Validity of the Polar H7 Heart Rate Sensor for Heart Rate Variability Analysis during Exercise in Different Age, Body Composition and Fitness Level Groups. - Adrián Hernández-Vicente; David Hernando; Jorge Marín-Puyalto; Germán Vicente-Rodríguez; Nuria Garatachea; Esther Pueyo; Raquel Bailón (Sensors, 2021)