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 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.