[trux]

It rather sounds you hyperventilate too much (you call it controlled breathing) - one of its consequences is a muscle tension or even muscle tetanism. Competitive freedivers mostly "pack" their lungs, often far above the physiological lung capacity. That technique bears some risks too, so be sure to enter "packing" into the search box above to get more information about it. Especially, if you hyperventilate during 2 minutes of your breath-up, and then strongly pack, you can black out at the very beginning of the breath-hold.