Some will barely shuffle; others will sprint. Some eyes will sparkle; others will stare vacantly ahead. Some shoulders will be square and solid; others will droop with exhaustion.

As the runners come down Boylston Street toward the finish line of the Boston Marathon or under the palm trees of the Honolulu Marathon, no two of the official entrants will look quite the same, even though they've all covered the exact same distance.That's on the outside. On the inside, their experiences will have been remarkably similar: Almost everyone will have a temperature above normal, be working at 85 percent of their maximum heart rate, be losing more fluids and energy than they can take in.

As Dr. David Martin, a physiologist at Georgia State University, explains it, whether you're in the lead or still somewhere back in the pack when the winner gets his laurel wreath: "I sweat, he sweats."

Granted, the conditioning of an elite athlete means his heart pumps more blood with each beat, so at equal heart rates there is much more oxygen-rich blood being propelled to the muscles. That's part of the reason he runs faster than your brother-in-law. (It's not quite that simple, but you get the idea.) And the top athletes have also learned to manage pain better, so they often don't appear to be hurting even when they are, whereas the 4-hour marathoner might have the stab of every footstep etched on his or her face.

But elite or not, almost all runners follow a pattern of physiological and even psychological ups and downs as they travel the 26.2 miles.

In the hours before the race, most marathoners will be trying to rest their legs and conserve energy, perhaps lounging on the village green near the starting line. But plenty is going on above the neck, says New York psychologist Maryellyn Duane.

"There's a lot of anxiety," says Duane, a clinical psychologist who has run 10 marathons. "You don't want to be without adrenaline, so a certain amount of anxiety is appropriate and will really help you run your best race possible. There is other anxiety that will take you over the top and make you dysfunctional."

As captain of the Psyching Team that works with runners in the hours before the New York marathon begins, Duane tries to help them find that optimal level of anxiety, urging them to follow simple breathing exercises and imagine positive things, such as crossing the finish line. To that end, runners are given a tiny piece of tape, like the stuff used to form the finish-line chutes, to pin to their clothes.

"We tell them it's magic, all they have to do is touch that chute tape and it will pull them to the finish line," says Duane.

The Psyching Team was formed about 13 years ago after medical personnel realized that many runners visiting the medical tent before the race complaining of headache, diarrhea, even leg pain, were actually victims of anxiety.

But once the race begins, anxiety fades - at least for the first 20 miles - as the mind focuses on the task at hand.

"Once the gun goes off, the first five miles are relief," says Martin, who has run 29 marathons. "You're feeling like you're on Cloud 9. You're starting the journey."

Because runners ideally take off from the starting line at whatever speed they plan to maintain the entire way - at about 180 steps a minute - many of the physiological changes they experience take place in the first mile or less. With variations depending on age, conditioning, and size, the heart rate jumps from about 60 beats a minute, or 32 percent of maximum, to 135-160, or about 85 percent; the number of breaths per minute goes from 15 to perhaps 40.

Body temperature begins to rise almost immediately, but it takes about 40 minutes to hit somewhere between 100.5 and 102 degrees, where it will hold steady if the runner is drinking enough fluids to stay well hydrated.

If dehydration sets in, it becomes more difficult for the heart to maintain its output - the amount of blood pumped over a period of time. Body temperature rises as blood volume decreases, and a severely dehydrated runner may have a temperature as high as 106 degrees when she crosses the finish line, regardless of the weather. More commonly, it's closer to 102.

Even though she is burning only about 100 calories a mile - at 2,620 calories, less than a pound of fat for the whole race - a runner may lose as much as nine pounds. Virtually all of that is fluid: a runner's stomach would not tolerate, nor could her system absorb, the amount of fluid she would need to stay even with the losses.

Then there's the matter of fuel, in the form of stored carbohydrates. "Even the skinniest marathoner is loaded with enough fat to get through 26 miles, but you don't have enough carbohydrates to get the whole way," says Dr. Paul D. Thompson, director of preventive cardiology at Hartford Hospital. Under normal circumstances, the body will have enough stored carbohydrates, or glycogen - the most accessible form of energy - in the muscles and liver to last about 20 miles. That's why runners "carbo load" in the days before a race: they're trying to save up another 6.2 miles worth of fuel.

If they fail, the glycogen runs out and a marathoner "hits the wall." Although he still has fat his body can burn, it requires much more energy to use it, and it shows. "It's a dramatic slowdown," says Martin.

Yet even if she hasn't drunk as much as she should, and even if she is running perilously low on glycogen, marathoners are usually able to summon up the energy to give it one last burst - or at least what passes for a burst after 26 miles - as they head toward the finish line.