Friday, February 02, 2007

What Does BMI Tell Us?

In January the Raleigh-Durham News and Observer published an article on the Body Mass Index that suggested it is a poor measure for who's overweight, particularly among athletes:

The paradoxical problem with athletes is simple: Muscle mass weighs more than fat. In a calculation that relies simply on height and weight, the buff athlete will lose out.
This is something I've always believed (or did I just have the need to convince myself of it?), but back in June, The Daily Erg conducted an interview that suggested BMI wasn't such a bad measure after all. (By the way, the N&O article uses the fact that the offensive line of a certain university's football team are considered obese by their BMI as evidence of the index's lack of credibility. To that I say that the index suggests that the linemen are obese, not that they are weak, or slow, or can't walk up a flight of stairs. If you've ever seen a major college football lineman, the fact that he is obese is inescapable.)

The Daily Erg interviewed Dr. Tony de la Mare who "is doing research into the use of the BMI both for health and in comparison to athletic performance."
Dr. de la Mare presented BMIs from a variety of elite athletes and all of their numbers were in a similar range - for men typically 20-24 and for women typically 20-22. Even ice hockey players and rugby players who are more muscular were in the 26-28 range - still well below so-called obese levels. There would seem to be an ideal BMI for aerobic performance at least, with runners (20), mountain bikers (22), and X-C skiers (23) all in the same range.
Also, The Daily Erg did some calculations of his own and found that the women on the 1992 Olympic team averaged a BMI of around 23.

In my (hardly scientific) experience, the average lightweight female rower is about 5'6". At 5'6" and 130 pounds a rower has a BMI of 21.0. What do you know, smack dab in the middle of the elite athlete range! Some more fiddling with numbers shows us that about the tallest a lightweight should go is 5'10", which gives a BMI of 18.7 (18.5 is underweight). On the other end, a rower would have to be under five feet and weigh 130 to be labeled overweight (pretty doggone generous if you ask me).

So BMI may not be some super accurate measure of who is overweight, but it is a reasonable indicator, even for athletes. As The Daily Erg says when talking about national team athletes, "being in the ideal range is necessary for membership in this elite club, but it does not predict how you will perform once you get there." My point, however, is that for a typical lightweight, the weight limit brings them into a range for which they should be striving anyway. The typical lightweight woman is not being asked to destroy her health to row lightweight.


Coach Jay said...

I agree that the typical light isn't dropping too low or risking health when rowing light. What keeps coaches up at night isn't the typical light.

The athletes "on the edge" that are "sucking weight" are the athletes that usually end up miserable, unhealthy and sometimes (terribly) in the headlines.

BMI isn't a measure that I believe in. Even with some sort of adjustment for athletes, I'm not willing to let some ratio in a computer tell me if somebody is "overweight" or "underweight". I'm using my eyes, the erg (sorry) and my judgement. If performance is slipping as a light gets closer to weight, (s)he takes more time to recover from workouts or (s)he looks like a concentration camp victim, then I'm getting worried as a coach.

It's a bummer that I'm not coaching lights anymore, because I really like that part of rowing. If the sport is going to continue growing, then light rowing is were the growth is going to be. There are only so many people who are in the 95th percentile in height who will row.

JW Burk said...

All good points, Jay. One of the things that bothers me is that many coaches allow lightweights to take erg tests above weight (this includes USRowing). Since the erg is an objective measure, it is the best way to know if performance decreases when an athlete gets close to weight (one of your tests). When these athletes do get to weight they're in boats which are usually set by then.