About a third of people across the world do not meet the minimum activity recommendations (which is generally about 2.5 hours of moderate activity per week). But countries vary greatly, with Malta topping the inactive list at 71.9%, and Greece faring best, at 15.6%. The situation is worse for adolescents (ages 13-15) across the globe, of whom about 80% fall into the inactive category.

Unsurprisingly, in the higher-income countries, jobs are providing less of a person's overall activity than they did decades ago. Calorie expenditure from work-related activity has fallen by about 100 calories per day, which would translate roughly to a weight gain of about 7.5 pounds/year. Luckily, the amount of activity gotten from one's leisure time has increased in recent years, filling this gap somewhat. But we're still in the red when it comes to energy intake vs. output.

At heart, the issue is not an aesthetic one. Experts estimate that 5.3 million deaths worldwide are the result of inactivity, which is about the same number attributed to tobacco use, making the situation seem even more grave. The World Health Organization (WHO) says that physical inactivity is the fourth leading cause of deaths due to non-communicable diseases worldwide.

The researchers point out that the publication of the studies at the time of the summer Olympics is "not a coincidence. Although the world will be watching elite athletes from many countries compete in sporting events requiring tremendous training, skill, and fitness, most spectators will be quite inactive." They hope that the timing of the series, along with Olympic excitement, will propel people into action. Literally.

And there's another element of irony to our growing inactivity that shouldn't be overlooked. Advances designed to make our lives easier and more enjoyable have made us less mobile. Not only do they contribute to the inactivity pandemic, but they may have the grimmer effect of shortening our lives.

As the authors put it, "Several behavioural and environmental factors, and megatrends (major forces in societal development that affect people's lives) affect population levels of physical activity. Rapid urbanisation, mechanisation, and increased use of motorised transport could have caused global changes in physical activity."

Getting back to basics, like walking or bicycling to work, would help the situation markedly, say the researchers. They calculate, for example, that if all of Denmark's non-cycling population suddenly hopped on bicycles regularly, about 12,000 deaths per year would be avoided. Of course, there's an important intersection between outdoor activity and environmental concerns, particularly cyclist safety and park safety, which can significantly affect our odds of engaging in healthy activities.

If people are put off the by idea of thought of the recommended 150 minutes of moderate activity per day (like brisk walking), at least we can take solace in the fact that even less activity offers a benefit: Getting just 1.5 hours per week can extend lifespan by three years.

Tracking physical activity is notoriously difficult, and because of the range of activity level within each country can be wide, getting a handle on what's going on even within a given country can be difficult.

Though the studies seem expansive, for about a third of countries in the world, there exists no data on physical activity, mainly countries in central Asia and "those of low and middle income in Africa." Striking, the authors say, is the divide between where the studies are done and where disease occurs. Closing the gap in which there exists no data is critical for getting a handle on people's habits, lifestyles, and activity levels, or lack thereof.