This is long, but found it to be an interesting read...comes from the Ironman website.
A “Magic School Bus” tour of your body's inner workings over 140.6 miles.
You may know what it feels like to swim 2.4 miles, bike 112 miles, and run 26.2 miles, but do you know what’s really going on inside your body?
Pre-Race
Even before you start moving, your body has begun preparing itself for the 140-mile course ahead. At the mere thought of exercise, blood starts rushing to your muscles and the pre-race jitters hit, due to the release of epinephrine—also known as adrenaline.
The Swim
While 2.4 miles sounds like a breeze compared to the entire event, the average swim takes four times more energy to complete than running the same distance. Your body expends a considerable amount of energy maintaining buoyancy and overcoming the drag of variables like the choppy open water.
As soon as you jump in the cool water, it begins to pull excess heat away from your body. This eases thermoregulation, or your body’s process of regulating its own temperature. Your muscles run on Adenosine-5'-triphosphate (ATP) but use it up quickly due to the strenuous activity. Because your oxygen system is a little preoccupied, your body has to start producing ATP without any oxygen, entering a stage called anaerobic glycolysis. You’ll feel this process as your legs and arms start to burn. (As you deal with competitors around you and swim to the finish, surges of epinephrine will push you forward and dull the discomfort.) Your body will continue working with a cocktail of fuel sources like oxygen, glycogen, lactic acid, and stored fat to create its much-needed muscle fuel.
The Bike
After shedding your wetsuit and getting on your bike, your body temperature will start to rise rapidly before leveling off. Your capillaries have widened and blood is sent to your sweat glands to start the cooling process. Sweat rids your body of water, sodium, potassium and other chemicals, keeping you from overheating in even some of the harshest climates. This rapid loss of fluids, while necessary for cooling, hinders performance: If you don’t stay properly hydrated, a loss in blood volume minimizes the availability of oxygen getting to your heart.
Muscle tissue stress starts setting in during the second half of the bike, as your body begins to choose burning fat for fuel instead of carbohydrates, which are in shorter supply. While fat is a nearly unlimited resource, it isn’t enough to keep you moving fast, so you’ll need to regain electrolytes quickly and build up a new source of carbs to work with as you approach the run.
The Run
One hundred and twelve miles later, you’re back on your feet to run the last 26.2 miles of the race. Just when you think there’s no way you could possibly run a marathon, another burst of adrenaline runs through your body when you hear the crowd. This propels you forward, but the jolt doesn’t last for long. Soon, your legs begin to feel the burn of glycogen depletion as your carbohydrate levels bottom out. Your body’s fat oxidation increases and it turns to one last fuel source to power your hardworking muscles the rest of the way: amino acids, or muscle protein. During this process (referred to as catabolism), you face muscle tissue damage since your body is taking the proteins straight from the muscle. This is the muscle fatigue or “burn” you’ll be feeling as you near the finish.
Post-Race
Both brain and body are exhausted after burning nearly 9,000 calories during 12 hours of intense exercise. Your body’s immune system reacts immediately, attempting to rebuild the muscular and cardiovascular systems. For the days following the race, you’ll be more susceptible to sickness since a lot of your amino acids are still at low levels.
Originally from: http://www.ironman.com/triathlon-news/articles/2013/04/zeasorb-physiology-of-an-ironman.aspx#ixzz2RO8hDRU9
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