The human body is an amazing instrument of motion and energy. When all its parts are tuned and working together, it plays a supernatural symphony of incredible complexity, sensitivity and strength. With intelligent guidance and healthy nurturing, the body is capable of reading a score of intense physical challenges, adapting and responding as one orchestra for incredible sustained performance. The bodies of the very best athletes in the world can run 100 meters in 10 seconds, one mile in fewer than four minutes, a 10km in just over 26 minutes and 26.2 miles in two hours and change.

Outside of these extraordinary accomplishments, if you knew just how many parts of the body must work together to simply swim a few strokes, ride a bike or trot around the park, you'd offer a standing ovation to everybody in motion, from the genetically average soul who walks a 5km to the aerobic artist who flies through an Ironman in under eight hours. And if you knew what happens to the body as a result of exercise, in particular, training for triathlon, you would never want to stop moving.

Body Basics

At birth, each of us is handed a Stradivarius of physical possibility—and a blissful obliviousness to our own complexity. If most people were aware of the enormity of our parts-25 trillion red blood cells carried by 60,000 miles of vascular highway and 280 million hemoglobin molecules that mix our food with oxygen to fuel the body's 650 muscles—we would spill our coffee while stretching for our morning five-mile run. And if we knew that the little pump in our chest that weighs 1.2 pounds and needs to work perfectly to keep us alive beats 100,000 times a day and 40 million times a year to move 38.3 million gallons of blood during the average 75-year lifetime, we might incur a permanent case of the jitters.

How Training Affects The Body

Indeed, the body is a gift that the human species has refined and evolved over countless millennia, from the days when hunters had to track down their prey to the current era in which modern technology threatens to make simple movement nearly irrelevant. But vestigial delight in play and motion still exists, exemplified by the growing ranks of triathletes and more-established tribes of runners, cyclists, swimmers and devotees of the gym who save themselves from death by sloth. Exercise, doled out in its most balanced form of triathlon's regimen of swim-bike-run, is a miraculous antidote to modern decrepitude. If you avoid injury and overtraining, aerobic exercise helps prevent heart disease, diabetes, obesity, stroke and several forms of cancer.

It improves strength and balance, makes your bones stronger and stimulates your marrow to crank out more red blood cells to transport nutrients and oxygen, helping muscles work longer and harder. Exercise causes the body to grow thousands of capillaries to connect the lungs, heart, skeletal muscles and brain so you can run faster and think more clearly. It also hones your metabolism, burns fats more quickly and cleanly, increases your blood volume and improves your immune system.

For those with a competitive streak who enjoy training for sport rather than mere exercise, vigorous and consistent aerobic activity does more for the body than stave off illness and fine-tune its physical systems. In short, training for triathlon can turn a rickety 1988 Ford Taurus of a body into a carefully crafted 2008 racecar, within the limits of genetic capability and age. Let's look under the hood.

The Heart

When you're training, your heart pumps more blood to your muscles, lungs and stomach to fuel working fibers, produce more oxygen and transport glycogen, lipids and amino acids to the mitochondria. Regular and consistent aerobic exercise increases the size of the heart's left ventricle, improving stroke volume while increasing the power of the contracting muscles. Regular, intense exercise also thickens the walls of the heart's chambers so the muscles can sustain a higher intensity output. After months of training, a previously sedentary person's resting heart rate might decline from 70 beats per minute to 40 or fewer as their overall blood stroke volume increases.

V02 Max

While many serious athletes are familiar with the concept of V02 max, most have a hard time defining the term. In short, V02 max is the body's maximum rate of oxygen flow, expressed relative to bodyweight in milliliters per kilograms per minutes (mL/kg/min). Age, gender, training, altitude and hereditary factors all influence individual V02 max.

For the ordinary athlete, increasing training and adding more exercise intensity improves V02 max, which usually results in increased performance. The V02 max of a sedentary person can range between 20 and 50, with an average of 35. While exercise increases V02 max, training like a world champion will boost your oxygen capacity by only 5 to 15 percent. This suggests that athletes with especially high oxygen capacities are genetic rarities rather than training geniuses.

In addition to V02 max, endurance training also increases your maximum breathing capacity, allowing you to take in more oxygen with each breath. While the average untrained college-aged youth inhales between 125 and 170 liters of oxygen per minute, a cross-country runner can breathe 200 liters in a minute. The more oxygen you can breathe with no additional effort, the more speed and power your body is capable of producing.

The Circulatory System

The body's circulatory system carries waste products, including carbon dioxide and lactic acid, away from working muscles and redistributes heat generated by exercise to the heart, where it is pumped to the surface and dissipated via radiation and sweat evaporation. Beyond everyday aerobic training, athletes preparing for long races in hot, humid climates engage in effective heat acclimatization. Training in a hot climate for two to four hours a day for eight to 14 consecutive days can cause several vital changes, including early onset and increased rate of cooling sweat, increased blood plasma volume, increased sodium chloride retention up to 25 percent and a 15 to 25 percent decrease in heart rate.

When faced with the positive stress of consistent training, the body's energy systems respond with amazing adaptations. Hard exercise can increase the number of capillaries that serve the specific muscles you train by as much as 40 percent. Running also stimulates red blood cell production and increases hemoglobin, the iron-protein compound that boosts the blood's oxygen-carrying capacity. Exercise augments the variability of heartbeat, reduces markers for inflammation and lessens the coagulability of the blood so it flows more easily to the muscles, lowering the body's risk of heart attack and stroke.

ATP And Fat Burning

Endurance training increases the number of mitochondria, the cellular power plants that produce adenosine triphosphate, or ATP, the main source of energy for cellular functions and the catalyst of chemical energy for metabolism. One of the key benefits of endurance exercise is the resulting shift in energy use. According to Ray Browning, a seven-time Ironman winner with a Ph.D. in exercise physiology, an untrained person who sets out for a half-hour bike ride and produces 75 watts of power at an average of 15 mph on a flat road will use mostly carbohydrate to fuel his or her efforts, up to 90 percent. After several weeks of training, if that individual sustains 150 watts of power, only 50 percent of his or her energy will come from carbohydrate while the other 50 percent is derived from stored and circulating fat.

Such an energy shift is critical for marathon, triathlon and other endurance pursuits, as more calories are required to fuel long-distance efforts. Why is fat better than carbohydrate? Muscle glycogen, your body's stored carbohydrate, tops out at about 2,000 calories, while even the leanest triathlete has between 5,000 and 10,000 calories of stored fat at any given time. Since the average energy needed to run one mile is 100 calories, training to burn fat instead of glycogen is vital to finishing a triathlon or 26.2-mile run in good shape.

Lactate Threshold

Another key benefit of endurance training is the resulting increase in your body's lactate threshold, or the point during exercise when blood lactate levels rise precipitously. While lactate has been mistakenly labeled as a negative metabolic event that creates a burn and shuts down muscle efficiency, it's actually a beneficial catalyst that reduces acidity in muscles and is reconverted in the liver to restock glycogen stores.

When your lactate production rises sharply, it's a good indicator that the intensity of the activity has surpassed your aerobic threshold and reached an anaerobic level. Like V02 max, lactate threshold, sometimes referred to as anaerobic threshold (AT), is another indicator of performance, although it's hardly conclusive. Untrained runners can reach their lactate thresholds at 55 to 60 percent of their V02 max, while trained runners can sustain an hour at 80 to 90 percent of their V02 max before hitting the lactate point. In a typical training scenario in which a new runner increases weekly mileage from 15 to 45 miles over a 12-week period, he or she may improve lactate threshold by as much as 20 percent, which means more speed for a longer period of time before reaching an anaerobic state.

Muscle Fibers

Endurance training causes adaptation to occur in the heavily used muscles of the legs and arms and, to a lesser degree, in the body's core area. As most learned triathletes know, there are two types of muscle fibers: fast-twitch and slow-twitch. Fast-twitch fibers contract quickly and have a high capacity to produce more anaerobic energy, while slow-twitch fibers contract slowly. On average, humans have a 50-50 ratio of fast- to slow-twitch fibers. Marathon champions tend to have a higher ratio of slow- to fast-twitch fibers (80-20), while sprinters and weightlifters have a higher percentage of fast-twitch muscles to operate in short bursts for anaerobic efforts (See chart on page 48). Extensive aerobic training tends to decrease fast-twitch muscle power, causing a veteran Ironman athlete who used to be able to dunk a basketball to have a difficult time springing off the ground fora jump shot due to his redistribution of muscle fiber.

Muscle Benefits Specific To Triathlon

Cyclists usually have strong quadriceps and glute muscles, runners primarily build up their hamstrings and calves and swimmers mainly muscle up their core and upper bodies. Triathletes, however, develop muscle groups fairly evenly. At the cost of some foot speed, high-end power output on the bike and maximum pool speed, a well-trained triathlete stays within the golden mean of musculature. By maintaining muscular balance, triathletes increase their fat-burning metabolism, lower their body fat percentage and boost the body's reliance on aerobic, fat-burning energy systems. Swimming and cycling's stronger core and leg muscles allow triathletes to better absorb the pounding of running, thereby limiting biomechanical injuries and helping maintain proper form during times of fatigue.

Overtraining And Injury

If you train continually with too high of a heart rate or too much speed—or with both—you will eventually lower your immune function, increase muscle damage and slow your body's fat-burning adaptations. Without a periodic, hard-easy training pattern and enough rest, your body can't recover, rebuild or respond positively to stress. Athletes who continue high-stress training when fatigued will eventually incur too much of the stress hormones cortisol and adrenaline, which, in excess, reap harmful effects on the body.

Furthermore, athletes who radically decrease their caloric intakes to try to lose fat and improve performance while training can cause their metabolisms to shut down and their fat-burning capabilities to virtually cease. Reduced calorie consumption can make you too exhausted to train to burn the calories you hope to, resulting in weight gain.

When you're laid up by injuries, illness or overtraining, your body's hard-won fitness can evaporate more quickly than it was gained. For example, the V02 max that took you six months to raise 20 percent can decline 7 percent in just 12 days of inactivity and by 20 percent in eight weeks of couch cuddling. The carefully wrought aerobic capacity that changed you from an inefficient carb-burning lunk to someone who cruises along torching fat stores can revert back to burning mostly carbohydrate within a month or two.

A resting heart rate of 35 beats per minute can jump to 60 beats per minute after a few months of chair-hugging immobility. Inactivity also causes a gain in fat and a loss in muscle mass, and pretty soon, the new capillaries your body has grown will start serving as fuel depots for your fat stores rather than finely tuned muscles. The good news, however, is that your body will adapt more quickly to training the second time around.

And hopefully, you'll be much wiser for the misadventure. According to Neal Henderson, coordinator of sports science at the Boulder Center for Sports Medicine and adviser to several top triathletes and Olympic cycling phenomenon Taylor Phinney, the Type-A obsessive tendencies of ambitious triathletes should be harnessed.

"I love triathletes' determination and desire to improve, but many times, the people I see are training harder than they should," says Henderson. "Their bodies simply tolerate the stress, but they get no better at what they do. I can ram my head into a wall and do that time after time, but because I can do that doesn't mean there will be a net positive outcome. That is the way some people overtrain."

So what causes these athletes to overtrain? Says Henderson, "What happens is they get addicted to the thing that allows us to cover so many miles at such a fast pace—the endorphins. When we push ourselves hard, the body creates endogenous morphine. In a clinical setting, it is used to dull pain acquired by some insult to the body. So if you are training at a level that is causing some insult and injury and requires endorphins, you are getting morphine and athletes become addicted to their own morphine. To actually get faster and stay healthy, you need to try something different and tune the body better."

Physical Hell

In the bigger picture, the American population is paying a hefty price to be the leader of the developed world. Estimates suggest that 67 percent of the U.S. population is overweight, with diabetes present in 20 percent of the population and heart failure rising as a result.

"I'm in awe of the amazing adaptability of the human body, but I fear for us," says Browning. "Our bodies adapt to and thrive on motion and play. Yet in modern, developed countries, machine-powered transportation and computers make walking and running virtually unnecessary, and we have adapted to this lack of motion. In addition, modern agricultural products fill our stomachs with high-quantity, low-nutritional-value fast food—and it's killing us.'

Browning sees two ominous canaries in the mine. "There are two absolutely alarming, horrifying statistics about the U.S. that don't receive the attention they should," says Browning. "The first is that life expectancy for children born now is lower than their parents'. That's due to genetics and the environment, but a significant part is also a lack of movement."

The second statistic relates indirectly to triathlon. "In the mid-'70s, approximately 60 percent of children either walked or rode their bikes to school," says Browning. "The latest survey now shows that only 17 percent do—and that number's dropping. What this means is that soon the next generation will not know how to ride a bike. Triathlon depends on a pool of people who grew up riding bikes, and if they don't ride as kids, once past childhood, they face a big hurdle in learning how to ride—and most people simply will not do it.'

Physical Nirvana

Chances are, you're already a triathlete. And if you've embarked on a physical quest for joyous bodily perfection through swimming, biking and running, you probably realized a while ago that so very few of us have the genetic capability to win an Ironman or make an Olympic team—or even make the age-group podium at a small, local sprint event. But in lieu of these dreams, anyone can unearth the real-life thrill of pursuing an absolutely perfect physical version of him or herself.

For many athletes, triathlon's about turning the body into a sports car, an amazing vehicle of fun and movement that can keep you healthy, fit and young beyond your years. And if you dare to go beyond simply moving and eating better, triathlon will inevitably engage you in the intriguing pursuit of peak performance.

If you're a numbers person, the numerical prospects of training for triathlon are very good. If you're lucky, you will watch your body fat lower from 25 to 13 percent, your resting heart rate plunge from 70 to 40 beats per minute, your muscle mass increase and your fat stores deplete as your body approaches perfection. Your V02 max may punch up from 45 to 60 mL/kg/min, your anaerobic threshold could rise from 50 to 85 percent, your blood volume will increase, your lungs will push more air, your leg and arm muscles will grow 10 to 33 percent stronger and your capillaries will turbo-boost more oxygen and nutrition to those muscles. You will sweat more when needed and leak fewer electrolytes to be better armed to go harder and longer in heat or cold.

Or if you simply don't care about numbers, think about this: Instead of measuring your progress through life against the dread of a scale and the fearful outcome of blood pressure and cholesterol tests, you can look forward to health, wellness and a body perfected by training, all gassed up and ready to go.

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