Hand grip strength. Balance tests. Standing up from a seated position on the floor. These physical challenges are often touted as measurements of healthy aging and even predictors of longevity. They can provide valuable indications of your stamina and strength. They can estimate the risk of falls, which increase older adults ' risk of death. And they're useful to doctors who work to address aging or people with injuries. But they're no crystal ball. “A poor or declining result is a reason a...