About 10 years after he first started studying the causes of heart disease, Jerry Morris started to jog. “I was the first person to run on Hampstead Heath, in the 1960s," he'd later tell The Financial Times:

Every Sunday morning, if the weather was at all possible, I took off my coat, and my little boy carried my coat, I took off my jacket and my little girl carried my jacket, and I ran for 20 minutes. People thought I was bananas.”

Morris had discovered something, though, that those skeptics weren't yet aware of: People who exercised—routinely, vigorously—were less likely to have heart attacks than people who spent their days stationary.

From the beginning of his medical training, Morris had been interested in health inequities and their causes. In 1949, back in England after a stint in India as a lieutenant colonel, he turned his attention to coronary heart disease. The number of people whose hearts were giving out was rising like never before and no one knew why.

Morris had a hunch that it might have do to with the way people worked. Heart disease affected the middle-aged—more men than women—and statistics the government had collected hinted that occupation could play a role.

He started collecting data on 31,000 men, 35 to 64, who resembled each other in class and lifestyle, but differed in one key way: Though they all worked in the public transportation system, running buses, one group—the drivers—spent most of their days sitting. The other group—the conductors—spent their time trotting up and down stairs—500 to 750, every single day. (Morris' team had sat on the buses and counted.)

When he compared the two groups, the difference was striking. Conductors had fewer heart attacks, later on in their lives, and the attacks were less likely to be fatal. Morris would later look at the bus workers' waistbands—Transport for London provided the data from records of pants provided to its workers—and found that, even though drivers were plumper 'round the middle, that correlated less strongly with their heart health than how much they moved.

The same pattern showed up in a different group of workers, too: Sedentary government clerks were more likely to have heart attacks than mobile postal workers.

"Can the hearts of men be seen to vary with the kind of work they have done?" Morris wrote in 1958. The answer, he had found, was "Yes." It was one of the first times that any doctor had shown that physical activity might be connected with health.

He looked, too, at the movements that 18,000 men made outside the jobs that kept them sitting down. Here, too, there was a striking trend: Those who did some reasonably serious exercise—biking, swimming, playing soccer—ended up with healthier hearts than men who spent their time puttering about.

So, Morris started to jog, and to tell the rest of the increasingly desk-bound world to try it out. In the years before he died, just a few months shy of turning 100, he spent 30 minutes of almost every day swimming or jogging. By then, he had plenty of company.