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A Physiological View Of What The Human Body Goes Through In An Ironman

  • By Matt Fitzgerald
  • Published Apr 5, 2013
  • Updated Apr 5, 2013 at 12:46 PM UTC
Photo: Kurt Hoy

There’s a Hole in the Bucket

The most visible effect of Ironman racing on the body is the production of tremendous amounts of sweat. Thank heavens for sweat. Perspiration is a vital cooling mechanism for the body. The blood carries some of the excess heat produced by the muscles during cycling and running away from the muscles to capillaries near the surface of the skin, where it leaves the body. Sweat glands then take up some fluid from the blood, and with it some heat, and release it onto the surface of the skin, where it evaporates, cooling the skin. Finally, cooled blood flows back toward the core of the body to absorb and distribute more heat.

The only problem with this mechanism is that it’s essentially self-sabotaging. The more you sweat, the more your blood volume shrinks, and the more your blood volume shrinks, the less heat your circulation can carry away from the working muscles. However, contrary to popular belief, dehydration only slightly increases core body temperature. Its greatest effect is on performance, because as your blood volume decreases, so does your cardiac efficiency, or the amount of oxygen your heart can deliver to your muscles per contraction.

In a typical warm or hot Ironman, athletes sweat in excess of one liter of fluid per hour on the bike and during the run. That adds up to more than 20 pounds of fluid loss for many athletes! If some of these fluids were not replaced through drinking, triathletes would not be able to complete Ironman events nearly as fast as they do. By the time they got to the marathon, their blood volume would be reduced to the point where walking or a painfully slow shuffle would be the greatest level of exertion possible.

Even with the availability of sports drinks and water, most triathletes finish their Ironman races weighing a lot less than they did when they started. Nevertheless, rather modest amounts of fluid intake appear sufficient to enable the body to maintain blood volume, as the body can also draw fluid into the blood from other compartments (and, for that matter, much of the weight lost during an Ironman comes from the metabolism of fuels and the release of water stored with glycogen, which does not contribute to dehydration). A 2007 study from the University of Cape Town, South Africa, found that while participants in an Ironman triathlon lost nearly 5 percent of their body weight, their blood volume actually increased.

Wear and Tear

Muscle tissue stress may be the single greatest challenge the body faces in an Ironman triathlon. Vast numbers of muscle cells are disrupted, damaged and deconstructed along the way. The main cause of muscle damage is mechanical stress, which is caused primarily by eccentric (pronounced ee-centric) muscle contractions. In an eccentric contraction, the muscle lengthens as it contracts (for example, during the lowering phase of a biceps curl) instead of shortening as in a concentric contraction (e.g., the lifting phase of a biceps curl) or staying the same length as in an isometric contraction (e.g., flexing to show off one’s biceps). The muscle is really being pulled in two directions at once during an eccentric contraction, like a tug-o’-war, so it’s easy to see the potential for tearing.

A second cause of muscle damage during exercise is the breakdown of muscle proteins for energy, called catabolism. Protein is not a preferred energy source during exercise, but when carbohydrate stores run low in the later portion of an Ironman, protein is called upon increasingly to take up the slack. As mentioned above, by the end of an Ironman, protein may supply as much as 15 percent of the energy your muscles use to keep moving. If you’ve ever finished a long workout or race smelling like ammonia, that’s a sign you’ve been burning a lot of muscle protein, as ammonia is a byproduct of protein catabolism. When your blood glucose level drops during exercise, your adrenal glands secrete the stress hormone cortisol, which assists in breaking down carbohydrates, fats and proteins to release energy. Most of the proteins that it breaks down are found in your muscles.

Muscle damage is also caused by oxidative stress during exercise. A small percentage (an estimated 2 to 5 percent) of the oxygen molecules that enter the body lose an electron while participating in energy release in the mitochondria, becoming “oxygen radicals.” This increases their instability and causes them to pilfer an electron from a living cell in order to regain stability. The result is often a chain reaction of “free radical” damage to cell membranes, DNA and various structural proteins. During endurance exercise the rate of oxygen consumption can increase up to seven times above resting levels, with a corresponding increase in the production of oxygen radicals.

Just how much muscle damage does your body experience over the course of an Ironman? One of the chemical biomarkers used to estimate muscle damage is creatine kinase (CK), which leaks into the bloodstream from ruptured muscle cells. According to Bryan Berman, Ph.D., an exercise physiologist with Carmichael Training Systems, in a recovered state, the typical athlete’s serum creatine kinase level is approximately 125 U/L. Twenty-four hours after completion of a half-marathon, the CK level doubles. A day after a bike ride to exhaustion at 70 percent VO2max (a little faster than Ironman intensity), CK levels are as high as 700 U/L. And one recent study found that 16 hours after finishing an Ironman, triathletes had an average serum CK level of 1500 U/L, or more than 10 times the normal level.

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