Gus, my 4-year-old son, was chasing me down the stairs on our way to the basement for our early morning workout. This particular morning, Gus jumped from the stairs onto the floor and took an extra step. Something seemed off, but he didn’t complain after he landed, so we moved on with our day. That afternoon, Gus complained about pain in his leg when his mom, Heidi, picked him up from preschool. But by that evening, he was running around again. The following morning, he was limping, though not...