Both the ancient Persian Empire and the ancient Roman Empire fell into demise, partly because the physical fitness of their peoples fell into demise. The demise of physical fitness in human cultures, therefore, is NOT a new phenomenon.
Physical Fitness DEvolution
The True Picture
Anyone caught up in the physical fitness movement of 20 or 30 years ago might be surprised to learn that human cultures have always gone through cycles of fitness and fatness. Even more surprising is the fact that the majority of people in highly developed countries today are no more physically fit now than they were two or three decades ago. Instead, people in developed countries today seem to be stuck in the fatness phase of the most recent fitness/fatness cycle.