The secret to longevity lies in the level of physical activity at age 40, a recent study reveals, showing that increased activity at this stage can extend lifespan.

Americans who increase their physical activity to match the top 25 percent of the population could add five years to their lifespan, the study revealed. Also, the least active individuals might gain nearly 11 years by reaching the activity levels of the most active, according to the study published in the British Journal of Sports Medicine.

To understand the impact of different levels of increased physical activity on life expectancy, the researchers used a predictive model based on data from the activity tracker from the 2003–06 National Health and Nutritional Examination Survey. Participants were aged 40 and older.

The study found that the most active 25 percent of Americans in this age group engaged in physical activity equivalent to 160 minutes of walking at a normal pace of 4.8 km (3 miles) per hour daily. The predictive model showed that if all Americans over 40 matched this daily level of physical activity, their average lifespan would increase by over five years. This means that their life expectancy would increase from 78.6 years to around 84 years.

Meanwhile, when the least active 25 percent of the population matched the activity levels of the most active 25 percent, they could gain nearly 11 additional years of life. However, this would mean they need to add 111 minutes of daily walking at 4.8 km/hour, or an equivalent effort.

"Our findings suggest that physical activity is associated with substantial gains in life expectancy for individual Americans and for the population. Moving the least active 25% of the population over age 40 to become as active as the top 25% could result in an average life expectancy gain of about 11 years for this group. The greatest gain in lifetime per hour of walking was seen for individuals in the lowest activity quartile where an hour's walk could add an impressive 6 hours to life," the researchers wrote.

While the greatest benefits were notable when there was an increase in physical activity to the least active group, on average, every extra hour of walking adds around 3 hours (169 minutes) to lifespan.

Since the study is observational and based on a predictive model, there are a few limitations. The researchers caution that there is the possibility that they might have "underestimated or overestimated the benefits of physical activity".

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