I've been trying to compare the energy expenditure (kilojoules) as calculated by my Powertap with the energy expenditure (kilo calories) that was calculated by my Polar RS800CX heart rate monitor. I wanted to see how comparable the energy expenditure calculation was between the two devices. In theory you can simply say that 1 kilojoule measured by the power meter is equal to 1 kilo calorie because of efficiency losses (instead of converting the kilojoules into calories).
The kilo calories burnt calculation of a heart rate monitor is highly reliant on the initial user values that you enter into the heart rate monitor (age, weight, exercise level, Max HR, VO2 Max etc.) The energy expenditure is then estimated using a formula based on these values. As a result the kilo calories burnt figure calculated using a heart rate monitor can be wildly inaccurate - often over estimating the numbers of calories burnt.
Have a read of this web page and try out the calculator to see how the estimated kilo calorie usage numbers change when you use different VO2 Max and age values (age in this formula adjusts your maximum heart rate). If you use an estimated maximum heart rate and estimated VO2 Max for the settings (as opposed to knowing your actual maximum heart rate and having a lab tested VO2 Max) then it introduces errors into how the heart rate monitor calculates energy expenditure.
Calorie Expenditure Calculator:
http://www.braydenwm.com/calburn.htm
Whilst I was looking at various articles the interview discussing energy expenditure below was linked. I've posted it here because it seems relevant when considering what a Powertap can be useful for.
Full Discussion:
http://www.bikeforums.net/showthread.php/179758-calories-burned-seem-low
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Energy In, Energy Out: What's the Magic Equation? (Part I)
A few weeks ago I was going through a thread on slowtwitch.com discussing caloric expenditure on the bike and the accuracy of the kcal numbers on our HRM's. I always assumed that the variables in caloric expenditure could never be accurately determined by heart rate alone; temperature, hydration, force, etc all have a direct effect on energy needs, right? In order to get a better understanding on our energy needs I contacted Richard Stern*, cycling and multisport coach of cyclecoach.com, Gordo Byrn of coachgordo.com, and my coach, Lauren Maule.
I'll begin this series with Richard. Reading his in depth explanations on the slowtwitch thread - "Fat loss question" are what set me on this quest. I began my inquiry by asking Ric simply, why is the Polar computer inaccurate and by how much?
Stern: Energy Expenditure (EE) is related to power output, as EE = power x time. This is actually the mechanical energy used to drive the bike but is actually close enough to actual EE. The mechanical energy is actually in kj (kilojules), but can be converted to kcal (kilo calories) -- the more usual metric for nutrition.
Supposing, that a cyclist averaged 200 watts (W) for 3-hours, the mechanical energy (ME) would be: ME = power (/1000) x time (secs) = (200/1000) x (3 x 3600) = 0.2 x 10800 = 2160 kj
However, 1 kcal is equal to 4.18 kj, thus, 2160 kj is equal to 517 kcal. Obviously, this would be a very low EE for a 3-hr ride. Interestingly, efficiency during cycling is ~ 20 - 25% in trained riders, so these two figures approximately cancel, and we can then say that 2160 kj ride is approximately a 2160 kcal ride.
Efficiency (thermodynamic efficiency), is a measure of the actual mechanical work accomplished divided by the input of energy. This is mainly affected by cadence in cycling. Paradoxically, efficiency is *highest* at lower cadences, and lower at higher cadences. This is because energy is required to just move the legs (with example, no chain on the bike) and higher cadences require more energy. However, as power output increases, efficiency also increases. Therefore, topographical conditions have an effect on efficiency as you tend to pedal slower uphill and faster on the flat.
The biggest retarding force in cycling is air drag (hence the reason why we all try to get as aero as possible). If for example, you have two bikes a TT aero one, and a standard road bike, at the same speed under the same environmental and topographical conditions you'll need to produce less power on the TT bike than the road bike.
Heart rate can vary for a multitude of reasons, e.g., temperature, topographical conditions, humidity, fatigue, stress, caffeine, etc. Heart rate can vary quite dramatically even at a constant power output. As an example, when I complete 20-min TT intervals at the same power (ridden indoors on a trainer) my HR can vary by ~ 15 to 20 b/min depending on the session the day before, what I've done just before, fatigue, etc. Heart rate therefore becomes 'unreliable'.
Therefore, knowing power is the only realistic way of calculating energy expenditure, as trying to estimate it EE from e.g., HR is just too variable. Thus a HR monitor won't give an accurate or useable EE for cycling as there are too many variables not accounted for.
KP: Without the benefit of having a power meter, how can one assess their caloric expenditure while cycling (assuming they use an HRM)?
Stern: Basically, unless you want to do a reasonable amount of analysis it's pretty much impossible to estimate EE without a power meter, and get a meaningful answer.
KP: I read on Gordo's site that Molina is of the opinion that the average well trained athlete burns 100 kcal per mile and an elite runner could burn as little as 50 per mile. Do you agree? Can you speculate on how this translates with swimming?
Stern: It's generally easier to calculate EE in running because speed varies less than in cycling and position isn't as important. However, running economy does vary between individuals.
KP: What do you suggest for your cycling and multisport athletes that you coach who lack power meters?
Stern: I don't suggest that they attempt to track EE. It would be very unreliable data.
KP: Do the foods consumed and their micronutrient content affect the balance of fat used to other fuel sources? How can one maximize fat usage as opposed to glycogen and protein?
Stern: Other than by training and getting fitter (i.e., increased VO2 max, increased LT) there is little or nothing that can be done to affect substrate usage. As we get fitter, we use more fat at a given intensity compared to glycogen. As the duration increases there can be a shift to greater fat usage. As relative intensity increases there's a greater use of muscle and liver glycogen (i.e., for an Olympic/sprint you'll be exercising at a higher intensity than an Ironman distance event, and using more carbohydrates).
During exercise, it's important to keep taking in carbohydrates and fluids. There's unequivocal evidence to support the use of carbohydrate during events longer than one hour (and some evidence for less than one hour). Taking in 0.7 g carbohydrate / kg body mass has been shown to unequivocally extend performance (e.g., 30 to 60 g / hr).
During exercise fluid intake is crucial to prevent dehydration: this can be achieved with 150 - 350 mL (milliliters) of a carbohydrate-electrolyte solution every 15 - 20-mins, starting at the beginning of exercise. The carbohydrate-electrolyte solution should be 4 to 8% and contain sodium. The electrolytes stimulate the thirst mechanism and can help prevent hyponatraemia in susceptible people.
KP: Can one determine optimal power to weight ratio without a power meter? Some try so vehemently to lose weight they end up costing themselves power in the process. Can one determine their optimal power to weight ratio by doing say, a specified hill or TT course over a period of weeks and comparing weight, time's, effort level, and HR?
Stern: Within the confines of losing typically small amounts of mass (e.g., ~1 or 2 kg) it will make little or no difference to performance. Unless you have a large amount of fat to loose, then it's virtually always better and easier to gain power compared to losing weight (as a primary goal). In fact, with a lot of the riders that I coach, whilst training intensely for increases in e.g., LT power output or power at VO2 max, they end up losing some fat due to the increased training energy expenditure. During flat ground cycling there's little to be gained by loosing small amounts of weight and even on hills it will only add a very small amount of time. Power to mass ratio is of more importance when e.g., cycling uphill.
More to follow in the next installment.
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Richard Stern is a cycling/multisport coach in UK and retains the copyright on the material used in this interview. For more information, visit
www.cyclecoach.com or click on the banner below.
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