watts to calories

Discussion in 'Power Training' started by JCF26, Jan 15, 2013.

  1. JCF26

    JCF26 New Member

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    I am a female 5'3", 100 pounds. I am not muscular but have endurance. When I use my indoor bike trainer, be it for 30 minutes or two hours, I can never generate average power higher than 130-140 watts. I have seen that exercise machines in gyms use body weight to determine calories burned. A smaller body, like a smaller engine, uses less energy, and generates less power. Am I right? The threads I have read have guys maintaining average output of 190-200 watts. I just don't think that would ever be possible for me. Yet I see charts on power output that categorize 130 watts average as light work, I am sweating bullets and working my ass off. How is this light? I'm not new to endurance sports, just not a competitor. Can someone explain the differences between a large and a small person and their ability to generate power and how that translates into calories?
     
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  2. daveryanwyoming

    daveryanwyoming New Member

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    Yes, smaller folks generate and sustain less power than larger equally fit folks and in general even when weight scaled women sustain lower power than similarly fit men.

    For instance world class male professional riders can sustain up to ~6.4 w/kg for roughly an hour where the top world class women sustain roughly 5.7 w/kg for similar durations. In both cases that's an awful lot of power. But down in the amateur levels it's similar with a lot of fairly well trained but recreational men sustaining perhaps 3 w/kg for an hour and women a bit less around 2.7 w/kg. Those numbers are taken off of the power profiling charts you can find explanations for here: http://home.trainingpeaks.com/articles/cycling/power-profiling.aspx

    The chart isn't hard and fast but it gives you a general idea of the range of power both men and women sustain from untrained individuals up to world class.

    In terms of watts to dietary Calories, take it in steps:

    - The kilojoules of energy you burn when averaging a certain power for an hour equals average watts for that hour multiplied times 3.6 (as joules = watts*seconds and there's 3600 seconds per hour or 3.6 kiloseconds per hour). This part is straight physics definitions of watts and joules of energy and not subject to any personal differences between riders male or female.

    - Technically there's 4.184 kj per dietary Calorie (or 4.184 joules per physics and engineering calorie but dietary Calories are 1000 physics calories, hence the capital 'C' in the dietary version). But that pretty much washes out because humans aren't that efficient in turning calories into productive work. A rider's individual Gross Metabolic Efficiency (GME) dictates efficiency in turning fuel combustion into useable work at the muscles of interest. GME ranges from around 19% to perhaps 27% across the human population and even pro level athletes see similar distributions. It takes a lab test to pin it down precisely but taking a swag that GME ~ 23.9% makes that conversion from kj to Calories wash out. IOW:

    Calories burned cycling ~= average watts * hours * 3.6 * 4.184 * 23.9% = average watts*hours*3.6

    So if you ride for an hour at 100 watts average power you'd burn ~ 360 Calories

    Sustain that 100 watts for 2 hours and you'll burn roughly 720 Calories

    Bump the intensity up to 150 watts and the burn rate climbs to 540 Calories per hour.

    So yes, larger folks tend to burn Calories at a faster rate as they tend to sustain more power. Fitter riders also tend to burn at a faster rate as they sustain higher power, ride for longer or both.

    Hope that makes sense,
    -Dave
     
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  3. vspa

    vspa New Member

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    wow you are really savvy on power, amazing explanation !
     
  4. maydog

    maydog Well-Known Member

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    Also conside how those calories are being measured. If you are using a heart rate monitor, the calorie count is guess based on a model of HR to caloric consumption. If you are well trained and have a lower heart rate than the nominal person at a given power output then the estimate will be low. In the end, its just a guess.

    If you are getting the calorie count from the machine, it may be more accurate. In this case, you have to rely on the manufacturer to get the calibrations and calculations correct, many aren't. The treadmills I have used are especially suspect - I think they way overestimate power output. To the typical gym patron, power output is not well understood or used, it doesn't sell equipment so there is not a lot of effort put into validating it.
     
  5. bgoetz

    bgoetz Member

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    I have always debated this, so your PM gives you the amount of energy utilized to power your bike in terms of watts and calories. However, it does not really tell you how many actual calories you are utilizing total, as there is a lot of other things going on that don't translate into power to the pedals. The biggest that comes to mind for me is calories utilized to repair and build muscle after the workout is over. Given this the calories burned to put power to the pedals is only a portion of the calories actually utilized from the activity. Has anyone actually came up with an estimate of percentage of actual calories utilized from riding a bike that the power to the pedal figure represents? Obviously there are lots of variables that would change this from person to person, workout to workout, but I am just interested in a real rough figure.
     
  6. daveryanwyoming

    daveryanwyoming New Member

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    The method described above is very accurate in terms of calories burned DURING exercise. It does not account for BMR, RMR or changes to those as the result of exercise or other things that might happen off of the bike. There are estimates for those things based on body mass, gender, and average daily activity level with or without exercise but in the end all of this is an exercise in estimation as is caloric intake even if you weigh each and every thing you eat and have accurate knowledge of every recipe which in general we don't.

    So yeah, there's plenty of room for error both on the caloric output and caloric intake part but when you compare typical RMR ranges to the kind of known work that happens during endurance exercise the latter often swamps out the former. IOW, for a mid size adult male athlete with a RMR on the order of 2000 Calories per day that goes out and trains for four hours at an AP of 250 watts you're comparing a known 3600 Calories burned during exercise to the 2000 Calories burned to support daily activities at normal exercise levels. Sure the latter is an estimation and will vary day to day based on a lot of things including any 'afterburn' from the exercise above 'normal daily levels' already estimated into RMR but still the former is a lot larger than the latter and some day to day estimation errors in the latter won't really change things much.

    The actual work done during exercise is one of the more precise estimations in a series of less precise estimations so sure if you want to 'credit' yourself additional burned calories for any afterburn effects and that works for you in terms of weight management then do what works. And in the big picture if you track your normal workout loads and your normal dietary intake and over time your weight isn't responding as expected then adjust RMR and intake goals as needed.

    -Dave
     
  7. tomw1974

    tomw1974 Member

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    I thought I read that the repair and refuel number was up to 10% of what was burned during the workout, but I cannot find my source for that for the life of me.
     
  8. bgoetz

    bgoetz Member

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    Good info Dave! I have basically taken the approach of trial and error and watching weight to see where I end up. I was just curious how closely this has been analyzed. For me the days where I do a hard AWC workout for say an hour and come up with 600-700 calories burned are the ones that make me wonder what is happening afterwards. I use My Fitness Pal to come up with my RMR and a target for weekly weight loss. With that they give you a goal for net calories per day. I then input my calories consumed and burned from my power meter. I am above somedays, below others, but tend to fall pretty close to the weekly target and my weight loss tracks pretty close to what they predict. So in that instance there is no accounting for and extra calories burned and it comes out to where I want.
     
  9. JCF26

    JCF26 New Member

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    Dave;

    It all makes sense. I am glad you bring me back to physics. It is the only way to get it. Reading digital displays doesn't cut it. Thanks for the awesome answer.

    JCF26
     
  10. huskyrider

    huskyrider New Member

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    Daveryanwyoming: where does the 3.6 come from in your conversion? Calories burned cycling ~= average watts * hours * 3.6 * 4.184 * 23.9% = average watts*hours*3.6 It sounds right based on what my Jym's bike meter is doing that is why I am searching for the conversion. In reality if you covert calories burned in 1 hour to watts (based on engineering conversion tables) the conversion is 720 kcal/hr = 837 watts that just does not make sense I do 720 kcal all the time but there is no way I am averaging 837 watts.
     
  11. JibberJim

    JibberJim Member

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    But you're not 100% efficient as an engine, you're only ~20% efficient.
     
  12. daveryanwyoming

    daveryanwyoming New Member

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    the 3.6 comes when converting watts to kilojoules per hour as there are 3.6 kiloseconds per hour. Or you an break it all the way down as I thought I did above:

    AP watts * 3600 seconds/hour = j/hour

    j/hour / 1000 = kj/hour

    so AP watts * 3.6 = kj/hour

    kj/hour * 4.184 = kilocalories/hour = dietary Calories/hour That's at the combustion level not the utilization level that we measure with a power meter as JibberJim indicates above

    dietary Calories/hour * Gross Metabolic Efficiency (~ 19-28% on a normal distribution across the human population) = Calories burned per hour from power measured at the pedals

    Taking GME `+ 23.9% makes the 4.184 and GME cancel out so:

    Calories = 3.6*AP*hours with some error depending on the individual rider's GME which requires testing with a gas exchange mask to pin down via the process of indirect calorimetry.

    -Dave
     
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  13. huskyrider

    huskyrider New Member

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    AHHHH its all clear now. Thanks folks.
     
  14. gudujarlson

    gudujarlson New Member

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    Not to mention that just because you put 100 calories of food into your mouth does not mean that your body absorbs it all. The substantial error and general hassle of measuring calories burned and calories eaten plus the fact that exercise leads to a basically linear increase in appetite (in my personal experience) has lead to be to believe it's better to rely on appetite to control the amount I eat and use my rational brain to control what I eat. Towards that end I try to eat large amounts of low calorie density food in each meal, e.g. vegetables, excepting post-workout meals.
     
  15. Tonka

    Tonka New Member

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    Just to clarify, the 'cancel out formula' works because the GME * 4.184 is approximately equal to 1. If your GME goes up or down the formula is not as easily condensible. Suppose GME goes from 23.9% to 34%. The GME does not cancel out and is now 1.42 (4.184*0.34) instead of 1. Calories burned now = 3.6*AP*hours*1.42. Using the correct GME results in a 42% difference vs cancellation and might produce some confusion. Just make sure your GME is close to equaling 23.9 if you want to use the cancel out method. Great information, kudos 2 Dave!
     
  16. Rxdml

    Rxdml New Member

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    Hello everyone.
    I don't have any clue about cycling, I just found this post while googling for information about mechanical power of cyclists and the calories needed for effort.
    I just wanted to say that this formula is wrong :

    Calories burned cycling ~= average watts * hours * 3.6 * 4.184 * GME
    It should be

    Calories burned cycling ~= average watts * hours * 3.6 / (4.184 * GME)
    Both formula give the same result for GME = 23.9%, because, as daveryanwyoming noted, 4.18*23.9% is almost equal to 1.
    The first formula doesn't work for any other GME though.
    A less efficient cyclist (say with GME = 10%) would need to eat much more in order to deliver 100W of mechanical power during an hour.
    He would need :

    100 * 1 * 3.6 /(4.184 * 0.10) ~= 860 Calories
    A much more efficient cyclist (GME = 27%) would deliver 270W of power during an hour with those same 860 Calories, or would only need

    100 * 1 * 3.6 /(4.184 * 0.27) ~= 319 Calories
    to deliver 100W during an hour.

    I hope it makes sense.

    PS: To check the unit conversion : http://www.wolframalpha.com/input/?i=100W%2F%2823.9%25%29+to+Cal%2Fh
     
  17. swampy1970

    swampy1970 Well-Known Member

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  18. sbatz72

    sbatz72 New Member

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    Wow, this was an excellent post. I am an electronics engineering student and have yet to have an instructor explain power quite like this. I think I finally understand power based upon your explanation. I am glad I read this thread. I have always heard there are instructors who teach different and of course each student learns different.
     
  19. stephen123

    stephen123 New Member

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    Would you please share some evidence based facts that validate you point of view or your personal experience because i don't agree with you on some points!
     
  20. dkrenik

    dkrenik Member

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    From a 3 1/2 year old post?
     
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