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Wednesday, July 06, 2011

Tour de France: Stage 4 power output analysis

453 Watts for the final 2km: A look at Stage 4

Just a very short post as a follow up to the earlier post on the power output during the Tour de France.  And for this, credit to SRM, whose website is an absolute must for anyone really interested in crunching the numbers on the Tour.  The truth is, they do such a great job of breaking down the power files of their riders that it sometimes feels unnecessary to do it again myself, but there are some insights I'd like to try share regarding what happened on the climb up to the Mur-de-Bretagne.

So again we look at Chris Horner - 10th last year, a podium contender this year, he's going to continue to be a great barometer for what is going on at the very front end of the race.

Below is his power output file for the entire 172km (again, click to enlarge).

It shows some of the features we mentioned in our post earlier today - a relatively low average power output of 216 W (still significantly higher than the 176W of stage 1), but particularly in the flatter sections of what was a "bumpy" stage.   For long sections, the power output is below 180 W (the green line in the above graph), but that is also dictated by the profile of the stage - the black line is the altitude graph and if you look at that, combined with the pink trace showing speed, you get an idea that yesterday was a pretty up and down stage, fast then slow, high intensity over rolling terrain.  These types of days are often more taxing than longer, but steadier mountain stages. In fact, at first glance, the power output profile resembles an interval-type session.

But the real story is the final 15% of the stage, and in particular, the last 2km, where the race broke up and Alberto Contador attacked.  Horner's average power output for the final 25 km (which took only 31 minutes) was 318 W (5W/kg), building progressively towards the finishing climb, where it really ramps up.

The Contador attack actually created a small gap over Horner, who ended up 8 seconds down on the stage, but his power output file for those final 2km reveal just how intense the effort was at the front of the race.

So, some numbers.  Horner covered the final 2km in 4:16, with an average power output of 453 W.  The mass being used by SRM is 64kg, and that means that his final 4:16 was done at 7.1 W/kg.  There was a 2 min stretch in the middle, shown in the figure above where the power output was 502W, an enormous 7.8 W/kg.  Note the spikes in power and the not insignificant period of time spent above 540 W, mostly towards the bottom part of the climb.

Some physiological implications

It's too tempting not to try to interpret the physiological implications behind those numbers.  Unfortunately, to do so means making some assumptions.  If we assume a cycling efficiency of 23% (which is reasonable, I think, based on the measurements in elite riders, including Armstrong), then we can estimate that the energy cost of riding at this intensity is 28.3 kCal per minute.  That actually gels pretty nicely with what the SRM is reporting - 116 kCal over 4:16, or 27.2 kCal/min.

It's impossible to know what kind of oxygen cost is involved, because there you have to assume where the energy is coming from.  And for such short, high intensity efforts, that is much more difficult to do than for the longer climbs.  

But if you go to the furthest extreme (which estimates the lowest VO2, call it the "low case scenario") and assume only carbohydrates are providing the energy, then you can estimate a VO2 of 5.6 L per minute, or 87.52 ml/kg/min.  The trouble with this estimate is that it doesn't account for energy production from anaerobic sources and lactate (and hence without oxygen), and so that 87.52 ml/kg/min is likely to be an overestimate.  And the reality is that for an all-out effort like that over 4:16, a cyclist is probably at supra-maximal intensities, or very close to 100% VO2max at least.  For this reason, it would be fascinating to see heart rate data as well, so that we at least get some indication of where Horner is relative to his own ceiling.

Once the climbs lengthen, and we can make a safer set of assumptions over the energy sources, then this kind of calculation will become much more valuable.  For now, it's just illustrative and highlights just how close the limit the finishing drives are, and how remarkable a Tour rider has to be just to hang on when the attacks begin to come at the end of the day

Hopefully that little illustration from Stage 4 contributes to the earlier post.  Enjoy the finish of Stage 5!