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Keith Mills stepped onto a treadmill at a Dorchester gym one day four years ago. It was apparently running, though he hadn’t noticed.

“I took one step and went face down and then I was thrown against the wall,” recalled Mills, 58, who lives in Milton. “The next thing I know, I’m bleeding from my nose and wrist, and am completely out of it.”

Mills suffered a concussion and broken nose. His wrist still bears a scar from where he fell on it.

Treadmill accidents injure thousands of Americans each year, but until this week, they went largely unnoticed. The death last Friday of Silicon Valley executive Dave Goldberg after falling off a treadmill and hitting his head has focused new attention on the risks of the hugely popular exercise machines.

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Goldberg, 47, apparently died of severe head trauma and blood loss Friday after working out in a gym during a family vacation at a beachfront villa in southwest Mexico, authorities in Nayarit state told USA Today.

Goldberg was chief executive of SurveyMonkey and the husband of Sheryl Sandberg, chief operating officer of Facebook, who wrote the best-selling, “Lean In: Women, Work and the Will to Lead.”

Though treadmill deaths are rare in the United States, injuries are not. In 2014, according to the Consumer Product Safety Commission, 24,400 injuries associated with treadmills were seen in hospital emergency rooms. For the 10-year period ending in 2012, the commission reported 30 deaths. In 2012, emergency rooms treated 62,700 injuries due to exercise equipment, treadmills leading the way.

And in 2009, the 4-year-old daughter of heavyweight boxer Mike Tyson died after she was accidentally strangled by a treadmill cord in her Phoenix home.

Details surrounding Goldberg’s death are still sketchy. He left his room about 4 p.m. Friday to work out and was discovered by his brother around 7 p.m. He was unconscious but still alive and was rushed to a hospital, where he died.

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Treadmills are the most popular type of exercise equipment in the nation and are especially common in places with long winters. “Treadmills can offer an excellent aerobic cardiac workout and can be used as alternative exercise for running outdoors during inclement weather,” said Dr. Frederick Basilico, a cardiologist and physician in chief at New England Baptist Hospital.

But Basilico warns that people should be aware of the risk of injury, staying within their physical limits and avoiding distractions. If they experience lightheadedness or weakness, they should get off the treadmill, he said. “Always use the safety device that will immediately turn off the treadmill in the event of imbalance leading to a fall,” he said.

Many fitness professionals view the proliferation of constant cellphone use — while huffing and puffing on the machines — as a significant contributing cause in injuries.

Tre Robinson, a customer representative at Planet Fitness in Mattapan, said his gym prohibits cellphone use on treadmills and other equipment. “They’re a distraction and if something were to happen, we would be responsible,” he said.

Mills agrees. He still uses the treadmill four times a week. But he’s very careful. “No cellphone, and I don’t listen to music or anything I think would be a distraction.”

Charles Hardesty, who owns Totally Natural Training in Quincy, has been in the personal training business for decades. “Treadmill accidents are very common,” he said. “The worst that I see is when people set up the back of treadmills against a wall. If they fall off, the treadmill keeps running and the tread acts as sandpaper that will just take your skin off.”

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Treadmillreviews.net offers nine tips for treadmill safety, including: look forward, don’t rely on handrails, start by straddling the deck, start slowly and increase speed gradually, don’t go barefoot, don’t step off a moving treadmill, keep children away, leave plenty of space behind the machine, and don’t push your own limits.

While the number of treadmill injuries is eye-opening, medical professionals note that all exercisers generally, and runners in particular, risk getting hurt while working out. Muscle strains are common, and many joggers suffer more serious injury when they trip or step into a hole.

Keith Foley, a veteran physical therapist who manages the Spaulding Outpatient Center in Cambridge, said he hasn’t seen any more injuries caused by treadmills than by running outdoors. “Usually why people get injured is because they’re not using equipment properly,” Foley said. “Maybe they’re talking on the phone or looking to the side. Head turning is a big thing we worry about because it creates a drift of the body and they can step off the belt.”

At his own health club, he’s seen people try to stop their treadmill workout by jumping off the back, and falling. He’s seen others ramp up the speed too quickly and fall.

Spaulding uses treadmills for some of its outpatients, including a program for injured runners and another one for the elderly to prevent falls called Live Long, Walk Strong. “We haven’t had any incidents with the elderly on treadmills,” Foley said. “They are a safe piece of equipment if you operate and use them properly.”

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Bella English can be reached at english@globe.com.