What Mattered Most and Least for LongevityCardio fitness tends to be the biggest predictor of mortality, as the study results suggest, Powers says. These fitness tests “look at things from a different standpoint,” Power says. Because the study was observational, it can’t prove that people with lower scores on fitness tests had shorter lifespans; it simply found a link. The participants’ physical fitness was measured at a single point in time, making it hard to know whether it changed over time. How to Improve Your Fitness With These MeasuresPowers doesn’t suggest trying to improve your fitness solely to excel at these tests.