In my last article I presented a formula that you can use to equate performance between sessions of different durations and (to some extent) intensities to come up with a global “VO2 Score” to assess general improvements in fitness over the course of a season — in essence an approximation of your VO2max in ml/kg/min.

The article prompted a spirited discussion on our Endurance Corner forum that centered not so much around the formula itself but rather the classification of athletes into “front of the pack,” “middle of the pack,” etc., based on the number that you come up with. While these were intended as very general fitness classifications, the implications that they have on actual ironman performance was appropriately questioned. Some good points were raised. Enough points that I thought it worthwhile to do a follow up piece on what that number tells us about potential ironman performance.

The most important point to come out of the discussion is that ironman performance depends on a lot of factors independent of the athletes VO2max (whether real or virtual). Gordo made the comment that he’d taken down guys with 20ml/kg/min higher VO2s than him over the HIM and IM distance. This is certainly possible given the many factors that combine to determine Ironman performance. Things like:

• Weather (wind, temperature, humidity)
• Equipment/aerodynamics
• Nutrition
• Pacing

These all afford the possibility of a less fit athlete beating a more fit athlete. But just as important to ironman performance is that the athlete is fit in the right way.

VO2max is a measurement of fitness that describes the maximal ability of the body to “suck up” and consume O2. It is a measurement of aerobic power, meaning that it describes the maximal power of the aerobic system. It’s like a drag race that rewards V8 engines but pays little attention to the merits of a Prius in its ability to generate power economically and for long periods of time — things that are critical to ironman racing.

So, while it’s a crucial element in producing the sort of aerobic power demands that are necessary for ironman racing, it does little to describe how long an athlete will be able to hold said power or given percentages of it. There are lots of physiological terms that describe this ability, for example, maximal lactate steady state, onset of blood lactate accumulation, anaerobic threshold. I prefer to look at data from the field over various durations to come up with a fatigue curve that globally explains how much an athlete’s power drops off as the duration increases. This number can range anywhere from 5-20%, meaning that an athlete with a low power output can beat an athlete with a high power output over longer duration events.

For example, let’s say athlete A has a 10 minute power (a good approximate of VO2max power) of 500W and athlete B has a VO2max power of 400W (20% lower) but athlete A’s fatigue curve is 10%, while athlete B’s is 5% — as the distance doubles athlete A loses 10% of his power while athlete B loses 5%. For a 20 minute event, athlete A still has the upper hand — at 450W versus athlete B at 285W. Ditto for a 40 minute event– 405W versus 270W. But what if we jump ahead to a 5 hour, 20 minute event? In this case, Athlete A is down to 295W, while athlete B is at 310W. In this way, an athlete with superior aerobic capacityy can beat an athlete with superior aerobic power.

In order to give the VO2 score some context as an ironman performance predictor it is necessary to also consider the athlete’s individual fatigue curve. I have provided some “real world” tables to the right to show how the two physiological constructs combine to produce various performance levels over an ironman course (in this case, Lake Placid — selected because it is a good power:weight course that ties in well with relative VO2).

Based on my calculations, Gordo’s comment is spot on. An athlete with a relatively “ordinary” VO2max of 60ml/kg/min can beat an athlete with a world class VO2max of 80ml/kg/min if he is adequately and appropriately trained for the ironman event.

These tables also qualify some of the terms I used in the last piece (back of pack, middle of pack, front of pack) with actual overall percentile finishing ranks. I also included percentile ranks for the female 35-39 age group and the male 60-64 age group to highlight the impact of gender and age on VO2 scores respectively. Generally, females will show a VO2 score 5-10ml/kg/min lower than males at the same respective performance level. Additionally athletes will lose 10-15ml/kg/min as they move from the 30-34AG to the 60-64AG for the same relative performance level (~3-5ml/kg each decade)

Hopefully this does give some greater context to the labels given in the last piece. As EC member Mimi suggested, I do need to be careful throwing labels around, especially when they are not adequately defined.

Until next time, train smart!