Converting Watts to liters of O2 - Is there a simple conversion factor?

Discussion in 'Power Training' started by earthman309, Dec 5, 2005.

  1. earthman309

    earthman309 New Member

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    Is there a straight forward conversion to go from output in watts on the Computrainer to liters of O2 used per minute. I understand that variable efficiency would play a part but I'd love an estimator ie an output of 70 watts requires the expenditure of 1 liter of O2 per minute.

    If the search function was functional I'd have checked that route first.

    Thanks for any informed replies.

    Hugh
     
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  2. robkit

    robkit New Member

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  3. earthman309

    earthman309 New Member

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    Robkit,

    Thanks for the link to your post. If I'm reading it correctly it looks like you are saying 366 watt output requires the use of 5 liters of O2 or 73 watts per liter.
    I'd love to have Andy C. chime in on this one.

    Hugh
     
  4. bikeguy

    bikeguy New Member

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    Hawley and Noakes equation: VO2 max=0.01141 x Wmax +0.435

    ie 400 watts: VO2 max =0.01141 x 400 + 0.435 = 5 liters a minute
    with an efficiency of 80 watts/liter O2. Note: VO2 max is protocol dependent,
    a faster ramp will give higher VO2 maxes.

    However, most cyclists don't have an efficiency of 80 watts/liter O2
    The more highly trained the cyclist, the higher watts/liter will be. Over
    >78 is considered elite (according to Asker Jeukendrup), 68-74 is more normal. According to the Padilla study on 24 world class cyclists, Miguel Indurain's watts/liter was 89 watts/liter (6.4 liters, 573 watts at Wmax), and at Wobla (521 w , 5.84 liters). Indurains maximal oxygen consumption was
    a mere 80 ml/kg/min. In the same study, 23 other world class cyclists were found to have watts/liter efficiencies of close to 90 watts/liter O2 at W Lt, W obla and W max. The ramp for this study was 35 watts every 4 minutes,1 minute recovery (at what wattage I wonder?) const 75 rpm.

    -Bikeguy
     
  5. frenchyge

    frenchyge New Member

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    Is it possible that the constants in that equation were intended to be rider specific? I ask this because Andy recently posted some of his own figures in the same format under another thread and they looked like this:

     
  6. bikeguy

    bikeguy New Member

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    If your efficiency is 80 watts/liter and you use a similar VO2 max ramp as the one used to determine that equation, then it is accurate. Otherwise it is not very accurate at all. VO2 max and efficiency values need to be measured directly and correlated to power output if you want accurate values. The equation is a very rough estimate, and since most cyclists (over 78 W/liter is considered very good) don't have as high an efficiency then changing the constants so that efficiency is 72 watts/liter is probably a better approximation (in general).

    I figure VO2 max= 0.0128 Wmax + 0.435 is a good approximation for a more
    typical rider with a efficiency of 72 W/liter. A bigger + const gives worse efficiencies at lower Wmaxes. I'd figure bigger riders would need a bigger + constant, since their bigger body consumes more O2, regardless of if they are at rest or cycling.

    -Bikeguy
     
  7. acoggan

    acoggan Member

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    The standard equation for converting from power to VO2 given by the American College of Sports Medicine is:

    VO2 (in mL/min) = 2 x power (kpm/min) + 300-400 mL/min

    Since 1 W = 6.12 kpm/min and 1 mL = 0.001 L, the above translates to:

    VO2 (in L/min) = 0.01224 x power (kpm/min) + 0.3-0.4 L/min

    The above equation, however, is for an individual pedaling a classic Monark mechanically braked ergometer, which tends to underestimate actual power output (input) at the pedals due to frictional losses in the drivetrain. Moreover, trained cyclists tend to be somewhat more economical than untrained individuals, although the difference isn't great and in fact this conclusion is controversial. Be that as it may, typically the slope of the relationship between power and VO2 is closer to 0.0105-0.0115 when you test a trained cyclist pedaling a modern electrically braked ergometer.

    Note also that:

    1) unless VO2 is measured directly, any estimates of VO2max from an incremental exercise test are only going to be accurate to w/in +/- ~10%, since not only do you not know the individual's economy, they will go to a power requiring 105-125% (usually 110-120%) of their VO2max before fatiguing (i.e., stricitly speaking a 'MAP' test doesnt really measure maximal aerobic power, but maximal aerobic power plus anaerobic capacity). Moreover, the ultimate power you reach in such a test will be protocol dependent, even though your actual VO2max is not.

    2) use of a constant conversion factor of, say, 80 W/L/min is convenient when doing rough calculations, but the precise value will vary even in a single individual, simply due to the positive y-intercept of the VO2-power relationship. Indeed, this is the major factor explaining why trained cyclists such as Indurain appear to be very economical: when you can sustain a power of >500 W for prolonged periods of time, the impact of your resting metabolic rate (or the cost of unloaded pedaling) on the instantaneous slope of the power-VO2 relationship (which is what that ratio reflects) is far less than in an untrained person who might only be able to sustain 200 W in a steady-state or quasi-steady-state manner.
     
  8. acoggan

    acoggan Member

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    One last comment: at some point or another, the Canadian Cycling Federation circulated a document in which it states that VO2max can be estimated from Computrainer data using a slope of 0.014. This was simply a typographical error, i.e., the exact value should have been 0.0114 instead. So, if you see such a slope in conjunction with Computrainer testing, you now know where it comes from, and that it is incorrect.
     
  9. bikeguy

    bikeguy New Member

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    Coggan, I'd assume the resting oxygen consumption of a 120 kg person would
    be considerably larger than a 70 kg person, so the +const should increase for larger individuals.

    As for the efficiency increase in trained individuals, don't pin it on the y intercept being positive and thus having less effect the higher watts go (partially does reduce it, but not a good explanation). In the Padilla study, the 23 other cyclists on average had a Watts/liter values of 90 even in the aerobic (W Lt zone) which were higher than Indurain, despite their avg Wmax being 478 watts, considerably less than Indurain's 573.

    The Coyle study of LA documents the efficiency increases from 1999 onwards.

    In my own latest VO2 max test, 1 month ago, I showed a remarkably consistent 76 watts/L through 150 w to 400 w, with a slightly higher value at 300 w. Last year, my efficiency was about 2 W/ liter less at higher powers, and 4 watts/L less in the low 150-200 w comparing the entire range of power outputs to this years, i.e comparing 150 to 150 watts, and so on.


    -Bikeguy
     
  10. acoggan

    acoggan Member

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    I agree, it should be (which is why the ACSM formula gives it as a range), but obviously that's not the only factor.

    Again, I agree: cycling economy (efficiency) tends to be greater in those who have been training on a bike for a long period of time versus those who have not. I was just trying let people know that 1) not every exercise scientists believes this is true (cf. the exchange of letters-to-the-editor between Ed and Asker Juekendrup, Dave Martin, etc.), and 2) how you express the data is important when attempting to compare different individuals. In particular, since the ratio W:L/min goes up with increasing intensity (as it must, since there is a positive y intercept), comparing that ratio in two individuals who are exercising at markedly different absolute intensities can lead to spurious conclusions.
     
  11. bikeguy

    bikeguy New Member

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    Mr. Coggan,

    regarding point 1) oh, ok...
    regarding point 2) I can see where you're coming from, at very low power
    outputs the fixed amount of basal O2 consumption will lower
    the W/L min value.

    -Bikeguy
     
  12. earthman309

    earthman309 New Member

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    "Hawley and Noakes equation: VO2 max=0.01141 x Wmax +0.435"

    Based on this, my 52 kg wife who just did a 27 minute computrainer time trial averaging 191 watts would have a VO2 max of over 50ml/kg., as she can certainly output a higher wattage in a step test than 27 minutes continous.

    0.01141 * 191 + 0.435 = 2.61L 2610ml/52 = 50.27l/kg for 27 minutes

    "so that efficiency is 72 watts/liter is probably a better approximation (in general)."

    or alternatively 191watts/72w/l = 2.6527777l 2653/52=51ml/kg for 27 min

    My reason for inquire realy relates to our impression that my wife may have a significantly greater potential run capacity than has been demonstrated at this time. Her running race performances translate to a Daniels V dot of ~40.

    Thanks so much for the wonderful replies,

    Hugh
     
  13. acoggan

    acoggan Member

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    Of course, the other explanation is simply that your Computrainer is miscalibrated - how old is it?
     
  14. earthman309

    earthman309 New Member

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    Andy,

    Its about a year and a half old. We used an ancient Nintendo version from the "dawn of Computrainer" for many years but upgraded to the new improved system to run on a PC. I know that Computrainer sells a "lab version" now with a load device that has undergone a more rigorous calibration routine. (Same unit more data points).

    We have VO2 max test available locally for a rather reasonable cost. That would be a quick way to give a bit of additional data.

    Thanks so much for the input.

    Hugh
     
  15. rmur17

    rmur17 New Member

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    I'm familiar with that as it told me my Vo2max was ~85 ml/kg/min! when I did my first CCA step test on my CT back in fall 2002.

    I thought "that's strange. How I'm not on USPS? ;)"

    I reported the error sometime last year when it was discussed on Wattage (to Dr. Thibault).

    rmur
     
  16. Exercise101

    Exercise101 New Member

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    OK, I know this thread is from 2005 and I found it via google. Nevertheless, it's an important topic and I thought I'd bump it to ask these questions:

    The formula that I found takes into consideration "Hawley and Noakes equation: VO2 max=0.01141 x Wmax +0.435" plus the weight in Kg


    VO2max = (0.01141 x Wpeak + 0.435)*1000/weight

    It's found here on this site in an user friendly calculator: milesj.com/cycling.vo2estimator.html

    Also, for any runners another calculator by the same site is found here: V O 2 M a x
     
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