Monday, March 17, 2008

More on Pacing

Since my post last month on negative splits in steady state events such as time trials and triathlons there have been a lot of questions about how the principles described there apply to courses with hills and wind. There have been several scientific studies done on this matter. Here is a brief summary of several of these studies. I'll let you draw your own conclusions.

* Using a mathematical model Swain found that when compared with a constant effort there was a significant time savings in a cycling time trial by slightly increasing power on the uphills and into headwinds and decreasing it slightly on downhills and with tailwinds. (Swain. 1997. A model for optimizing cycling performance by varying power on hills and in wind. Med Sci Sports Exercise 29:1104-1108.)

* This study involved a review of other research such as Swain's above using a mathematical model to predict how hills and wind affect performance in a cycling time trial. The authors then revised the previous models slightly but the results were largely the same as the others: Increasing cycling power on uphills and decreasing it on downhills, and increasing power into the wind and decreasing it when riding with the wind improved time trial times significantly. (Atkinson et al. 2007. Variable versus constant power strategies during cycling time trials: prediction of time savings using an up-to-date mathematical model. J Sports Sci 25(9):1001-1009.)

* Seven male cyclists did a 16.1km (about 10 miles) time trial on a CompuTrainer 3 times each. There was a simulated 8km headwind in the first half of the ride and a simulated 8km tailwind in the second half. The pacing of the 3 rides were: a) self-selected pace, b) constant power and c) variable pacing with 5% higher power into the wind and self-selected and constant with the wind. Times were significantly faster in b and c compared with a. The fastest was c. Variable pacing based on power should be used when there is a headwind. (Atkinson and Brunskill. 2000. Pacing strategies during a cycling time trial with simulated headwinds and tailwinds. Ergonomics 43(10):1449-1460.)

* Seven cyclists did 3 time trials of 800 kilojoules each. A kiloJoule (kJ) is a measure of mechanical energy expended; 1 kcal = 4.184 kJ. Considering that an experienced and fit cyclist is about 23% efficient, this means that they were riding as intensely as they could until they expended roughly 880 kcal. For most such athletes this would take about an hour. The 3 courses and conditions were 1) flat with a self-selected power, 2) hilly with 5% grades ridden at the same constant power as course #1, and 3) same course as #2 but ridden with power varying - 5% greater than #2 on uphills and 5% lower on downills. The overall power was equivalent for all 3 courses and conditions. But the finish time was significantly faster with pacing #3. The results were: #3 - 3670 +/- 589 seconds vs. #2 - 3758 +/- 645 seconds. So varying power by 5% on hills did not significantly change the power output but saved, on average, 88 seconds in a 1-hour time trial. (Atkinson et al. 2006. Acceptability of power variation during a simulated hill time trial. Int J Sports Med 28(2):157-163.)

* So does varying power with hills and wind as suggested in the above studies cause additional physiological stress? That could be quite detrimental for a triathlete who needs to come off the bike and run. But according to Liedl et al this is not a problem. They found that variations of +/- 5% had no negative consequences for the riders' performance when compared with a constant power output. (Liedl et al. 1999. Physiological effects of constant versus variable power during endurance cycling. Med Sci Sports Exercise 31(10):1472-1477.)

* Speaking of triathletes, what is the effect of varying power on the run? A recent Aussie study addressed this issue. Eight triathletes did 2 bike-run workouts. In #1 they they rode steadily for 30 minutes at 90% of their lactate threshold power (another way of identifying FTP). In #2 they alternated +/- 20% (!) of the power they rode at in #1 every 5 minutes for a total of 30 minutes. So they did 5 minutes at FTP + 20%, 5 minutes at FTP - 20%, 5 minutes at FTP + 20%, etc. After each 30-minute ride the triathletes immediately started a run to exhaustion at 16.7 kph (about 6 minute/mile pace). The times for the run to exhaustion improved significantly after the variably paced rides. Following the steady power ride the average time to exhaustion on the run was 10 minutes, 51 seconds (10:51). After the variable-power ride the average run was 15:09 - an imporvement of nearly 50%. That's huge. However, the improvement may well have been because in #2 the last 5 minutes of the ride nefore starting the run was done at 20% below FTP. Coming off of the bike somewhat rested would have been an advantage, especially given that the power for the last 5 minutes was a whopping 20% below FTP. (Suriano et al. 2007. Variable power output during cycling improves subsequent treadmill run time to exhaustion. J Sci Med Sport 10(4):244251.)


At March 17, 2008 9:47 AM , Blogger Sarah said...

These are really interesting studies. Thank you for sharing. I've always thought I should really power on the downhills and with tailwinds given the theoretical advantage of having gravity/wind on my side.

I was especially interested to see what the triathlete studies showed; 50% improvement is huge.

At March 18, 2008 5:18 AM , Blogger Hilton Meyer said...

I really like these scientific features. I've been trying to put the negative splits into action during races but unfortunately I don't have split times on my HR monitor. this powering uphills would also be difficult to check without some sort of power meter but will keep it in mind.

At March 18, 2008 10:08 AM , Blogger Bahzob said...

Very interesting and consistent with my subjective experience.

But I am puzzled about this in relation to calculations of TSS and Normalised Power in WKO. As I understand it the rides with varying power would have increased values of both. (e.g. for 300W FTP the TSS is 55 vs 50 and normal power 316W vs 300W). But the last study seems to show that, at least in terms of immediate impact, riding with variable power is in fact less stressful (in the sense of having an impact on running), though as you mention perhaps with the proviso that the variation is well timed.

At March 18, 2008 12:35 PM , Blogger FatDad said...

I'm curious if there is a similar study relating to output up and down hills for running rather than cycling. If you know of any and would pass along the info, that would be amazing. Keep up the great posts.

At March 18, 2008 2:06 PM , Blogger Joe Friel said...

FD--None that I've come across.

At March 21, 2008 1:53 PM , Anonymous Anonymous said...

Re. Bahzob's question: the variations in power in the study of Atkinson et al. (2007) were actually quite small, and would have resulted in the normalized vs. the average power. Indeed, based on the mean values presented I calculate the difference to be <1 W.

Andy Coggan

At April 3, 2008 5:52 PM , Blogger Fran said...

Joe - I know that FT power can be estimated as follows:

FT = (CP20 - 5%)

What equivalent estimates of sustainable power for periods of 90 and 120 minutes?

The reason I ask is because I am trying to develop a pacing strategy for a 17.5 mile uphill time trial (5357 ft of climbing). I believe strongly in your 49/51 approach, and want to preserve my power for the 2nd half. The problem is I have no idea how to estimate my sustainable power for this effort. I also do not know how long I should estimate this to take me.

FYI - My FT power is 265.

Any ideas greatly appreciated!!


At April 8, 2008 1:38 PM , Blogger Joe Friel said...

Fran--As the duration doubles the power decereases by about 5%. The farther away from the reference duration you get the less likely this formula is to be correct.


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