Contractions
Contractions are actually part of the hypoxia reflex (which is a subset of the diving reflex). Early attempts at breath-holding may not result in contractions, the body needs to 'remember' how to have them. A contraction's purpose is twofold 1) To force extra O2 into the blood by both circulating the air in the lungs and increasing the pressure in the lungs, and 2) to increase O2 availability to the brain by temporarily increasing cerebral blood pressure...there may be other reasons as well, but due to these reasons they definitely make you 'feel' good, which is bad because they burn lots of O2. If you resist the contractions for a really long time, you start to get light headed, and your arms and legs may start tingling and feeling ugly, and the lack of O2 to the brain will cause your heart rate to slow dramatically. Then, if you allow a contraction, your brain O2 goes up, and your heart rate goes up a bit, and you feel better in general.
Eric Fattah
BC, Canada