As threatened, lesson 2.
Body position and locomotion have a lot to do with air consumption. For the following very long post, I am only talking about swimming at a constant average depth.
For this we need to picture three general body positions for a moving diver; the snorkeler body position, like an airplane wing at takeoff, with the snorkel as the highest point, the horizontal body position and the formula one race car body position, with fins slightly higher than snorkel.
Beginners will probably do better with traditional up and down finning until more control is achieved. Using the term kick when the proper term is finning may be one reason so many beginners bend the knee too much when using fins. Most people "see" a bent knee when they "hear" the word kick, because we kick balls by first bending the knee. Finning is mostly accomplished by moving from the hip. Try to keep the knee straight; let the water pressure bend the knee slightly, but do not bend the knee with your brain/muscles. Move the fins up and down 2' - 3', gracefully from the hip, and not anything like the fast flutter kick of competitive swimmers.
For most beginners, using the arms is a waste of energy except when making dead stop turns. Your hands are really small paddles and do not move you very much. The biggest problem with hand/arm swimming is all the muscles we flex in the process. Swimming hard with the arms fires nearly every muscle in the upper body, and all those muscles then need more oxygen. A big increase in bottom time comes by only using the muscles you need to use. Just use your fins; we go only as fast as the slowest diver fins, diving is not a race. I teach grabbing one wrist with the other hand; the only muscles flexed are those needed to hold the other wrist. Let the upper body be as relaxed as possible. Release the tension in your shoulders, do not hunch them up.
The only way you will be able to relax is if the BC is supporting you, let the air in the BC hold you up, the fins only move you forward, the arms do not need to flap like wings to keep you from hitting the bottom. When you stop finning you should only move up and down slightly with your inhalation/exhalation. Neutral buoyancy is a fleeting moment; we are mostly not neutrally buoyant. With every inhalation we are increasingly positive as we continue to breath in. With every exhalation we are increasingly negative as we continue to breath out. Timing our inhalations to catch the falling caused by our exhalations. Similarly timing our exhalations to arrest the rise caused by our inhalations. The net is swimming neutrally buoyant, but we are only neutrally buoyant for fractions of a second during every inhalation and every exhalation.
Now picture the diver with a head up "airplane wing at takeoff" body position. The thrust of the fins pushes you in the direction of the head, which is angled up, so some of the thrust is also up. Combine that with an inhalation, which increases the buoyancy of the chest, causing the chest to rise and the angle upward to increase. Now your BC expands because you have ascended, so now the BC is lifting you AND you are finning to the surface, so you must vent air and struggle to control your buoyancy, flexing muscles more and using more air. Then when you get under control at your previous depth you need to replace the vented air so more air added to BC, requiring more muscle flexing.
The horizontal body position is better, but the inhalation still causes the chest to rise and you have both the frontal drag of the head and shoulders as well as the frontal drag of all the gear attached to the tank.
With the head slightly lower than the fins, the thrust has a downward component, so the lift of the inhalation is countered easier. Drag is also now a blending of the two separate drags of horizontal and even gives some down force as we move forward, like a race car. Not only that but our eyes are close to the bottom, to easier see the cute critters better, and our knees/fins are not so close to the bottom. No hitting the bottom with knees or fins, no turbulence from fins causing the rototiller dust cloud behind us.
