The scientists who wrote the paper in PLOS ONE look really serious: https://www.researchgate.net/publication/333333571_Validation_of_Polar_OH1_optical_heart_rate_sensor_for_moderate_and_high_intensity_physical_activities But is the treadmill protocol they have chosen really serious? The protocol of the papers is listed as paper [90.8.6] in the following list of the protocols choose in the papers: We wonder why they have chosen zero inclination on the treadmill, …