it.
Last edited: