I have noticed a problem with HR in calm condition. My resting heart rate RHR is 37-39bpm (49 male athlete and without heart issues). When Iam in a calm condition sitting or laying on the couch reading or watching tv, the HR is 45-55bpm. If I make a flexion or move my hands towards a thing, I notice spikes in HR even +20 to +30bpm into 2-3secs, which turn to the normal after 30-90secs. This occurs also when I take some steps from my initial calm position. I have also noticed the same inaccuracy when I have my resting period between weight lifting training sets. If I take some calm steps, the HR is increasing almost +1bpm per step. I guess this is a fault of the algorythm, because after some seconds standing still and not moving my arms, the false HR is diving and the true HR is restored (during the excercise sets the HR is accurate). I use the watch according to the instructions and the comparison is made with medical oxymeter or manual detection.