Body Mass Index Versus IM Performance

How important is your Body Mass Index in determining your IM performance? Body Mass Index (BMI) is calculated by taking your weight in Kg and dividing by your Height (in Meters) squared. Thus for myself, Weight = 75Kg, Height 1m77 my BMI is 57 / 1.77 / 1.77 = 23.78. Typically the advised range for the normal population is between 18.5 and 24.9. Below this range is “underweight” and above this range is “overweight” with Obesity defined as greater than 30.


A large number of triathletes from the Slowtwitch community responded to my question on this and posted their weight, height and best Ironman (IM) performances.


IM Performance.

Not all Ironman races are the same. Well you could even say that none of them is the same. There is a considerable difference between the fastest races on the circuit (Austria and Germany) where the top pros are crossing the finish line in around 8h15 versus the slowest races (Wisconsin, Korea) where the pros are taking almost 9 ˝ hours to complete the same distances. Swim conditions, course elevation and weather conditions are the major factors accounting for these differences. Thus in order to effectively determine what the performance of the respondents was I compared the time of each response to a reference time for each individual race. This reference time is the average finishing time of the top 5 finishers in each race. Thus each respondent’s performance is expressed as a percentage over this time. Example, my 10h46 at IM Louisville in 2007 was 22.3% above the reference time for that race.



I have plotted all the responses on a scatter graph (on the right for Males) and you can determine for yourself whether there are any conclusions to be drawn. I have also added the BMIs for the top male pros that I could find (the top 20 male finishers at Hawaii in 2006)and I have given them a performance rating of 0%. i.e. They are the standard by which the rest of us are measured.


My comments

Well the distribution of the results was far wider than I anticipated. As the IM performance nears that of the elites, there is a narrowing of the data as you would expect, however there is still a large number of Age Groupers who have put in some impressive performances with BMIs which the British National Medical Association would define as Overweight. The first reaction to this is that the BMI is not applicable to Ironman triathletes, nor athletes in general, because of the larger amount of muscle that they carry compared to the normal population. Muscle is heavier than fat so it would seem logical that this would be the explanation. But this explanation seems to lose credibility when you look at the Top Professional Athletes. From some googling on their, or their sponsor’s websites I was able to obtain the data on many of the top athletes. Simon Lessing, Luke Bell, Chris McCormack, Faris Al-Sultan, Cameron Widoff, Ellios Llanos, Normann Stadler, Chris Lieto and Spencer Smith. All these athletes fit into a BMI of between 20.35 and 23.01 with the notable exception of the Mars bar-loving Spencer Smith.


Now if you argue that as you improve your performance as triathlete then your BMI will cease to decrease because of the muscle mass that you develop then how do you explain the lower BMIs of the world’s top IM racers.


Of course the harsh reality for me in all this is that if I truly want to improve and look like a pro (well more than just getting atop a Cervelo P3C) then I need to lose another 7Kg or gain 9cm in height without putting on more weight. I’ll think about which option to chose over a Nutella sandwich.



I have separated the data for the female survey respondants on this chart, however I believe that some more data is required before any comments can be made

Raw Data