Polar s720i Pedaling Index

Discussion in 'Power Training' started by chuy, Aug 2, 2003.

  1. chuy

    chuy New Member

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    Just out of curiousity how accurate is this? I seem to have a max PI of around 90% however my average is around 20-30% ? Does this mean it's time to do One-Legged Pedaling Drills?
     
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  2. steve

    steve Administrator
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    What is pedaling index?
     
  3. chuy

    chuy New Member

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    The s720i manual says the following: PI %, Pedalling Index*: Helps you to analyze how evenly the power is distributed. The smoother the pedalling motion, the closer the figure is to the ideal 100%. A pedalling index of 100% means that power is applied evenly throughout the whole pedal revolution.
     
  4. ric_stern/RST

    ric_stern/RST New Member

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    Distribution of pedal forces can only be measured with force instrumented pedals, which the polar (or any current power meter cannot do). In studies comparing better cyclists to less good ones, the better cyclists (elite) showed that higher power outputs (compared 40 km TT performance) were acheived by pressing down harder on the pedals and generated less force on the recovery phase of the pedal stroke.

    Forget PI and left/right distributions.

    Ric
     
  5. HandyAndy

    HandyAndy New Member

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    Ric does the computrainer not have that spinscan feature that measures individual power per leg?Ric is the computrainer a very useful training tool in your experience?
     
  6. ric_stern/RST

    ric_stern/RST New Member

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    The Computrainer doesn't actually measure power, it estimates it. The *only* way to measure power from each leg is with instrumented force pedals.

    I find that all trainers are useful. I've only used a Computrainer a few times. I have a trainer that has graphics of other riders and resistance alters up and downhill etc. i find this very motivational, e.g., when training outdoors is curtailed due to inclement weather.

    Ric
     
  7. Feanor

    Feanor New Member

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    I'm just taking a total layman's stab at this one, but doesn't the polar s720i power measurement unit measure the force imparted to the chain? and wouldn't that be at least a very strong indicator of force imparted to the pedals? Isn't it akin to grabbing a string and pulling on it and having someone determine how much force your hand motion was exhibiting by measuring the force at the end of the string?

    The s720i also has the cadence sensor so at one time you could measure both the full cycle time in one revolution and "superimpose" that data on the profile of the forces imparted to the chain during one revolution. Although that would not be as _directly_ connected to pedaling force as measureing it from the pedals themselves, isn't it only one small step removed from the data?

    I'm probably missing something, but though the data may in part be "muffled" by measuring cadence vs chain force to determine Pedalling Index, wouldn't it be like listening to a conversation thru a door? You don't hear it as well as if you are in the room, but you still get the completely accurate sense of what is being said...

    Now I'm not sure how the computer would know left from right (unless it is somehow indexed at the start of motion, but by measuring cadence you just have to split revolutions into alternate "sides" you overlay the power profile data of the chain during each stroke and you have relative effeciency data, and relative side to side variation data...

    Isn't that accurate enough? or are there issues with the Polar power measuring device? I reliaze that the polar system would be more accurate with measuring variations, and the pedal system would be more accurate at hitting the actual power "number"...

    Have a good one!

    Feanor
     
  8. chuy

    chuy New Member

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    There was a website that explained how the polar unit works, but I can't seem to find it.

     
  9. gregke

    gregke New Member

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    Jusy a little point about "how the computer would know left from right". The fact that the cadence sensor is in the power meter unit means that is always detecting the right pedal on its up stroke, so i guess it should be able to tell right from left.

    I am interested if anyone knows more about Ric's comment "higher power outputs (compared 40 km TT performance) were acheived... " suggesting that a higher pedalling index may not necessarily be a good thing. Though his data does sound conclusive? I guess you would need to analysis fitness level against both pedalling index and average power to really know what is going on with the data. It may be that its simply harder to maintain a high pedalling index when working at higher power outputs or closer to ones limits. I wonder if actually a higher sustained power output could still be achieved through a higher pedalling index.

    Greg
     
  10. ric_stern/RST

    ric_stern/RST New Member

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    they're not really 'my' comments but from the research of Coyle et al 1991

    ric
     
  11. acoggan

    acoggan Member

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    To add/expand on Ric's comments: the limitation of the Polar Pedaling Index and Computrainer SpinScan functions is that the power delivered to the chain (Polar) or rear wheel (Computrainer) is always the sum of the two legs acting in unison. Thus, to the extent that you do generate some of your power on the upstroke (which is a minor amount in almost all individuals), this will be 'seen' as power being generated by the other leg pushing down. Failure to recognize this limitation, as well mistakenly believing that the 'rounder' the pedal stroke the better, has led some people to develop sub-optimal pedal force application patterns, all in search a pretty SpinScan picture.

    The Coyle et al. paper, BTW, can be found here: www.midweekclub.com (click on 'Articles'). Note that this study is one of many examining the pedal force application patterns of cyclists (both elite and novice), and the results are consistent with the general literature.
     
  12. 9606

    9606 New Member

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    Andy/Ric - in a nutshell, what does the general literature imply about improving, or training, a better pedal stroke? Are isolated leg exercises worthwhile? Should we pedal simply as it feels comfortable?
     
  13. Squint

    Squint New Member

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    The studies are showing that there is no association between a "better" pedal stroke and how fast you can ride. So don't bother trying to train it. One-legged drills are retarded. Only do them if you're doing one-legged bike racing or only have one leg and have no choice.
     
  14. dkxkvtr

    dkxkvtr New Member

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    The polar meter analyzes 1 to 5 o'clock area of the pedaling stroke and compares it to the whole stroke. The reading that you get is really 1/3 (4 out of 12 hours on the proverbial clock) the total. if you were getting 30% with the polar that would mean 90% on the computrainer.
     
  15. beerco

    beerco New Member

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    Just to throw a monkey wrench in here, is it possible that the sub-elites were pulling up on the upstroke to make up for a poor downstroke? I.e. they can't push down hard enough so the pull up to yield a higher average power.

    A very interesting follow up study would be to try and get some of those elites (one of whom is arguably the best bike rider on the planet right now) and see if they still pedal that way. Phase two would be to try and teach the sub-elites to "stomp harder" and stop lifting to see if their FTP improves or gets worse.
     
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