BOSTON -- His legs trembled slightly. A bead of sweat inched down his cheek.

Brendan R. Doyle held his body in a squat as his physical therapist counted the seconds. His breathing deepened.

“Down,” physical therapist Christine Stone said again, and Doyle squatted a little lower. “Stop,” she said. He held still and exhaled.

Outside Spaulding Rehabilitation Hospital next to the Charles River on Wednesday, a few patients pedaled by on cycles. Out on the river, others windsurfed. A train clanged by and boats chugged along. Doyle stayed focused.

He was due to be discharged from the hospital today — well ahead of expectations. He was getting stronger.

A state police badge is affixed to the gray helmet Doyle wears to protect his head, indented from the missing piece of skull. A St. Brendan medal that his boss, Col. Brendan Doherty, gave him hangs around Doyle’s neck. The 25-year-old trooper is determined that his career, just three years old, won’t be ended by the severe brain injury he suffered when he was assaulted June 16.

The blow that knocked him backward onto the back of his head should have killed him. Even the doctors who saved his life say they wouldn’t have been surprised if he had died. When Doyle arrived at Spaulding on July 9, his doctors told him to expect to stay until at least early next month. The average stay is about 40 days.

No way, Doyle thought.

He went from needing help walking, to hustling on his long runner’s legs up and down the stairs to his room on the eighth floor. He went from learning to balance on a line on the floor, to walking heel-to-toe on a balance beam a few inches off the floor. Though not a boater, Doyle went canoeing for two miles on the Charles River this week.

He’s lost about 20 pounds since he was injured, and he still has a feeding tube. But he tasted his first meal Monday, a peanut butter-and-jelly sandwich, and he looks forward to eventually having a steak and a beer. He is being discharged today to his mother, Maureen Adams, in Seekonk, and will continue with outpatient rehabilitation.

While his strength has improved, his right side is still weak and he has double vision. But he is regaining his balance. “When I read his chart, I was absolutely floored that he is where he is right now. It’s just really incredible,” Stone said, as Doyle rested for a minute and grinned. “Part of it is he’s a really hard worker, part of it is he’s really fit to begin with. And, part of it is much more like a miracle.”

The wail of a police siren suddenly cut through the rumble of traffic — a reminder of Doyle’s life just a little over a month ago, and where he wants to return.

“I love being a state trooper, love it, absolutely love it,” Doyle said. “Coming back from this injury, safety is a huge priority in my eyes. And I just wish I could come back to work tomorrow, I really do. It’s going to be a couple months now, until I get my skull put back together and my vision comes back. But I look forward to being a cop again.”

In the early hours of June 16, Doyle was off-duty when he tried to stop a reckless motorist on a street crowded with pedestrians. Doyle told the driver he was a trooper and ordered him to stop. The police said the driver got out and sucker-punched Doyle, who fell back and struck his head.

Providence firefighters rushed the unconscious, bleeding man to Rhode Island Hospital, where Dr. Christopher Demers met them at the emergency room doors.

Doyle’s head was bleeding profusely, Demers said. One eye was dilated, the other barely responsive. One arm had stiffened. He had trouble breathing. “He was close to death,” Demers said.

An emergency CAT scan in those first minutes showed bruising and bleeding inside Doyle’s skull. The doctors needed to open up his skull quickly to relieve the pressure.

Demers, a surgeon, called then-chief resident Dr. Steven Cobery and attending physician Dr. Deus Cielo, and the trauma team. Doyle’s mother and older brother arrived just as the trooper was being wheeled to the elevator. We need to take him to the operating room right now, or he will die, Demers told them.

The family crowded into the elevator, telling Doyle they loved him. As the doors opened, the medical team ran with Doyle to surgery.

In the first 10 minutes, the doctors cut down a flap of the temporalis muscle, pulled it from the skull, and drilled three nickel-sized holes at each corner of an incision around where the blood clot was. They used a saw to cut an incision line and remove a hand-sized piece of skull.

They opened the dura mater membrane encasing the brain, and coagulated blood spilled out. As they washed away the blood, the doctors noticed the deep purple blush of the bruised brain and faint pulsation from the beating of Doyle’s heart.

It doesn’t look good at all, Demers thought.

They closed the incision with 30 to 40 stitches and 40 staples. The brain was swelling, a natural reaction to trauma, and an opening in the skull would give it enough room to expand. The skull piece was placed in a freezer to be reattached months later.

After an hour and a half, the surgery was over. “We knew even though we relieved the pressure, he wasn’t out of the woods yet,” Demers said. The doctors talked to Doyle’s family about organ donation.

As Doyle remained in a coma, his family was warned that he could be paralyzed, lose his memory, or have a different personality. But the man who slowly began to wake up was still their Brendan.

“It was pretty impressive,” said Dr. Marc Goldman, the neurosurgeon in charge of Doyle’s care at Rhode Island Hospital. “You would not have predicted that recovery, from the way he was.”

Doyle regained his voice weeks later while he was at Spaulding. He tells cop stories — chasing a suspect over a fence in Providence, getting slammed from behind by a drunken driver on Route 146, and pulling a loaded .357 Magnum and cocaine off a passenger. He likes stopping cars and the intrigue of who may be behind the wheel. It could be a harried soccer mom. It could be a man with a warrant and a gun and a pack of lies. It’s a guessing game — with public safety at stake. Even off-duty.

“To be a difference-maker, to me that’s important,” Doyle said. “And someone’s got to do it, to protect and serve. And I like doing that.”

The doctors say Doyle’s youth and conditioning as a runner helped save his life. The next 14 to 16 months will decide his recovery, said Dr. David Burke, medical director of Spaulding’s brain injury rehabilitation program. There can be some slight measurable improvements recorded over five years, he said. “But if he can’t get back to work in 16 months,” Burke said, “he will never get back to being a trooper.”

There are many hurdles. The neuropsychological tests don’t measure how someone functions while fatigued and under great stress, Burke said. The state police are “an elite group with elite behavioral and physical needs,” Burke said. “Vision is critical and he’ll need to deal with situations where people are lying to him and conniving.”

These issues lie ahead, but Doyle’s challenges now are regaining his vision and balance and strength. Burke knows Doyle wants to return to his job.

“I don’t want to rob him of that goal,” Burke said. “His trajectory is he’s doing better than most [brain injured]. I have great hopes he’ll get back to where he needs to be.”

In between exercises outside, Doyle teased his physical therapist. “She thinks it’s pretty cool to torture me,” Doyle said. “Gotta do something with my day,” Stone bantered.

The last exercise was the hardest — balancing his body sideways with one arm on the ground. He strained, grunting, on his right side, as Stone ticked off the seconds.

At the academy, Doyle could hold this pose for three minutes. Now, his right arm is just strong enough to hold his body for a few seconds. Still, he says, just three weeks ago, he couldn’t move his right arm at all.

This week, Doyle is walking.

Next week, he says, he will run.

amilkovi@projo.com