Alan Couzens, MS (Sports Science)

In the last article on power meter basics, I outlined the importance of having an up to date FTP (functional threshold power) number. I also suggested some easy methods of estimating it, using tests that you can run relatively often without tiring you out. One of these methods, critical power testing, also affords the athlete some very useful additional information that can be used to effectively direct his or her training. First, let’s revisit the test:

If you have a 5 and 20-minute best effort, you can estimate your FTP. Some sources suggest that simply taking 95% of your 20 minute value offers a reasonable estimate. I have found that this significantly over-estimates the FTP in most circumstances and offer this alternative method:

• Identify the % drop off from your 5 to 20 minute test
  • e.g. if 5 minute = 400W & 20minute = 300W, the drop is 100W  or in % terms, 100/400 = 25%
• Multiply this % by 0.75 = 18.75%
• Take this % off the athlete’s 20 minute value to get a good estimate of FTP
• So, for this athlete, we would multiply his 20 min test of 300W by 18.75% = 56W and subtract this from 300W to get a good estimate of FTP, in this case, 244W

In addition to estimating FTP, this method takes into account the very real differences in each athlete’s power-duration or ‘fatigue’ curve. The fatigue curve is an assessment of the balance between any given athlete’s power and endurance. While the magnitude of the curve is very specific to each athlete, the general shape of the curve is similar among athletes and relatively predictable. For the majority of athletes power drops off by a given percentage as duration doubles.

For example, if we take the example above, the athlete lost 25% of their power in 2 ‘jumps’ where duration doubled (from 5-10min and from 10-20min) Therefore, their fatigue rate is 25%/2 or 12.5%. We would expect other (best effort) points on their power duration curve to follow a similar pattern, like in the graphic below, which applies to a triathlete’s biking power over different distances:

The fatigue curve is an assessment of the balance between any given athlete’s power and endurance. While the magnitude of the curve is very specific to each athlete, the general shape of the curve is similar among athletes and relatively predictable. For the majority of athletes power drops off by a given percentage as duration doubles.

For example, if we take the example above, the athlete lost 25% of their power in 2 ‘jumps’ where duration doubled (from 5-10min and from 10-20min) Therefore, their fatigue rate is 25%/2 or 12.5%. We would expect other (best effort) points on their power duration curve to follow a similar pattern, like in the graphic below, which applies to a triathlete’s biking power over different distances:

If we compare the 12.5% example given above with this table, we’ll see that the athlete’s current profile falls in the ‘elite sprinter’ range. Indeed, if we extrapolate along the curve we find a max power of ~1170W - a number we would typically see in an elite (Category 1/2) sprint. The athlete’s projected Ironman output of 154W is, for a 75kg athlete a number that will yield a flat Ironman bike split in the 6 hour range – a middle of the pack performance. In this case, if the athlete is targeting an Ironman competition, his long (‘shallow end of the curve’) endurance is most certainly a limiter & something that the training should be focused on.

These numbers provide some useful standards to help to better direct the nature of your training towards your weaknesses with respect to your event (and also possibly, to identify events to which you are best suited). While general fitness, i.e. a constant ‘northward’ elevation of your whole fatigue curve, is always desirable for any event, small shifts in the specific nature of the fatigue curve can have a large impact. For example, an Ironman athlete with an FTP of 300W and a fatigue curve of 7% will have an 8-hour output of 241W. A small shift in the fatigue curve of only 2% will increase their 8-hour output to 257W, a number that could translate to 10 minutes or more off a typical Ironman bike split - at the same FTP! The effects are similarly powerful for a sprinter looking to shape the ‘deep end’ of the curve – he/she might not have to improve FTP to do better at the target event.

So, how do we go about ‘shaping’ the curve? What types of training should we emphasize to address a specific weak point on the curve? We’ll take a look at some of those questions in our next article.

Until then, train smart!

Alan Couzens