Correspondence to Dr W Müller, Human Performance Research Graz, Max-Mell-Allee 11, 8010 Graz, Austria; wolfram.mueller{at}uni-graz.at If you wish to reuse any or all ...

Thanks to the ever-improving world of fitness trackers and apps, you've never had more access to personal health data. But which of the dizzying number of markers and metrics should you really be ...