Beyond the Fatigue Curve: Bringing Lab Testing to the Field – Part I: Fat Oxidation

As regular readers will know, I’m a big fan of using fatigue curves as an indicator of the relative top end power versus submax endurance strength of an athlete. By looking at how an athlete’s power decays as event duration increases, we are able to make some conclusions as to the endurance capacity of that athlete and we are able to extrapolate down the curve to make some pacing goals/predictions for event durations which the athlete may infrequently attempt. This is especially useful for ironman athletes.

However, despite the usefulness of the fatigue curve, it still only represents a general impression of the athlete’s endurance. It lumps together such physiological qualities as aerobic power, lactate threshold and metabolic efficiency into one index of the athlete’s endurance. To dig a little deeper and truly discern the “make up” of the athlete’s endurance it has been necessary to get the athlete into the lab and test these qualities separately. In this article, I want to present an alternative way of looking at field data that can enable you to begin to estimate the relative strength of metabolic fitness/fat oxidation, aerobic capacity and anaerobic capacity without booking a lab slot by breaking the fatigue curve down into its linear constituents.

Below is a typical example of a fatigue curve (in this case, 8%) with duration in hours along the x axis and power in watts along the y axis.

To break the curve down let’s start with the basic assumption that given a long enough duration of exercise, glycogen stores run out. A sporting event like ironman racing is fueled with significantly greater exogenous carbohydrate (sports nutrition taken during the event) and fat oxidation than a higher intensity activity like an open marathon. Extending this logic further, if we extend the event duration to a very long period of time, the power output will be fueled almost entirely by the body’s fat stores along with any exogenous energy taken in along the way. Or, put in terms of the fatigue curve, the asymptote of the curve would represent a power level indicative of only fat oxidation plus exogenous carbohydrate. For practical purposes, let’s call this number one week of continuous exercise (or 168 hours), a similar duration to the Race Across America. Let’s extend the above curve to that duration…

The expected power for this athlete over one week of continuous exercise (neglecting the effect of sleep deprivation) would be 154W or, on economy numbers of 75W/L, a total metabolic output of about 10kcal/min. Assuming maximal exogenous carbohyrate oxidation rates of 4kcal/min (Jeukendrup & Jentjens, 2000) we can assume that maximal fat oxidation for this athlete is about 6kcal/min. This metabolic output (less 4kcal/min of exogenous carbohydrate oxidation) is shown on the secondary y axis. This would represent a strong level of fat oxidation (elite for an average size ironman, good for a larger ironman athlete) based on the testing that I’ve done. The table below shows fat oxidation based on this estimated “one-week power” for a number of different power levels.

I’ve written a lot on metabolic fitness and fat oxidation in the past and you can do a search at the top of the page to dig deeper. To help you get more context on these numbers, check out these past articles as a starting point: Fat Burning Essentials and Fitness-Constrained vs Fuel Constrained. Our How to Qualify for Kona section also puts the numbers and terms into perspective for the aspiring Kona qualifier.

In future article, I’ll look at what the fatigue curve can tell us about aerobic power (VO2 max) and anaerobic power.