FREMONT -- When a student in the engineering design class he teaches at San Jose High School told David Haslet last year that two bombs had just exploded at the Boston Marathon, Haslet briefly mentioned that he had family in Boston, then the discussion quickly turned to a modeling project the class was working on. He didn't think about his younger sister again until he returned home later that afternoon and found a stack of urgent phone messages from family.

Adrianne Haslet-Davis is four years younger than her twin brothers, David and Timothy, and she was always the excitable child, the one who threw herself into any interest that seized her. At her Boston home not far from the marathon finish line, Haslet-Davis turned to her husband that morning after watching Ethiopian runner Lelisa Desisa glide to victory on TV.

"I'm still in my pajamas, and this guy just won a marathon," she said, a fateful remark she would recount ruefully to her brothers from a hospital bed. "Let's go out and cheer on the other runners."

When the second bomb went off, it took most of Haslet-Davis' left foot with it. Her husband, Air Force Maj. Adam Davis -- on leave from a deployment in Afghanistan -- also suffered shrapnel wounds from the blast. But it was Haslet-Davis' more grievous injury that changed her entire family. When a record field of 36,000 runners sets off Monday in the first renewal of the marathon since the bombing, David and Timothy Haslet -- neither of whom, at 37, has ever competed in a marathon -- will be running for their sister, "sharing a burden in order to share a triumph," David says.

Miracle of science

Haslet-Davis had vowed to run in this year's race, but six months ago she realized her recovery was not happening fast enough to permit it. That's when she decided to ask her brothers to run in her place. If all goes well, Haslet-Davis will meet the twins about a mile from the finish line on her prosthetic running leg and finish the race arm-in-arm with them.

If it happens, it will mark a complete reversal from the bloody aftermath of the day of the bombing. In those dark moments immediately after the blast, Adrianne pleaded with first responders for help. "She was telling anyone within earshot, 'I'm a dancer, I'm a dancer. You've got to save this foot,' " David says.

To save her life, doctors amputated her left leg at the knee, transforming her in the process from an obscure instructor at an Arthur Murray Dance Studio to a miracle of biomechanical science. Not long after her operation, Haslet-Davis was invited to appear on "Dancing With the Stars," where she was told she'd be welcomed back to dance on the show as a competitor when she's ready. She's expected to make a special appearance on the show this fall, and her inspiring comeback performance of a rumba on her prosthetic leg following a TED Talk by MIT bionics wizard Hugh Herr -- who crafted a bionic dance foot for her -- could make Haslet-Davis one of America's most celebrated hoofers.

Her determination not to be cast as a victim has made Haslet-Davis one of the most visible survivors of the bombing. She recently walked off a planned appearance on the NBC News program "Meet the Press" because producers declined to agree to her ground rule that the bombers' names not be mentioned.

Haslet-Davis has done her best to serve as a positive counterweight to the evil the bombers did, embracing her role as one of the many faces of Boston Strong. But it remains to be seen whether even she can turn her brothers into long-distance runners. Timothy, a landscaper and artist, says his sister called David first because she knew he would say yes, which meant his twin couldn't say no.

'A transformative year'

David Haslet ran a few 10Ks in college, but neither he nor his brother -- who lives on Whidbey Island in Washington state, near where they grew up -- are serious runners. "Who would turn down this opportunity?" David says. "This is a way of helping someone in a tangible way, of standing in for them. It's something that means such a great deal, being part of a transformative year. When people ask about motivation, I think I have it easier than others because I'm so motivated to do my best for my sister."

He had been training himself into shape using the website www.marathonrookie.com.

"Three and a half weeks ago, I went off script," Haslet says. "I finally got a chance to move away from the treadmill and run outside on the Alameda Creek Trail." Running near his Fremont home, he completed 26½ miles -- a little more than the full marathon distance. "I was so excited I went too fast," he says, "and I ended up straining my right knee. I couldn't run for a week and a half."

A P.E. teacher at the school helped him stretch the knee to relieve the pain, and another teacher tried to offer some coaching expertise.

"The main thing is just for him to finish," says Rick Carreiro, who teaches math and science in the dropout-prevention program at San Jose High. "It doesn't surprise me he hurt himself. He obviously got a little carried away about going out there, about being with his sister and the family."

Adrianne recently changed her Facebook profile picture for the first time since before the bombing, and though you can only see the outline of the prosthetic limb through her jeans, she acknowledged that her recent dance performance changed her mind. "I couldn't let go of that pic with two legs," she wrote on her wall. "After my TED experience I feel stronger and braver than ever."

It runs in the family. The Haslets are descended from the Scottish Ferguson clan, whose motto is Dulcius ex asperis (Sweeter after difficulties). "We believe it," David Haslet says.

Contact Bruce Newman at 408-920-5004. Follow him at twitter.com/brucenewmantwit.

she can dance
To watch Adrianne Haslet-Davis' dance performance at the TED Conference in Vancouver, British Columbia, go to http://tinyurl.com/ma68ajn.