Fitness-Constrained vs Fuel-Constrained

by Alan Couzens, MS (Sports Science)

You know you’re fuel constrained when….

• You take a bottle of Gatorade with you for your trip to the mail box
• You "hit the wall" in a 50 meter swim race
• Your top tube starts looking like a Vegas buffet…

If any of the above resonate with you just a little bit you might be an athlete who is more "fuel-constrained" than "fitness-constrained." You will often hear the coaches tossing around these terms so I felt it prudent to do up a little article explaining what we mean by this in a little more depth.

An average-sized athlete will have 2000-3000 kcal of stored glycogen within their muscles and liver. Once they use up this energy store, the body must operate almost solely from the combustion of fat, which it can do but at a very slow rate. One could think of it as the difference between throwing a piece of pine wood on the fire versus a piece of coal.

Furthermore, the brain depends solely on glucose as its fuel supply, so when the body starts running short on glycogen/glucose, folks can start acting a little cuckoo -- disoriented, poorly coordinated, slurred speech, etc. We’ve all seen pictures of this on the Kona finish line. If glycogen starts getting low, most of us will ease up before we get to this point of meltdown and will give in to the experience of low energy/motivation that accompanies low blood glucose.

Because of this fact, sometimes it’s hard to determine just what the cause of slowing down late in a longer race is. Many athletes will just feel like they don’t "have it" during the run and will attribute this to a lack of general fitness or the need to HTFU. In reality, the interaction between physiology and psychology is complex and a bit of self-talk and a positive mantra won’t cut it over the long term when blood sugar starts to fall.

And so, we’re left with a situation in which the athlete must meter out his glycogen stores over the race duration in order to arrive at the finish line at the appropriate pace and, more importantly, on his own two feet. Going back to our hypothetical average athlete toeing the line with 2000 kcal worth of glycogen in the tank, the question becomes, how long will I last? At the typical high efforts associated with a sprint- or olympic-distance triathlon, an average-sized athlete may find himself putting out 250-280W or 900-1008 kj/hr. Assuming normal economy numbers, this is equivalent to 830-930 kcal/hr. In other words, if most of this energy comes from glycogen (as it does at very high intensities) he’ll be done in a little over two hours. Thus, for competitive age group athletes, races up to an olympic-distance tri can be considered a fitness-constrained rather than a fuel-constrained event, as the athlete is somewhat likely to cross the line with a little gas in the tank.

So, if the athlete isn’t running out of fuel, what causes him to slow down? The fitness variable that prevents the athlete from going faster in this case is the accumulation of lactate. In other words, athletes who are not limited by their fuel stores are constrained by how high their maximal lactate steady state or "threshold" is. While having reasonable metabolic fitness is important to olympic-distance racing (and to athletes' general tolerance to training load), it is athletes' threshold numbers that separate the sheep from the goats.

Stepping up a distance to the half ironman, things get a little more variable. At similarly high power outputs of approximately 830kcal/hr at 250W for four or more hours, it takes a pretty special athlete to not be metabolically limited. In the longer races, athletes can expect to get 240kcal/hr from "on course carbohydrate," but this still leaves them about 500kcal/hr short. Unless they’re going under four hours (for a total endogenous output of 2000kcal) fat oxidation is likely to come into play. Much less so for a small, fast athlete, but for a larger, more powerful athlete it can become a key limiter to racing fast in a half ironman.

Finally, stepping up to a full ironman, just about everyone -- even the elites -- is fuel constrained. Even if we use the same output -- 250W/830kcal/hr for an elite ironman race, the writing is on the wall when it comes to running out of carbs. If an elite ironman wants to meter 2000kcal of reserves over eight hours, even with an "on course" carb contribution of 240kcal/hr, this still means that he can only dip into the reserves for 2000/8 = 250kcal/hr. This leaves the athlete about 240kcal/hr short meaning he must generate greater than 4kcal/min from fat. Now, this is assuming a pretty small athlete in a Boardman like position to be going anything close to eight hours on 250W. You can imagine the situation for bigger athletes!

I ran some numbers on the sort of output that Lance Armstrong would be forced to put out in order to be competitive in Kona for a recent Inside Triathlon piece written by the G-man. As a bigger athlete, with an obvious bike strength, I hypothesized that Lance would be putting out over 1000kcal/hr on the bike. Even as a bigger athlete, likely starting with glycogen stores closer to 3000kcal (about 360/hr) and with on course carb contribution of 240-280kcal/hr, this still leaves him being forced to generate 400+kcal/hr or about 7kcal/min from fat. While not impossible, these numbers are freakish in a different way to his freakish threshold power of 6.7W/kg. While Lance can generate almost 50% more relative power at threshold than the bulk of elite ironman athletes, this type of fitness is not specific to ironman racing. Lance would be moving from an event that is fitness-constrained to an event which is fuel-constrained.

When it comes to "key limiters" in ironman racing, understanding what category you fall into -- fitness-constrained versus fuel-constrained will go a long way towards directing you towards the most effective training for your personal limiters.

Train Smart