As a Philadelphia firefighter, Steven Mesete often is required to spring into action at a moment's notice, abruptly sending his heart into rat-a-tat mode.

"You can be sitting still, and then running at 100 miles an hour," he said.

One day last week, Mesete ramped up his cardiovascular system in a much more controlled fashion, walking on a treadmill with wires stuck to his chest and a cardiologist standing nearby. The 41-year-old firefighter with Engine 49 in South Philadelphia was among the first in the 2,200-member department to undergo what is now a mandatory physical exam every two years.

The Philadelphia Fire Department has long conducted medical exams for new hires and before promotions, but regular physicals have not been required for several decades, said deputy chief Ted Mueller, the department's health and safety officer.

That was a cause for concern among management and labor alike, given the extreme physical demands of the work. In addition to sudden stresses placed on the heart, firefighters contend with all manner of unknowns, such as exposure to blood-borne pathogens and the inhalation of smoke and other airborne contaminants.

Regular physicals were not required for so long partly because the International Association of Fire Fighters Local 22 was concerned that the results could be used to force its members off the job.

The new requirement, negotiated as part of the labor contract signed in 2015, includes safeguards. Firefighters with medical conditions that prevent them from doing their regular jobs can be placed on light or restricted duty, where appropriate. Only those deemed unfit for any duty would have to go on leave, using their own paid leave time.

"The new commissioner made it known that this was nothing punitive," Mueller said of Adam Thiel, whom Mayor Kenney appointed in April. "The intentions are all aboveboard."

Philadelphia's lack of required physicals was not unusual. Nor is the change. Required physical exams are a growing trend in fire departments nationwide, administered with varying degrees of frequency. New York and Phoenix mandate a physical every year, while Los Angeles has no requirement, though its union will cover the cost.

Camden does not require physicals except for those in its hazardous materials unit.

No hard numbers are available, but among the 3,000 fire departments with paid members, perhaps three-quarters have some sort of requirement for regular checkups, said Jim Brinkley, director of occupational health and safety for the International Association of Fire Fighters, a national union.

A few more join the tide each year, said Ken Willette, responder segment director for the National Fire Protection Association, a nonprofit fire-industry group that developed standards for medical exams such as the ones being given in Philadelphia.

Some volunteer departments also pay for physicals, though that is less common, said John K. Murphy, an attorney and retired deputy chief in North Bend, Wash., who advocates testing.

It used to be that about 100 firefighters died from a duty-related injury each year in the United States, although the annual toll has mostly been lower recently.

The big fear is heart disease.

From 2002 to 2012, heart attacks were responsible for 46.5 percent of firefighter deaths, up from 43 percent during the period 1990 to 2000, according to a 2015 study in the Journal of Burn Care & Research. (The analysis excluded 2001 because of the hundreds of deaths among responders to the 9/11 attacks.)

The mortality rate from trauma remained about the same during the two time periods, at just under 30 percent of the total, while death from asphyxiation decreased from 12.1 percent to 7.9 percent.

That shift may be due in part to advances in breathing apparatus technology, said Deborah L. Feairheller, an assistant professor in health and exercise physiology at Ursinus College. She was not involved with that study, but researches firefighter health and volunteers as a firefighter with several area departments.

Another factor may be that firefighters have become more judicious about entering buildings when no lives are at stake, Willette said.

Philadelphia's physical exams are conducted in trailers on the grounds of the city's firefighter academy. They are administered by Professional Health Services Inc., an occupational health testing firm based in Havertown.

In one trailer, clinicians test cardiac function with a stress test like the one Mesete did on the treadmill. In the other, tests include hearing, vision, and lung function, and also blood work to measure cholesterol and blood cell counts. Mayoral spokeswoman Lauren Hitt said the process eventually will include a screening for hepatitis C, a potentially life-threatening virus that firefighters can be exposed to on the job.

Last week was the second pilot week of testing, and already there have been success stories, said Jack Eltman, who represents the union on a committee overseeing the program.

During the first week, in late July, a firefighter was diagnosed with severe heart disease after failing a cardiac stress test. Within a week he had two stents put in and was back at work, Eltman said.

"We were able to potentially save a life already," Eltman said.

The union also is teaching some of its members to serve as personal trainers, to work with any of their peers upon request.

On the inclined treadmill last week, Mesete marched for seven minutes until his heart rate reached 152 beats per minute - about 85 percent of maximum for a typical 41-year-old male.

No abnormalities. Mesete, who will have been on the force for 10 years as of January, said he tries to eat a balanced diet and stays active with yard work and other off-duty exercise. He welcomed the added scrutiny of medical professionals.

"It's definitely good stuff," he said.

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@TomAvril1

Staff writer Allison Steele and Jillian Condran contributed to this article.