In 1968, Kenneth Cooper, M.D., introduced the revolutionary concept of doing aerobic exercise to maintain well-being in his best-selling book, Aerobics. Cooper famously said, “We don’t stop exercising because we grow old; we grow old because we stop exercising.”

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Fifty years ago, Cooper accurately identified the ‘life-changing’ and ‘life-saving’ benefits of staying physically active across the lifespan. Anecdotally, Cooper is living proof of the benefits of staying active. At age 87, he’s still going strong and doesn't show signs of becoming inactive any time soon, according to a recent report in the Dallas News. Unfortunately, new research shows that during middle age, inactivity creeps up on most of us. During , we are prone to start moving less and sitting more.

For the first time, a decade-long study has identified some nitty-gritty details about how people tend to become more sedentary and less active during middle age. This paper, “Ten-Year Changes in Accelerometer-Based Physical Activity and Sedentary Time During Midlife: The CARDIA Study,” was published August 23 in the American Journal of Epidemiology.

Over the course of 10 years, the researchers found that study participants between the age of 38 and 50 at baseline (2005–2006) decreased light physical activity (such as walking) by an average of about 30 minutes per day and increased sedentary time by roughly 40 minutes per day. These changes in activity levels during midlife were observed equally in both men and women.

According to the researchers, this is the first time fluctuations in the quantity and intensity of physical activity have been examined in a large scale, longitudinal study of a diverse group of participants across a 10-year span of midlife.

For this study, investigators from the University of Texas Health Science Center at Houston used massive amounts of data from the Coronary Artery Risk Development in Young Adults (CARDIA) study which began following the daily activity of 5,115 participants using an accelerometer over three decades ago. Most recently, the researchers followed up 10 years later when participants were ages 48 to 60.

"We know higher intensity physical activity tends to decline with age. But these findings show just how much even gentle forms of activity that are part of daily routines, like casual walking, slip in midlife, which doesn't bode at all well for future health and should serve as a wake-up call to us all," first author Kelley Pettee Gabriel said in a statement.

World Health Organization and CDC guidelines discourage prolonged sedentary time and recommend at least 150-minutes of moderate intensity physical activity per week, 75-minutes of vigorous aerobic exercise weekly, or an equivalent combination of these totals doing some type of moderate-to-vigorous physical activity (MVPA).

"Making relatively small lifestyle changes and taking advantage of missed opportunities to be physically active could have a crucial bearing on how people are setting themselves up for being more during older adulthood," Gabriel concluded. "Simple changes all add up, like walking to the corner coffee shop, parking further away from the store entrance or taking the stairs rather than the escalator."