Activity also triggers a cascade of effects that make us stronger. “Exercise is a type of stress,” Lieberman told me. It slightly tears the muscles and strains the blood vessels and organs. In response, an extensive body of scientific study on exercise shows, our body activates a series of cellular mechanisms that repair tears and strains and, in most cases, rebuild the affected parts. “It’s like when you spill coffee on the floor, you wipe it up and the floor ends up cleaner than it was,” Lieberman said. This internal overreaction is likely to be important when we are older, he continued. Without exercise and the repairs that go with it, aging human bodies function less well. We wear out. We can’t take care of the grandchildren.
