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Value of a Low VI: The Science

Al TruscottAl Truscott
in General Training Discussions

This month's Lava had an article on "economy", covering all tri disciplines. They referenced a study which was intriguing, the conclusion of which says: "A highly variable power distribution in cycling is likely to impair 10 km triathlon run performance." I searched, and found the original article, from Australia in 2013. While they studied an hour's cycling followed by a 5 mile run, they found what we've been preaching here for over a decade: the lower your VI while cycling, the faster you'll be able to go in a subsequent run. They kept *average* power the same, and had one group ride like roadies, and another group ride like an EN ninja. Running times were 6% faster in the ninja group. Duh. Here's the link to the full text:

http://www.researchgate.net/publication/256501349_Cycling_attributes_that_enhance_running_performance_after_the_cycle_section_in_triathlon

Comments

  • Peter GreaggPeter Greagg
    Of course this result is conceptually derived as follows.

    Given the physiological cost of increasing watts above a given reference point is (almost certainly ) exponential in nature, and that Pav is what actually pushes you down the road, then when you solve the optimisation problem, you find the solution is:

    maximising Pav for a given Pnorm, then the lower the VI, the faster you will go for a given physical cost.

    Or alternatively, minimising Pnorm for a given Pav, will give you the minimum physical cost for a given speed.

    Which give us the optimal strategy for the bike - which is, of course, the EN Ninja strategy of minimising your VI.

    And as I would expect, this is what Al said!
  • vickaiservickaiser

    A couple months ago, I talked to CoachP during office hours about how to ride with a low VI.  Being relatively new to power, I was having a hard time staying on a small range of watts and keeping VI low.  CoachP had a great thought that has helped me immensely.  He said, another great metric that correlates to steady power is steady HR.  As a noob to power, instead of watching the 3s watts, I watch my HR.  I find a number 145-147 and just try to keep it in the range.  This has yielded some pretty awesome VI's of 1.03-1.07 on steady rides, as opposed to the 1.10-1.17 rides where I was trying to chase a range of watts, fighting my gearing, changing cadence to try and stay on 150 - 155.  Very helpful.

  • David LeventhalDavid Leventhal
    The area that I've struggled with is tackling hills while maintaining a low VI. In Buffalo Springs a few weeks back I rode a 1.10 - this is a course with 8 climbs - some short and some less short (none are what I would call "long" - i.e. >2 miles). I've been riding with power for over about a year and a half and I'm definitely improving but still struggling to find how to not spike my watts on hills. I suspect that it will continue to improve over time but if anyone has any suggestions or things to focus on I'm all ears. The HR correlation makes sense to me - feels intuitive that a steady power would equate to a steady HR.
  • Al TruscottAl Truscott

    Do you need more gears? If you are reduced to 60 rpm or even less just to get up hills with some semblance of "steady", you may need more teeth in the back, or fewer in the front.

  • David LeventhalDavid Leventhal
    I've got 50/34t in the front and a 10 speed 11/28 in the back. I put the 11/28 on last year before IMCDA (& was glad I did!). I just took a quick peek at my bike file from Buffalo Springs and my RPM on the hills never dropped below the mid 80s & most of the time it was in the 90s - so maybe it's just that I need to continue to practice at not sucking so bad when climbing!!
  • William JenksWilliam Jenks
    One of the difficulties that you CAN face is to compare the low VI that a very strong biker can have on most any course or that a reasonable biker can have on a fairly flat one, to what a mid-grade biker will have on a hilly course (e.g., Wisconsin). The simple reason for this is mathematical, i.e., that if you just HAVE to get near your FTP to get up a hill, that will raise your VI as a deterministic mathematical certainty. With a stronger biker (higher W/kg) or flatter course, then the variations are more related to your own "voluntary" efforts. So, yes, minimize the VI...but if you're a relatively low W/kg rider on a hilly course...you may want to make a little more allowance for what you really expect that number to be. No excuse for not executing as best as possible...but that 1.02-1.03 VI race my not physically be possible for some of us.
  • Al TruscottAl Truscott

    Posted By William Jenks on 17 Jul 2015 04:29 PM P
    One of the difficulties that you CAN face is to compare the low VI that a very strong biker can have on most any course or that a reasonable biker can have on a fairly flat one, to what a mid-grade biker will have on a hilly course (e.g., Wisconsin). The simple reason for this is mathematical, i.e., that if you just HAVE to get near your FTP to get up a hill, that will raise your VI as a deterministic mathematical certainty. With a stronger biker (higher W/kg) or flatter course, then the variations are more related to your own "voluntary" efforts. So, yes, minimize the VI...but if you're a relatively low W/kg rider on a hilly course...you may want to make a little more allowance for what you really expect that number to be. No excuse for not executing as best as possible...but that 1.02-1.03 VI race my not physically be possible for some of us.
    EG, Last year, I did 1.03 on IM AZ, yet only 1.06 on IM CDA which is a lot like Wisconsin. Elev gain/kJoule were: 2140/2700, 5650/3130. Of further interest, here are my peak power #s for short intervals, AZ on left, CDA on right: ((FTP was 21X, IIRC)

    3' 155. 157
    2'. 160. 169
    1'. 182. 192
    30". 204. 227
    5". 303. 344
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