IF PAIN TOLERANCE varies wildly, our pain threshold, the first point at which we experience discomfort, is surprisingly uniform. In my first lab test, Dr. Rohlen placed a small black paddle on the tender skin of my inner forearm and connected it to two tubes circulating water from a bluish device wired to an IBM ThinkPad. The nickel-sized paddle heated incrementally from 35 to 52 degrees Celsius (95 to 125.6 degrees Fahrenheit), and my job was to click a computer mouse when the sensation turned from pretty warm to "That freaks me out a bit." Easy enough.

"Five seconds," Dr. Rohlen announced. "Heating!"

I chuckled to myself. Then I felt the heat. It swelled from warm to itchy to tingling. At the tipping point it turned to a sting, like icy feet in the hot tub, and I clicked. We repeated the test dozens of times, and my body pinpointed its threshold at 117.5 degrees, give or take a degree and a half.

This temperature is common, it turns out. Humans seem genetically programmed to recognize 117 degrees as an important warning point—much higher and skin will burn. But how much burn (or freeze or pressure) you can accept seems to be significantly affected by past experiences. The lab has found that people who've experienced significant amounts of pain—say, soldiers, athletes, or women who've given birth—tend to have much higher tolerances, some of them capable of distinguishing up to 11 intensities of pain.

What this means is that (1) the easiest way to recover from a fractured arm is to have already broken your back. Relatively speaking, you feel fine. (2) Most people probably can't even imagine what it's like to lose a leg; the pain is off their charts. And (3) Lance wasn't lying when he said he could pedal harder: The tearing in his quads was nothing compared with the slow, devouring pain of chemo. At Indiana University's Simon Cancer Center, Armstrong had been forced to radically recalibrate his pain tolerance, so three years later, even if he was hurting as badly as other riders, he knew he could withstand more.

"He was contextualizing the pain," Dr. Rohlen explains. Indeed, Rohlen himself has done it. As a competitive mogul skier, he didn't understand the true depth of pain until years of bumps led him to the OR for lower-back surgery. "That was the kind of pain that made me sweat, that made breathing hurt, that made walking impossible," he says. "Now, as a physician, I see people in pain and I respect the truth in their hurt."

All of which is fine for Rohlen, but the worst accident in my grand parade of mishaps was splitting my chin open on the monkey bars when I was 11. On the toughness scale, this puts me just ahead of Miss Teen USA and well behind rodeo clowns. And I'm not about to leap off a speeding train to "contextualize" future pain.