“The more we study physical activity, the more we realize that it doesn’t really matter what you do,” Lieberman says. “You don’t have to do incredible strength training … to get some benefits of physical activity. There’s all different kinds of physical activity, and it’s all good in different ways.”

*On how chairs with backs have contributed to our back pain.

*On the idea that running is bad for your knees.

*On becoming frail with age