Dr. David Shau is in a plank position on his living room floor, and his toddler is sitting on his back, giggling. This is core training , orthopedic-surgeon style—and it’s one of the milder things Shau, who specializes in hip and knee replacements, does to keep his own body in working order. Ask an orthopedic surgeon—doctors who specialize in the musculoskeletal system, including bones, joints, ligaments, and muscles—what they do to stay healthy, and the answers get strange fast. They do toe yoga at their desks. They hop around like pogo sticks at the bus stop. They brush their teeth in a lunge, stretching the Achilles tendon they know is often first to go. They sit on the floor with their legs crossed and try to stand back up without using their hands. (People who can do this, it’s been shown , have a five-times lower risk of dying in a given time frame than people who can’t.) These aren’t quirks: They’re the byproduct of a job spent watching what happens when people stop maintaining their bodies.…