Hi there Omartinez17,
There is really only one person who is in a good position to help answer your questions, and that is your freedive instructor.
He/she is the only person who can watch exactly what you're doing for your whole dive - how you're breathing before the dive, what your technique is like during your dive (body position, finning, equalisation, relaxation and so on), your buoyancy, your recovery breaths, etc.
Also, there are so many questions I'd want to ask to try to better pin down what you're doing that might help give a better reply...
- questions about your pre-dive breathing (e.g. typically how long? what position? -different divers have different preferences, and maybe you'll find trying a different position could help you relax more, allowing you more awareness and control of how you're breathing)
- questions about the 20m dives you mentioned you've done (e.g. what type of dives? -using fins? -using arms to pull down line? how long did dives take? how long between dives?)
- questions about your 4mins static (e.g. was this dry? if so have you done any static in pool to compare how you feel when in water? when did contractions start? have you done breathholds much before? have you often done well over 3mins static before, or was it fairly hard working up to and breaking 4mins just the once? how did you feel afterwards?)
...and numerous other related questions on various aspects of breathhold and freediving (haven't mentioned dynamic yet!)
However, I don't think you'd want to wade through so many questions, nor have to spend lots of time answering (and, again, your instructor is in a better position to work through these with you anyway), so I'll make a few guesses and try to give some rather general initial thoughts.
Also note that I'm certainly not a freedive instructor myself, so I'm speaking largely from my own limited knowledge & experience - but hopefully some of what I say might happen to end up vaguely useful for you (or for anyone else in a roughly similar situation), or at least give you some ideas where you might start looking for answers...
First thing I would say (apart from the main things above) would be that if you've really only been freediving four days, and you're already diving regularly to 20m, and you have some questions like this, then you probably don't need to aim to go deeper right now - slow down and take a careful look at everything you're doing, alongside your instructor. (Especially since you've experienced this light-headed feeling after a 20m dive.)
Instead of doing more of the same dives, aiming to reach 20m and beyond, I'd suggest you first aim to do some shallow but longer more relaxed dives, particularly when you first start each new dive session. (Maybe you've been doing dives like this already, and/or you'll do more during your course, so you know what I mean... but I'll carry on anyway and let you take from this what you want...)
For example:
• Try practicing slow, relaxed free-immersion dives (slowly pulling down line using arms, equalising every one or two pulls), and only go down to about 10m (maybe 15m later on, but start with 10m).
• Once at that depth, hang there, and try to relax until your first contraction (-you must have experienced contractions during your statics, so you already have some idea how they feel?) and take note of the time (assuming you have a dive watch).
• Then start to pull up slowly back to the surface, trying to stay relaxed and not rushing.
• If you feel you need to start coming up before you reach first contraction then do so.
• Once you've done this a few times, and feel relaxed and fine during and at the end of the dives, try extending the hang a bit, so you come up after maybe 3rd contraction.
• Keep track of times for your first contraction and total dive times (-note that first dives of a session will likely be shorter, with time increasing over the first 3 or 4 dives as your dive reflex kicks in more), and see if your first contraction gets delayed more as you practice over several sessions.
Taking more time for slower dives can help you to feel more relaxed in the water, and help you practice staying relaxed for longer. It also gives you more time to check everything that you're feeling & experiencing during the dive itself - it's good to be more aware of what you're doing, so you can correct anything that needs it, and then it'll becomes more 'automatic' as you do it more.
Also, I'd recommend you have someone film some of your dives, so you can watch your technique (applies especially to body/head position, equalisation, and finning when you do that). Being able to see yourself is often even more helpful than just someone else watching & trying to tell you how to change what you're doing at the time. (Better to have both - someone like your instructor telling you about your technique at the time, and discussing later as you watch playback of your dives.)
Regarding the 7.5l of air per minute (and 5s inhale + 10s exhale), it's clearly only a very rough guide, and not particularly useful unless you have equipment attached that can measure the volume of air as you're breathing!
It will also vary from person to person, and even in different situations (e.g. dry static while lying down vs static in a calm pool vs trying to relax pre-dive on a sea surface that has some waves & swell) - just getting used to the surface movement may take a bit of time & practice to relax as much. Also, even just wearing a thicker wetsuit (when in cooler water) could make you feel a bit more restricted while breathing (also, your weight-belt position - see comment later).
Furthermore, I wonder if such long inhale/exhale might potentially mean breathing too deep, and maybe unnecessary stretching of your chest muscles? (That may mean you're not relaxing as much as you could, and your heart rate ends up a bit higher.)
Again, without seeing what you're actually doing, it's impossible to know for sure what's going on, but I'd suggest a couple of other things for you to check...
• For inhale, just make sure you're not taking in so much that you feel like your chest is stretching. Concentrate on relaxed 'belly breathing' (i.e. diaphragm only, not getting to the point that it feels like you're stretching muscles that much), and stop before your chest expands. (Hope your weight-belt is in the right place? -shouldn't be so far up that it restricts your diaphragm during inhale...)
• For exhale, don't 'push' out the air beyond what you would for normal everyday tidal breathing.
Basically, try to keep pre-dive breathing relaxed with as little muscle use as possible - don't breath in/out so much that you start feeling as though you're using muscles to 'force' air in/out.
If your static breathholds have only been dry then it'd be interesting to see how your static compares when in water. Also, doing that will help you practice relaxation breathing while in water, and is a little more similar to the pre-dive situation on the line. (If it's static in a pool then it's still missing the movement you may experience from waves/swell if your open water is out at sea, but at least it's more similar than dry.)
Of course, finally, it goes without saying (yet I'm saying it anyway
) that you should only try all of the above suggestions while with another freediver, someone who is trained how to do rescues, etc. Again, ideally you're with your instructor, who can help more by giving you personalised guidance throughout.
Wow, that ended up being an enormous ramble! -but I hope there's something hidden among it all that could be useful for you (even if you have to spend some time 'digging' to find it...)
Hope your course is going well!
