New Heart Rate Monitor...questions! LOTS of math. Need help!

Options
itsMcKay
itsMcKay Posts: 131 Member
in Fitness and Exercise
So, I have serious math issues and need your help. Thank you in advance for anyone who takes the time to even be bothered reading this. I would REALLY appreciate it! :D

I just got my new Polar FT4 yesterday. I wore it all day, a rest day with no exercise, just normal, mostly-sedentary day-to-day activities, for 12 hours total and took it off to sleep. (I sleep on my stomach and the sensor was killing me.) So, the HRM says I burned a total of 1849 calories just existing for 12:17:47. Let's round that down to 12. my awake average HR was 82 bpm.

My resting heart rate is 71 (sitting up working or playing video games, not true resting) and generally about 68 when I measure before getting out of bed, so let's average that out to 70 for my sleeping HR. I sleep, on average 7 hours.
  • 1849 calories divided by 12 hours is 154 calories burned per waking hour.
  • 70 is 85% of 82. So my sleeping calories are 85% of my waking calories.
  • 85% of 154 is 130.
  • So I burn 130 calories per sleeping hour.
  • 154 = waking = w
  • 130 = sleeping = s

So, the formula for my daily calories burned just existing with no out-of-the-ordinary activity and no exercise is 17w + 7s, right?

Plugging in the values for w and s, I get:
17(154) + 7(130) = 2618 + 910 = 3528

So, I'm burning over 3500 calories per day just being alive, right? Is that right?

So, at a calorie net of 1200, I'm burning 1300 per day. So, I should be losing a bit more than a lb. every three days, which is just over two a week. Correct?

Plus any exercising. I tend to eat back a lot of my exercise calories and shoot for that 1200 net but if I burn 1,000 in a day (common), I may only eat 1700 total instead of 2200. And I'm 100 lbs. overweight. Point being, I'm not dying of starvation over here.

Okay, thank you in advance to anyone who can check my math and my logic. :D

Replies

  • MeanderingMammal
    MeanderingMammal Posts: 7,866 Member
    Options
    Ok. An FT4 isn't designed to be used in that way, essentially you're using your HR as a proxy for calorie expenditure well below the range where there's a meaningful relationship.

    On that basis, none of your subsequent calculations are meaningful.
  • itsMcKay
    itsMcKay Posts: 131 Member
    Options
    MeanderingMammal wrote: »
    Ok. An FT4 isn't designed to be used in that way, essentially you're using your HR as a proxy for calorie expenditure well below the range where there's a meaningful relationship.

    On that basis, none of your subsequent calculations are meaningful.

    Can you tell me where that information comes from or why that is? I'd like to be as accurate as possible so if i'm way off base, it's important for me to understand why. Thanks!
  • itsMcKay
    itsMcKay Posts: 131 Member
    edited January 2015
    Options
    So, I did the Harris Benedict Equation for my BMR. That reads 1777.

    1777 x 1.2 for sedentary = 2132.4

    That's a large difference and seems more reasonable.

    So, round it to 2100 -1200 calorie net = 900 additional calories burned per day. Roughly 3500 calories per pound equals 1 lb. loss every 3.8 days, so just under 2 lbs. per week.
  • 3dogsrunning
    3dogsrunning Posts: 27,167 Member
    Options
    itsMcKay wrote: »
    MeanderingMammal wrote: »
    Ok. An FT4 isn't designed to be used in that way, essentially you're using your HR as a proxy for calorie expenditure well below the range where there's a meaningful relationship.

    On that basis, none of your subsequent calculations are meaningful.

    Can you tell me where that information comes from or why that is? I'd like to be as accurate as possible so if i'm way off base, it's important for me to understand why. Thanks!

    There is no direct relationship between HR and calories burned. There is a relationship between VO2 max that allows for estimation under certain conditions - steady state cardio activity. Outside of that, it is not accurate.

    Here is a good blog that explains it.
    http://www.myfitnesspal.com/blog/Azdak/view/the-real-facts-about-hrms-and-calories-what-you-need-to-know-before-purchasing-an-hrm-or-using-one-21472
  • itsMcKay
    itsMcKay Posts: 131 Member
    Options
    Thanks, 3dogs! :D
  • MeanderingMammal
    MeanderingMammal Posts: 7,866 Member
    Options
    itsMcKay wrote: »
    Can you tell me where that information comes from or why that is? I'd like to be as accurate as possible so if i'm way off base, it's important for me to understand why. Thanks!

    3DR has given you what you need here, but it's also worth appreciating that both the calories in and calories out sides of the releationship are awash with approximations.

    What will give you meaningful information is your own progress. As long as you're consistent in your approach you can tune the figures to reflect your progress against goal loss rate.
  • dearannna
    dearannna Posts: 60 Member
    Options
    I have an FT4 and use it like this: I use MFP to calculate how many hours an hour it thinks I burn during day to day life. (MFP tells me to eat 1664 calories a day set to a 2lb weekly loss. That is a deficit of 1000 calories a day so to maintain MFP thinks I need to eat 2664 kcals a day.) I divide that by 24 to give me an hourly burn (of 111 in this case). I only recalculate this once every month or two rather than after every weigh in.

    I wear my HRM for all workouts (cardio, lifting, circuit classes, whatever) and log the calorie burn shown - 111ph. So if I worked out for 45 mins and my HRM showed a calorie burn of 450 kcals I would do the following: 111/60*45 = 83, then 450-83 = 367.

    So would then log 367 as my calorie burn.

    I know that it's inaccurate for many reasons both in the way I've worked out my base calories to deduct, and the fact that the HRM isnt particularly accurate when lifting or interval training like circuits. However, it's the best I can do with the resources I have, and it works for me in that if I eat back my exercise calories worked out that way I lose about 2lb a week which is what is expected.

    I will say that if I didn't routinely eat back my exercise calories I wouldn't bother working it all out :-)
  • itsMcKay
    itsMcKay Posts: 131 Member
    Options
    Dearannna: Thank you so much! That's precisely how I think I'll be logging using mine, too. That's really good advice. And I'm finding that's how a lot of people interpret the data from their HRMs. Thank you! Very helpful post! :D
  • mrsmammahunter
    mrsmammahunter Posts: 221 Member
    Options
    I usually just skip the math and subtract 200 cals from what my heart rate monitor says i do during workouts. I have lost 11 pounds since mid december so I guess its working.
  • brianpperkins
    brianpperkins Posts: 6,124 Member
    Options
    "skip the math and subtract " ... that's still math.
  • WalkingAlong
    WalkingAlong Posts: 4,926 Member
    Options
    dearanna- You could probably simplify things and just use your per minute BMR (instead of TDEE) for subtracting, and it'd be more accurate of an estimate of your net burn. Your hourly TDEE is 111 but your BMR is much lower and is probably not a whole lot more than 1 calorie per minute. So if you worked out 40 minutes, subtract 40 calories. Though what you do is more conservative. I'm not sure the rationale for subtracting per minute TDEE, I've only seen BMR used to arrive at a net calorie burn estimate.
  • dearannna
    dearannna Posts: 60 Member
    Options
    Hi walkinglalong, the reason I choose to do it the way I have been is that I figure the 111 have already been accounted for by mfp so I'd only want to log the 'extra'. To be honest though because of the potential for inaccuracies and my blatant use of the hrm during exercises it's not intended for, everyone would need to keep an eye and find something that works for them anyway. I like numbers etc so it helps me :smiley:

Categories