The (Major) Physiological Impact of an Ironman Triathlon
Matt Fitzgerald— From the outside, swimming, cycling and running appear as movement. But from inside the triathlete’s body, swimming, cycling and running appear as an acceleration of time.
Blood gushes through veins and arteries like traffic through night highways in a time-lapse video. Within muscle cells, glucose and triglyceride molecules are tossed into the fiery furnace of mitochondria at a breakneck pace, as though someone has put a DVD of the process at rest on 4x fast forward. Armies of oxygen radicals punch holes in muscle cell membranes, causing a general deterioration that calls to mind those computer animations that show a person aging 20 years in 10 seconds.
Indeed, from an internal perspective, completing an Ironman is a bit like sitting on a sofa for 12 hours and aging two decades. In other words, the changes the body undergoes in 12 hours of extreme exertion are similar to some of those that occur in the body over the course of two decades of non-exertion, as a result of normal aging. Fortunately, though, those years are restored to you within a few weeks. Then it’s time to start thinking about tickling the reaper again.
Let’s take a more detailed look at how racing an Ironman affects various parts of your anatomy. There’s no particular lesson in this exercise, but it may give you a greater appreciation for the accomplishment of crossing an Ironman finish line.
Something “Wicked Hard” This Way Comes
An Ironman begins to affect your body even before the starting horn—or cannon, in one notable event—sounds. Research has shown that the mere anticipation of exercise increases blood flow to the soon-to-be working muscles, as well as oxygen consumption and the release of 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 gray area, which is responsible for regulating the cardiorespiratory response to exercise.
That internal roiling you experience when you step out of your car at the event site on race morning, surrounded by your fellow competitors and the electric Ironman atmosphere, is essentially the same feeling your dog experiences when you show him the leash.
When the race begins, the biochemical state of every system in your body changes as each responds first to the challenge of swimming 2.4 miles, then to that of cycling 112 miles and finally to that of running 26.2 miles. Among the greatest physiological challenges are core body temperature regulation, dehydration, fuel supply and usage, muscle damage, nutrition absorption and processing 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 very effectively. But the bike and run legs of the race are a different story, especially on hot days. Almost three-quarters of the energy that your muscles release during cycling and running takes the form of heat waste. If this heat were allowed to accumulate in the muscles, it would eventually cause serious tissue damage.
Your body has various means of preventing this heat from accumulating. The best known and among the most effective is perspiration. But the most effective of all is simply regulating your exercise intensity. The more you slow down, the less heat your muscles produce. That’s why you don’t race as fast on hot days.
You might assume that your core body temperature begins to rise gradually at the beginning of the bike leg and continues to rise throughout the rest of the race, reaching its highest level when you cross the finish line. This is seldom the case.
“Within the first 10 to 20 percent of the race, the core temperature rises relatively quickly,” says Jonathan Dugas, Ph.D., an exercise physiologist at the University of Chicago. “For example, it might rise from 37.5 to 38.75 degrees C in that timeframe. And then for the remainder of the event it stays within a very narrow range—maybe 0.1 and 0.2 degrees Celsius.”
The maximum safe core body temperature is 40 degrees C, or 104 degrees F. Even on the hottest days, Ironman participants seldom cross this threshold. According to Dugas, that’s because the brain constantly monitors the core body temperature and produces feelings of discomfort and fatigue that force you to slow down and generate less heat if things look like they’re about to get out of hand. Triathletes sometimes mistake these unpleasant symptoms as indications of heat illness, but they are actually products of a self-protective mechanism that prevents heat illness.
But this self-protective mechanism has been known to fail. That’s probably because the brain itself can become too hot to function properly during exercise in hot environments. When this happens, the central nervous system begins to malfunction and the athlete becomes dizzy, disoriented and uncoordinated, and may collapse.
Running on Empty
The most celebrated physiological challenge of Ironman racing is supplying the muscles with enough energy to cover the distance as quickly as possible. The average Ironman competitor burns more than 6,000 calories between the start and finish lines.
These calories come from fats stored in adipose tissue and within muscle tissue, 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 carbohydrates.
The balance of fuels shifts over the course of the day. During the swim and the first portion of the bike leg, carbohydrates are likely to provide almost half of the muscles’ energy, with fat providing an equal amount and protein just a sliver. As the body’s carbohydrate stores decrease, the carbohydrate contribution to forward progress diminishes and fat takes up the slack. By midway through the marathon, at the latest, muscle glycogen will have reached critically low levels in the calves, quads and hamstrings. Consequently, total carbohydrate contribution to continued running drops further, fat oxidation increases, and amino acids may provide as much as 15 percent of the muscles’ energy.
The inability to supply sufficient energy to the muscles is one of the main reasons individuals who do not train for an Ironman cannot complete an Ironman. Endurance is strictly limited by the availability of glycogen in the liver and working muscles. When these stores fall too low, your day is done. Endurance training greatly increases the body’s capacity for glycogen storage. But even the fittest triathlete cannot store enough glycogen to fuel an entire Ironman. Thankfully, training also greatly increases the capacity to burn fats, which allows the athlete to conserve glycogen, making it last longer.
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. 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 two to five 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.
I Think I’m Going to Puke
Australian professional triathlete Chris Legh had emergency surgery to remove half his colon after the 1997 Hawaii Ironman. A good chunk of the organ had literally died during the event due to inadequate oxygen supply. While this type of crisis is extremely rare in Ironman racing, and in Legh’s case was probably related to a congenital heart defect, completing an Ironman is stressful to the gastrointestinal system of every competitor. Common problems include stomach discomfort and bloating, nausea, vomiting and diarrhea.
Exercise scientists do not fully understand the causes of such symptoms of GI distress during intense physical exertion. But they have identified some of the contributing factors. In a 2005 review published in the online International SportMed Journal, authors Stephen Simons, MD, and Gregory Shaskan, MD, wrote, “To date, contributing theories mainly focus on the mechanical agitation of the gut, fluid shifts, decreased splanchnic blood flow, dehydration, increased sympathetic and parasympathetic tone, endotoxaemia, changes in bowel transit time, hormone shifts and autoimmune changes. However, none of these adequately explain the full range of GI pathology.”
Triathletes also bring many of their GI troubles on themselves by trying to consume too much fluid or nutrition or foods that are too difficult to digest while competing in Ironman events. The gastrointestinal system cannot tolerate the same rates and types of nutrition intake during vigorous activity as it can at rest. Studies have shown that athletes who take in the most nutrition during endurance events are most likely to suffer gastrointestinal mishaps.
The Final Stretch
If you’ve ever completed an Ironman, you know that the last few miles of the marathon are a unique experience that is only hinted at by the experience of running the last few miles of a regular marathon. Your body is so impaired from the beating it has taken over the course of the day, it’s almost funny. The simple act of lifting your foot off the ground to take the next stride feels akin to performing a heavy squat with a weighted barbell on your back. Research from the National Institute of Sport and Physical Education in Paris confirms that the energy cost of running at the end of a triathlon is significantly greater than that of running at the same speed without swimming and cycling beforehand. And that’s an Olympic-distance triathlon.
There are probably multiple causes of the “weightlifting” effect of an Ironman marathon’s closing miles. Stride form is measurably different at the end of a triathlon run than it is in the same athletes in an independent run. The stride changes that increase the energy cost of running at the end of a triathlon are themselves caused in part by local fatigue in specific muscles, which necessitates a change in form in much the same way you might start running with a locked right knee to protect a suddenly cramping right calf muscle. It’s neither efficient nor pretty, but it sure beats the alternative.
In triathlons and independent runs alike, fatigue and loss of mechanical efficiency are associated with increasing ground contact time. The closer you get to the finish line, the harder it becomes to pry your feet off the road. This bit of bodily mutiny is caused by a weakening of motor output from your brain to your working muscles. It is your brain’s way of preventing you from running faster—and perhaps even forcing you to slow down—in response to feedback from your body.
Your brain itself may become tired by the end of an Ironman—a phenomenon known as central fatigue. Like your muscles, it runs low on critical fuels and accumulates increasing levels of metabolites that interfere with its functioning, resulting in feelings of discomfort, loss of will to continue, fractured thinking, declining mood and reduced ability to fire the motor neurons that activate the muscles.
It takes a good while for the body to recover from the stress of completing an Ironman. An Austrian study found that blood levels of antioxidant enzymes remained significantly reduced, while biomarkers of muscle damage and inflammation remained significantly elevated in triathletes nearly three weeks after they had crossed an Ironman finish line.
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 such an experience, greatly increasing the athlete’s susceptibility to viral and bacterial infections. The causes of this effect 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 during exhaustive exercise. It seems that 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 very downregulation impairs the immune system’s ability to fight foreign invaders.
Triathletes also commonly suffer from a malady known as the “post-Ironman blues” in the weeks after an Ironman. It is likely that such mood depression is to some degree just another symptom of the general overtraining syndrome that commonly affects endurance athletes after such a test. Overtraining is known to disrupt brain neurotransmitters that influence mood. It has been hypothesized that as a symptom of overtraining, depression is your brain’s way of discouraging you from overexerting yourself again—in this case, doing your next Ironman—for a while.