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A Physiological Look at What the Body Goes Through in an Ironman

The changes the body undergoes in 12 hours of extreme exertion are similar to some that occur over 20 years on inactivity.


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A good word to describe how triathletes feel after completing a 140.6-mile Ironman triathlon—regardless of age—is old. If you’ve done an Ironman (or multiple) yourself, you know this feeling. You’re stiff all over and your energy level just isn’t what it used to be. Getting up out of a chair is a two-step process, and you’re more inclined to take the elevator than the stairs. An Ironman can be hard on the body.

From a physiological perspective, there’s something to this perception of Ironman completion as a kind of sudden aging. A lot of overlap exists between the processes that make you feel old after an Ironman and the actual aging process. Cellular damage caused by free-radical molecules is just one example. The good news is that, unlike biological aging, Ironman aging is reversible. After a few weeks of taking it easy, you’re back to your old—er, young—self. Then it’s time to start thinking about doing it all over again.

Let’s take a closer look at what happens to your body after racing an Ironman and how it various parts of your anatomy. If nothing else, it will give you a greater appreciation for the accomplishment of crossing an Ironman finish line. But we’ll also sprinkle in a few practical tips for “aging gracefully” in your next Ironman.

What Happens To Your Body in an Ironman?

An athlete prepares for in the swim leg of Ironman Copenhagen on August 22. (Photo: Joern Pollex/Getty Images)

Something “Wicked Hard” This Way Comes

An Ironman begins to affect your body even before the starting horn (or cannon, if you happen to be in Kona) sounds. Research has shown that the mere anticipation of exercise increases blood flow to the muscles, elevates the breathing rate, and releases hormones including epinephrine (adrenaline) that prime the muscles for activity. This anticipatory response is mediated largely by a primitive part of the brain called the periaqueductal grey area, which is responsible for regulating the cardiorespiratory response to exercise.

Race anticipation affects our emotions as well, for better or worse. Studies point to a U-shaped relationship between emotional arousal (nervousness, excitement) and athletic performance. For a majority of triathletes, it’s good to be moderately keyed-up before the start of an Ironman, and not so good to be either totally calm or terrified. A 2016 study led by Jorge Sanhueza of Universidad Mayor in Chile and published in the Journal of Sports Science and Medicine found that triathletes who possessed certain “anxiety genes” underperformed in an Ironman 70.3 event compared to peers who lacked these genes. If you’re get really anxious before races, consider working with a sports psychologist to develop strategies for finding your optimal pre-race arousal state (which differs somewhat between individuals).

RELATED: Determine Your Ideal Mindset for Triathlon

Once the race begins, your body quickly gets thrown further out of homeostasis as it responds to the challenge of swimming 2.4 miles, cycling 112 miles, and running 26.2 miles. Among the greatest physiological challenges encountered in an Ironman are core body temperature regulation, dehydration, energy depletion, muscle damage, nutrient absorption, and brain fatigue.

On a Hot Streak

Core body temperature regulation is not a big issue in most Ironman swims, as cool (and even somewhat warm) water transfers heat away from the body quite effectively. But the bike and run portions are a different story, especially on hot days. Almost three-quarters of the energy your muscles release during cycling and running takes the form of heat waste, which causes a rise in core body temperature.

Things start to get interesting when the core body temperature exceeds 38.5 degrees C (or 101.3 degrees F), which is considered hyperthermic. Athletes who breach this threshold during competition are likely to experience extreme thirst, fatigue, and gastrointestinal distress, along with just plain feeling uncomfortably hot! At the 2014 Ironman World Championship, fifteen participants volunteered to ingest a telemetric pill that enabled a research team headed by Guillermo Olcina of the University of Extremadura to monitor their core body temperature during the race. A majority of the athletes became hyperthermic during the marathon, with the average core body temperature reaching 38.55 degrees C.

Notice, however, that the athletes just barely dipped into hyperthermia. That’s no accident. The human body is equipped with various mechanisms that help prevent heat from accumulating to dangerous levels. Among these is perspiration, which I’ll say more about momentarily. But the most effective mechanism is simply regulating your exercise intensity in response to perceived effort. Studies have shown that heat perception is linked to effort perception—the hotter athletes feel, the more conservatively they pace themselves, which keeps them from getting too hot. Interestingly, while it was once assumed that core body temperature itself was behind this protective mechanism, it now appears that the conscious sensation of heat is the main factor, as athletes tend to slow down even when their skin is warmed but their core temperature remains normal.

RELATED: The History and Science of Heat Acclimation

This mechanism is not failsafe, however. Highly motivated athletes can consciously override it, pushing past hyperthermia into heat exhaustion, where the core body temperature reaches a dangerous 40 degrees C (104 degrees F). When this happens, the central nervous system begins to malfunction and the athlete becomes dizzy, disoriented, and uncoordinated, and may collapse, as Scottish runner Callum Hawkins did less than 2km from the finish line at the 2018 Commonwealth Games Marathon. Hawkins recovered, but take a lesson from him and don’t ignore warning signs of overheating.

Sponges are used to cool off Skye Moench at the Ironman European Championship. (Photo: Thomas Frey)

Running on Empty

In 2019, Duke University scientists announced that they had discovered the ultimate limit of human endurance. By studying runners competing in a transcontinental race, they were able to calculate that no human can sustain a daily rate of calorie burn exceeding 2.5 times their resting metabolism. Why not? Because no human can eat enough to support that level of exercise! In short, the limit of human endurance is metabolic.

An Ironman isn’t quite as extreme as a transcontinental running race, but it presents a significant metabolic challenge of its own. The average Ironman competitor burns more than 6,000 calories between the start line and the finish. These calories come from fats stored in adipose tissue and muscle cells, glycogen stored in the muscles and liver, amino acids released from the breakdown of muscle proteins, and calories ingested during the event, usually in the form of carbohydrate.

The balance of fuels shifts over the course of the day.  During the swim and the first portion of the bike leg, carbohydrate is likely to provide almost half of the muscles’ energy, with fat providing an equal amount and protein just a sliver. But as the body’s limited glycogen stores dwindle, fat begins to take up the slack. By the midpoint of the marathon, glycogen levels will have reached critically low levels in the calves, quads, and hamstrings. At this point, protein comes to the rescue, with some muscle cells sacrificing their proteins for use by other muscle cells (a process known as catabolism). By the time the athlete sees the finish line, as much as 15% of the energy for that final sprint will come from muscle proteins.

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. Here’s how it works: Blood flowing through the circulatory system 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. 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 causes dehydration. 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. Dehydration also impacts 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.

RELATED: Sweat Testing 101

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! Unless these fluids are at least partially replaced through drinking, completing an Ironman will be almost impossible. By the time the athlete 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 manageable.

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 to be 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 by researchers at the University of Cape Town found that, while participants in an Ironman triathlon lost nearly 5% of their body weight, their blood volume actually increased.

Although not to be taken lightly, dehydration isn’t quite the bugbear it was once made out to be in the 1990s, when ACSM guidelines (since revised) urged athletes to drink “as much as tolerable” during events like Ironman. In fact, in a 2017 editorial written for Orthopedics Today, Ironman medical volunteer Dean Matsuda noted that hyponatremia—aka water intoxication, resulting from overhydrating—was the number-one reason athletes wound up in the medical tent.

RELATED: Are You Doing Thirst Right? The Science Says Probably Not

An athlete runs during Ironman Chattanooga. (Photo: Jacob Kupferman/Getty Images )

Wear and Tear on the Ironman’s Body

It’s the muscles, of course, that do the real work of getting athletes to an Ironman finish line, and they do so at a cost. Vast numbers of muscle cells are disrupted, damaged, and dismantled along the way. The main cause of muscle damage is mechanical stress, which is caused primarily by eccentric muscle contractions. In an eccentric contraction, the muscle lengthens as it contracts (as during the lowering phase of a biceps curl) instead of shortening as in a concentric contraction (as during the lifting phase of a biceps curl) or staying the same length as in an isometric contraction (as when 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 one we’ve mentioned already: catabolism, the breakdown of muscle proteins for energy. 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 noted above, by the end of an Ironman, protein may supply as much as 15% 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 gland secretes 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. An estimated 2-5% 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.

What all of this biochemistry means for you is an Ironman participant is that you’re likely to feel very sore in the latter part of the race and for some time afterward. A 2008 study conducted by scientists at the University of Vienna found that blood markers of tissue damage and inflammation remained elevated in triathletes on the fifth day after completing an Ironman. Nineteen days post-race, however, most of these markers had returned to normal levels.

I Think I’m Going to Puke

The human body was not designed to take in nutrition during physical activity. Yet triathletes must do exactly that to reach the finish line of an Ironman triathlon. An almost unavoidable consequence of fueling during intense and prolong cycling and running is some degree of gastrointestinal discomfort. A 2011 study led by Helen Wright of the University of Cape Town reported that 59% of participants in an Ironman event experienced at least one GI symptom. The most common complaints are mild, and include bloating, nausea, and flatulence (oh, grow up!), but potentially race-ruining symptoms such as vomiting and diarrhea and are not exactly rare.

The causes of exercise-induced gastrointestinal distressed have been heavily researched. The main culprits are believed to be reduced blood flow the gut, which can be exacerbated by dehydration and slows the rate of nutrient absorption, and the jostling of stomach contents that occurs during running especially. For reasons that are still poorly understood, some athletes are more susceptible to GI distress during competition, as the number-one predictor of such complaints is a history of such issues in races.

Other, more controllable, predictors of mid-race GI symptoms are anxiety, lack of fueling practice, and speed (with faster athletes having more issues, likely due to more severe stomach jostling). If gastrointestinal symptoms have impacted your performance in past races, you may be able to reduce the likelihood that they will do so again by performing anxiety-reducing breathing exercises prior to racing and by practicing your race nutrition plan routinely in training, particularly at race speeds.

The Final Stretch

The hardest-working organ during an Ironman triathlon, arguably, is not a muscle but your brain. It takes brain work to drive muscle work, after all, but making the muscles contract is not the brain’s only responsibility during prolonged aerobic exercise. It also must perform the cognitively demanding job of pacing, manage emotional stress, and hold up under an ever-increasing burden of discomfort as the race unfolds.

Insights into the cognitive demands of Ironman and other endurance events come from studies in which subjects are required to perform mental tasks during or after prolonged exercise. In one such study, endurance runners completed a standard card-sorting test both while running and at rest, and were found to perform significantly worse while running. If you’ve ever wondered why you lose the ability to calculate your splits toward the end of an Ironman—well, now you know!

Granted, triathletes tend to feel a lot more muscle-tired than brain-tired during and after an Ironman, but what feels like muscle fatigue is actually largely nervous system fatigue. Remember, it’s the brain that drives the muscles to contract, and research has shown that the loss of contractile capacity in the muscles after exhaustive exercise is caused mostly by a reduction in the ability of the brain and motor nerves to make the muscles do what they want them to.

For example, a 2014 study by French and Canadian researchers found that voluntary muscle contractions in runners who had just completed an ultramarathon were 26% weaker than muscle contractions caused by transcranial magnetic stimulation, which overrides brain fatigue. In essence, this means that, even after running 68 miles, the muscles were still able to work but the brain was too tired to give the order.

A triathlete collapses his fatigued body after an Ironman.
An athlete is drained at the finish line of his race. (Photo: Alexander Scheuber/Getty Images)

What Happens to Your Body in an Ironman: The Aftermath

Given all of this, is it any wonder that it takes a while for the body to recover from the stress of completing an Ironman? An Austrian study found that nearly three weeks after they had crossed an Ironman finish line, triathletes exhibited elevated biomarkers of muscle damage and inflammation. Three weeks!

The immune system plays a major role in helping the body recover after exhaustive exercise, but the immune system itself is overwhelmed by the stress of endurance racing and its aftermath. Immune cell function remains depressed for as long as three days after competition, greatly increasing the athlete’s susceptibility to viral and bacterial infections. The causes of this phenomenon appear to be multiple and are not fully understood. Part of the problem is that the immune cells’ main fuels, such as the amino acid glutamine, are depleted by exhaustive exercise. It seems the immune system also downregulates its inflammatory response to tissue damage to avoid out-of-control systemic inflammation that would otherwise result from the high muscle damage incurred. But this downregulation impairs the immune system’s ability to fight foreign invaders.

Unfortunately, there’s not a heck of a lot you can do to accelerate post-Ironman recovery. In 2018, a research team led by Jenna Bartley of the University of Wisconsin subjected triathletes to cold-water immersion after they completed the Ironman World Championship in the hope that it would reduce inflammation, but it did not. An earlier study by Brazilian scientists found that a post-Ironman massage did reduce perceived muscle soreness and fatigue, but similar experiments have shown that these perceptions are not linked to faster physical recovery. In other words, post-Ironman massage feels good but it doesn’t accomplish anything.

By far the most effective way to promote recovery after an Ironman is rest. It might feel as though your body has aged 20 years, but it hasn’t. Give it time and it will bounce back. In the meantime, treat yourself to some ice cream. Heck, you’re an Ironman!

This story originally appeared in the January/February, 2009 edition of Inside Triathlon magazine. It has been updated to reflect the latest science.