SAN ANTONIO — After the explosion, Cpl. Sebastian Gallegos awoke to see the October sun glinting through the water, an image so lovely he thought he was dreaming. Then something caught his eye, yanking him back to grim awareness: an arm, bobbing near the surface, a black hair tie wrapped around its wrist.

The elastic tie was a memento of his wife, a dime-store amulet he wore on every patrol in Afghanistan. Now, from the depths of his mental fog, he watched it float by like driftwood on a lazy current, attached to an arm that was no longer quite attached to him.

He had been blown up, and was drowning at the bottom of an irrigation ditch.

Two years later, the corporal finds himself tethered to a different kind of limb, a robotic device with an electronic motor and sensors able to read signals from his brain. He is in the office of his occupational therapist, lifting and lowering a sponge while monitoring a computer screen as it tracks nerve signals in his shoulder.

Close hand, raise elbow, he says to himself. The mechanical arm rises, but the claw-like hand opens, dropping the sponge. Try again, the therapist instructs. Same result. Again. Tiny gears whir and his brow wrinkles with the mental effort. The elbow rises and this time the hand remains closed. He breathes.

Success.

"As a baby, you can hold onto a finger," the corporal said. "I have to relearn."

It is no small task. Of the more than 1,570 U.S. service members who have had arms, legs, feet or hands amputated because of injuries in Iraq or Afghanistan, fewer than 280 have lost upper limbs. Their struggles to use prosthetic limbs are in many ways far greater than for their lower-limb brethren.

Among orthopedists, there is a saying: Legs may be stronger, but arms and hands are smarter. With myriad bones, joints and ranges of motion, the upper limbs are among the body’s most complex tools. Replicating their actions with robotic arms can be excruciatingly difficult, requiring amputees to understand the distinct muscle contractions involved in movements they once did without thinking.

To bend the elbow, for instance, requires thinking about contracting a bicep, though the muscle no longer exists. But the thought still sends a nerve signal that can tell a prosthetic arm to flex. Every action, from grabbing a cup to turning the pages of a book, requires some such exercise in the brain.

"There are a lot of mental gymnastics with upper limb prostheses," said Lisa Smurr Walters, an occupational therapist who works with Gallegos at the Center for the Intrepid at Brooke Army Medical Center in San Antonio.

The complexity of the upper limbs, though, is just part of the problem. While prosthetic leg technology has advanced rapidly in the past decade, prosthetic arms have been slow to catch up. Many amputees still use body-powered hooks. And the most common electronic arms, pioneered by the Soviet Union in the 1950s, have improved with lighter materials and microprocessors but are still difficult to control.

Upper limb amputees must also cope with the critical loss of sensation. Touch — the ability to differentiate baby skin from sandpaper or to calibrate between gripping a hammer and clasping a hand — no longer exists.

For all those reasons, nearly half of upper limb amputees choose not to use prosthetic devices, functioning instead with one good arm. By contrast, almost all lower limb amputees use prosthetic legs.

But Gallegos, 23, is part of a small vanguard of military amputees who are benefiting from new advances in upper limb technology. This year, he received a pioneering surgery known as targeted muscle reinnervation that amplifies the tiny nerve signals that control the arm. In effect, the surgery creates additional "sockets" into which electrodes from a prosthetic limb can connect.

More sockets reading stronger signals will make controlling his prosthesis more intuitive, said Dr. Todd Kuiken of the Rehabilitation Institute of Chicago, who developed the procedure. Rather than having to think about contracting both the tricep and bicep just to make a fist, the corporal will be able to simply think, close hand, and the proper nerves should fire automatically.

In the coming years, new technology will allow amputees to feel with their prostheses or use pattern-recognition software to move their devices even more intuitively, Kuiken said. And a new arm under development by the Pentagon, the DEKA Arm, is far more dexterous than any currently available.

But for Gallegos, becoming proficient on his $110,000 prosthesis after reinnervation surgery remains a challenge, likely to take months more of tedious practice. For that reason, only the most motivated amputees — superusers, they are called — are allowed to undergo the surgery.

Gallegos was not always that person.

His father, an Army veteran, did not want him to join the infantry, but it was like him to ignore the advice.

Gallegos grew up in Texas, raised in poverty primarily by his divorced mother. He was smart, ambitious and a bit of a know-it-all, said his wife, Tracie, who attended high school with him. A college scholarship seemed assured.

But the idea of military service called louder.

"I felt I was too immature to go to school and be some brat in college," he said.

The Marine Corps seemed the perfect challenge.

He loved the Corps and the Corps seemed to love him. Before deploying in 2010, he was made leader of a three-man fire team and sent to learn basic Pashto, the language of Afghanistan’s largest ethnic group.

His unit, Lima Company of the 3rd Battalion, 5th Marines out of Camp Pendleton, arrived in Helmand province that September and immediately faced some of the toughest fighting of the war, losing 25 men in seven months, most from improvised explosive devices, or IEDs.

In October, Gallegos, was walking second in a patrol through the Sangin district when he stepped into an irrigation canal, heard a boom and blacked out. When he awoke, he found himself anchored to the bottom by his body armor and weaponry. He tried to pull himself out with his right arm, not realizing it had been virtually severed just below the shoulder.

On an evacuation helicopter, the corporal glimpsed his intact arm wrapped in bandages, giving him hope that doctors could reattach it.

That hope was dashed at Brooke Army Medical Center, where he began the long process of recovery. His attitude, he admits now, was negative, influenced by another Marine who rarely used his prosthetic arm because it was so uncomfortable.

But then Gallegos met an Air Force amputee who was among the first at Brooke to receive targeted muscle reinnervation surgery. The airman warned him that rehabilitation would be frustrating and painful, but that the payoff would be huge.

"You wouldn’t notice, unless you were looking right at him, that he was missing his arm," Gallegos said. "I was like: ‘I want to be better than him."’

First, though, he had to learn to cope with phantom-limb pain. A pulsing sensation like having a tourniquet applied to the arm, the pain is sometimes powerful enough to keep the strapping corporal in bed, leaving him unable to concentrate or converse.

"He’s in constant pain," said Tracie Gallegos, who is in nursing school. "But he just won’t complain, because he doesn’t want people to ask: ‘Are you OK?’ That question really bugs him."

Over time, medication and surgeries dampened the pain enough that he could throw himself into practicing on a robotic arm. The device, he found, was a brain teaser, frustrating his efforts to make it obey. More than once, he threatened to throw it out the window.

To motivate himself in those moments, he thought about his Marine Corps friends. Eventually he had a skin-tone silicone sleeve for his prosthetic arm engraved with the names of all 10 Marines from Lima Company who died in Sangin. Now, when he needs a lift, he looks at the arm — in the very place he once wore his wife’s hair tie — and recites their names like a personal prayer.

As he began wearing mechanical arms longer each day, his prosthetist, Ryan Blanck, decided Gallegos might be ready for targeted reinnervation surgery. The procedure works off the natural ability of muscles to amplify nerve signals. By rerouting nerves into healthy muscle and reshaping the tissue to bring it closer to sensors in the prosthesis, the procedure strengthens the brain’s signals, and hence its ability to control the machine.

Wearing the same type of prosthetic arm he had used before, Gallegos noticed the difference almost immediately. No longer did he have to think so hard about contracting various muscles: When he wanted the arm to move, it did, faster and more fluidly.

That did not mean, however, it behaved as he wanted. He still has problems with "cross talk," where certain nerves dominate over others. If a wrist nerve dominates, for instance, a patient may have to think about bending the wrist to make the hand close. But with repeated use, the nerves sort themselves out and the need for trickery fades, Kuiken said.

For all his gains with the prosthesis, however, Gallegos has not overcome the embarrassment he feels when wearing his robotic arm in public. Once the hand fell off in a crowded restaurant, eliciting gasps from a nearby child. In darkened movie theaters, the Terminator-like sounds his arm makes draw startled whispers. And to this day, he will not wear short-sleeve shirts to restaurants.

"Even if it is 75 degrees out, I’ll wear a jacket just to avoid getting stared at," he said.

For a year after nearly drowning in Afghanistan, Gallegos could not go near water, any water, even the River Walk, a restaurant-lined path along the San Antonio River. But a therapist pushed him to overcome his anxiety, first by swimming, then kayaking, then surfing.

Ben Kvanli, a former Olympian who runs a kayaking program for disabled troops, said Gallegos was an ambivalent paddler at first. But his technique was good, partly because the prosthesis forced him to use his core muscles more. And he was fast.

Fast enough, indeed, that Kvanli is encouraging him to try out for the national Paralympic team next year.

"Independence is a big part of this," Kvanli said. "He is proving something."

Fiercely self-reliant from childhood, he has struggled with losing independence after losing his arm. Suddenly, he had to ask for help with buttons, zippers and shoe laces. And he loathes asking for help.

There are holes in his living room wall that testify to his failed attempts to hang things using his prosthetic arm. And he still cringes at the memory of barking orders at his wife while she assembled a living-room furniture set that he could not assemble himself.

"Stuff is a lot more complicated," he said. "I’m still figuring out what my norm’s going to be, just on a day-to-day basis."

For that reason, he no longer makes big plans for the future, as he once did. Keep it simple, he tells himself: Get out of the Marine Corps. Go to college. Learn how to tie his shoelaces with a robotic hand.

And maybe, just maybe, become a Paralympian.

So there he was one recent afternoon, kayaking down the sun-dappled San Marcos River, using the wrong prosthesis because he had broken his kayaking limb while surfing. Normally he is at the front of the pack, but today his arm kept slipping off and he seemed in pain as he struggled just to keep up.

Yet he said nothing that could be heard as a complaint. And at the end of the six-hour trip, he went over the 14-foot Graduation Falls, the first time he had done so in a boat. After dropping vertically into the frothing water, his kayak momentarily disappeared beneath the surface before popping out like a cork.

Eyes smiling below the brim of his helmet, Gallegos paddled to shore, hefted his boat onto his good shoulder and started the trudge upstream.

He did not ask for help.

© 2012 The New York Times Company