Sunday, October 01, 2006

The Navy Weigh

It’s the day of my monthly weighing and the scales tell the story – bang on 85 kg (187 lbs) or a loss of nearly four pounds in the past month. A somewhat pleasing development but when I measure my waist I don’t see the expected improvement so questions arise. I could have incorrectly measured my waist (a definite possibility I’ll get to later) or my weight loss was perhaps only the result of the temporary weight fluctuations which happens to everyone, sometimes up to as much as 1% of total body weight, each and every day. But you can avoid most of this fluctuation by weighing yourself at the same time of day (which I did) so I must believe the 1½ kg loss is fairly accurate of my true mass. The problem is clearly my earlier waist measurement. Whereas in September I took my waist measurement around my girth’s smallest circumference (belt level) this time I measured myself at the level of my navel, and the different location resulted in no change in waist measurement month to month. Why the change in location? Well, I’ve found a new way of calculating body fat. I wasn’t really happy with my first BFM site I used last month as it only took measurements in whole inches. Given the estimates involved in these calculations such ‘coarse’ inputs are perfectly reasonable, but I wasn’t happy about having to wait around for an entire inch to be lost before I could record progress, so I went back the internet to find another method. And I found a great one. At the US Military’s method of calculating body fat is explained (the preamble to the tables sets out the history of the methodology from its Navy origins to adoption by DoD for all five services as well as the algorithms employed) and the necessary tables to calculate body fat based on tape measurements are provided. Simply put the Navy discovered using the neck circumference as the base for body type showed a very high coefficient to actual variability in body fat in large populations. Therefore subtracting the neck measurements from those which best assess overall fat levels gives very accurate body fat percentages. For men this requires only two measurements – neck and waist (measured at the belly button level); and for women three measurements – the neck, hips (at the greatest circumference around the hips) plus the waist (determined by measuring the smallest circumference). The net measurement is then looked up in the tables at the individual’s height to give their body fat percentage. Some caveats apply however; because these tables are derived from a military population the database does not adequately provide for individuals under military age and post-menopausal women. For me neither applies, so after plugging in my October measurements, the tables say my body fat is basically unchanged at 21%. Clearly my September reading was off. Can’t wait until next month when I can make an accurate evaluation of my progress.

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