Somewhere around 400 BC, Hippocrates, long-considered the father of Western medicine, declared that “walking is man’s best medicine.” But it wasn’t until the 1950s that doctors and researchers began to collect data suggesting he was right. It started with London’s iconic double-decker buses.

One of the first, and oft-cited, studies to demonstrate the impact of regular physical activity on heart disease, health, and longevity was conducted in London in the early 1950s. Researchers sought to “uncover social factors” that might be contributing to a notable rise in heart disease. To do so, they evaluated the incidence of heart disease between two groups of approximately 31,000 men between the ages of 35 and 64 who were employed by London's transportation system.

Group one included the bus drivers, whose work was largely sedentary. Group two included the bus conductors, whose work involved walking around the bus collecting tickets, including climbing the stairs to the buses' upper level. In most other respects, the two groups were similar, sharing similar socio-economic status, work hours, and ages.

The researchers considered the incidence of heart-disease as well as the outcomes: how likely was an individual to suffer a heart attack and how likely was that heart attack to be fatal? The results were clear: the conductors had considerably less heart-disease than the drivers. Moreover, when conductors did develop heart-disease, they were likely to do so later in life and the severity of the disease was likely to be less than for the drivers. All in all, the drivers were twice as likely to die from a heart attack than the conductors.

The key difference? The drivers sat for most of their work days while the conductors walked and climbed the stairs of their double-decker buses.

The researchers went on to hypothesize possible explanations for the difference. Perhaps men who chose to become conductors were physically stronger than those who chose to become bus drivers. Perhaps being a bus driver was more stressful than being a bus conductor. Ultimately, the researchers rejected these other causes and concluded that the most likely explanation was that “the physical effort in the conductors’ work may be a protective factor, safeguarding them in middle age from some of the worst manifestations of heart-disease suffered by less active workers.”

Yup, more walking, less heart disease.