How to Qualify - Fatigue Curves for the Kona Athlete

by Alan Couzens, MS (Sports Science)

In my last article in our How to Qualify series I looked at how some typical benchmark workouts may progress across the course of the qualifying year for an athlete who is on track for a Kona slot. In this piece, we’re going to dive into these benchmarks in a little more depth to look at some of the implications of being strong in some benchmarks while struggling to hit others.

I’ll address such questions as:

What do the benchmark tests tell us about strengths and weaknesses of the athlete and the sort of training we may want to include in the athlete’s program?
What do they suggest about how the athlete may want to approach a pacing plan for the event?

The benchmarks provided in the last article are based on a fatigue curve of 7%. This basically means that as the duration of a given workout/race is doubled, we expect the athlete’s power to drop off by 7%. For example…

1 hour best = 300W
2 hour best = 279W (93% of 300W)
4 hour best = 259W (93% of 279W)
8 hour best = 241W (93% of 259W)

What does the fatigue curve tell us about the athlete’s physiology?
The fatigue curve is a good indicator of the athlete’s general endurance. A fatigue curve of 7% implies a threshold of approximately 85% VO2max. This is a level of aerobic capacity typical of elite endurance athletes across all sports (such as swimming, cycling, long-distance track).

However, it goes a step beyond the standard measures of aerobic fitness. Because the fatigue curve incorporates data from some very long sessions, it also incorporates a measure of the athlete’s metabolic fitness. Specifically, for an athlete with an FTP of 300W, a fatigue curve of 7% implies a fat oxidation of 5kcal/min at 65% VO2max (about AeT). This is a level typical of mid-sized elite long course athletes.

For the reasons above, I recommend shooting for this 7% number as a good general target. Keep in mind here that more is not always better. Aerobic capacity and aerobic power share an inverse relationship. If you get too much relative endurance you lose absolute power. So, while the very best ironman athletes may be closer to a 5% fatigue curve, it is important to be generally powerful before putting the finishing touches on your endurance. This principle is the same reason that we tend to see track athletes progressively distance up as they gain endurance but lose power. Power first is the order of the day.

How do I determine my fatigue curve?
Software such as WKO+ and web applications like Training Peaks offer a chart of the athlete’s maximal power numbers (from all data collected) plotted across the span of durations. For an athlete who has opportunities for maximal efforts across multiple durations, the fatigue curve can be derived from simply plotting the athlete’s best data from multiple durations and identifying an exponential trend line that best fits the data. Excel offers this option (show trend line with equation) on its charts.

However in the case of the ironman athlete who usually has limited true maximal short duration efforts, context is important. It’s no good concluding that an ironman athlete has a 5% fatigue curve because he simply never does any strong efforts over the short durations. Therefore, only best efforts, or at the very least, strong effort data points should be included in the derivation.

One of the best ways to ensure strong effort benchmarks across the power duration curve is to include them as specific workouts -- enter the benchmark workouts. Below I have expressed some of the
benchmark workouts from the last article for different fatigue curves (5, 7 and 9%)

To assess your fatigue curve, simply look at the relationship between the test workouts above. If your 5-hour best is less than 60% of your 5-minute best then your endurance may be considered weak relative to other top age group athletes. If your 5-hour best is 60-70% of your 5-minute best then, in my opinion, you have an appropriate balance of strength and power for a top age-grouper. If your 5-hour best is greater than 70% of your 5-minute best then your aerobic power could be considered relatively weak.

What does the fatigue curve tell me about my strengths and weaknesses and the type of workouts I should be including?
We do these assessments in the middle of the build (June, in this case) for a reason -- so we can have sufficient time to change the course of the training to address any weaknesses.

If an athlete is lacking strength/power, we may elect to include a block of higher intensity power training. If the athlete is lacking endurance, we may choose to simply keep building the aerobic base and remove a real specific prep period.

It should be noted that even a "power block” will consist of a majority of aerobic training (it’s an aerobic sport). It’s a question of degrees -- while most of the year may consist of very little threshold and VO2 work, for an athlete with very strong endurance, we may include a short period where threshold and VO2 make up about 30% of the workload. This level of high intensity work while very useful for some, will fry many athletes, hence the importance of getting a firm fix on where the athlete falls on the fatigue curve.

What does the fatigue curve tell me about optimal IM pacing?
The fatigue curve also has direct implications on optimal pacing for the event and should therefore be assessed both mid season (before specific preparation begins) and late season (prior to the race simulation workouts).

Looking at the chart above, extrapolating to 10-hours by taking a further 5,7 or 9% from the respective fatigue curve has direct recommendations on appropriate pacing for the race. If you arrive at the end of season with a best 5-hour of 54% of your best 5-minute number then it makes little sense to adopt the oft touted, generic ironman pacing plan of an IF of 0.73-0.8 (in this case, equivalent to 53-58% of CP5). It is unlikely that an athlete will be able to put out 10 hours of work at the same intensity that they were only able to put out five hours of work in training!

Similarly, for a 5% athlete, an IF of 0.73-0.8 equates to 61 to 67% of CP5. In this case the athlete may be low-balling his or her optimal power which, extrapolating from the fatigue curve should be in the range of 70% of CP5 for 10 hours.

The point being that the further down the curve we get, the wider the swing between what an athlete with strong endurance should do versus an athlete with less endurance. This is shown graphically below.

The fatigue curve affords the athlete an additional means (along with the race simulation workouts) of estimating their optimal output over race duration and highlights the importance of plain old endurance (independent of FTP strength) to this sport.

Now that you have a good sense of what your ironman duration power output is likely to be, in the next article we’ll take a look at how you can best distribute this power between swim, bike and run to give you the most speed for a given energy output.

Until then…

Train smart