Most all day activity tracking devices do not measure heart rate. It is a fairly common misconception that heart rate alone can determine calorie burn and activity level. Fitness Heart rate monitors were originally made as training tools for endurance athletes who would want to work within certain zones each workout for their training purposes--they might mix high, low and medium intensity workouts in their weekly plan depending what they are doing. The clever manufacturers somehow got the idea to include a calorie burn estimate which made it desirable for more fitness exercisers and people wanting to lose weight. The calorie burn estimates they give are assuming you are doing aerobic exercise and the reason your heart rate is beating faster is to supply your muscles with oxygen so you can sustain the activity. It is estimating how much oxygen you are using to estimate calorie burn. In a lab setting, the measurement is usually oxygen use--they hook you up to a helmet/mask that is the source for the air you breath in and collects whatever you exhale. I do use a heart rate monitor for exercise calorie burn, but I understand that estimate is pretty specific to aerobic exercise where my heart rate is elevated enough. I wouldn't use it for a calorie burn for normal, daily non-exercise activities and it is a very flawed estimate for a lot of types of exercise too.
The activity trackers usually are tracking movement throughout the day. When fitbit was first developing their product they also did the lab. oxygen consumption tests for walking and running. But their calorie burn estimates are based on applying the movement data (and profile stats) rather than heart rate. I am sure the same or similar is true for the other good activity trackers out there. Bodymedia uses several sensors--temperature, movement, perspiration to come up with calorie burn. Note none are heart rate though I believe some users may also use hrm's for their aerobic exercise. Withings just came out with a device that looks like the Fitbit One but also measures pulse--the issue in my opinion from the specs and reviews is it is a pulse capture similar to the apps people can get that use the camera on a smart phone and would not provide continuous heart rate and looks awkward. You have to take the device out of the clip to use the pulse sensor. It may be nice for people who like to check their pulse throughout the day though. But for non-aerobic activity, I guess I would trust movement based estimates ore than heart rate based estimates. For aerobic exercise I personally trust my HRM more. I am not sure there is any one perfect device, they all have their strengths and weaknesses.
But my Fitbit One (I don't have the bracelet style)... The calorie burn estimate it gives me for step based aerobic exercise usually matches or is very close to the estimate my HRM gives me personally. So I do trust it with a lot of activity--on balance more than my HRM unless it is an activity I feel the fitbit cannot track well. Of course they both may be wrong--as it is all estimates. The activities I get matching or close estimates on include: very brisk walking (slow walking my HRM is lower than fitbit since my heart rate isn't high enough), jogging, cardio kickboxing, aerobic dance with hops or traveling steps, jumping rope and cardio drills like jumping jacks. It doesn't do well with water exercise (well because it is dry in the locker), rowing machines, cycling, some types of dance where the movements are very soft or isolated, any kind of resistance training, etc.
Right now there are so many options in just about every price range so it is probably worth doing a little research if you are interested in such devices. I like fitbit because it seems accurate for me, it can link with a lot of other apps and programs, and I am use to it so understand the information, etc. I haven't really tried the others (other than an old school pedometer which didn't work for me) so hard to say. There are a few that interest me so I follow the reviews and articles about them, though. When I found fitbit three years ago, there were not so many options. I was originally considering body media as it sounded like it would be most accurate and I had seen people wearing the armbands--but on reflection I realized I would personally tire of wearing an armband all the time. Then I found the fitbit and it looked very small and discrete like something I could clip to my bra or waistband and forget. I never saw anyone with a fitbit, but I ordered it and it works well for me. So when you research maybe think about how likely you are to wear that device consistently and how you feel about that.
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