I was directed to this blog from searching the body mass topics, but have since read quite a few of your posts. All very interesting stuff. I think I share the same completely random interests as you. In this blog I cannot pull up the all important body density calcuation in the first white block. Could you forward to me or post here.

Thanks,

Paul Stone

I calculated my body fat percentages using this method of hydrostatic weighing and compared the results to the skinfold measurement method and the BMI calculation. Here are the results:

Weight – 111.7 Kg
Height – 1.93 m
BMI – 30.0%
Skinfold – 29.0%
Hydrostatic – 25.3%

I had previously noted that skinfold measurements gave a lower bodyfat reading than the BMI. I expected this since BMI does not take into account higher than average muscle mass. I expected to see a closer correlation between the skinfold method and hydrostatic weighing. The skinfold method is an empirical relationship derived by comparing the total readings (in mm) of three body sites with hydrostatic weighing results. A curve is then fit to the data points and an empirical equation derived from the curve. This process is repeated for different ages since the relative percentage of subcutaneous fat changes with age. I don’t have the equation; the book I have simply has nomographs to solve the unstated equations.

So, for the $64 question, “Which of my two measurements was more accurate?” I don’t know. The vague estimate of residual lung capacity bothers me a little. How accurate is this? and How do the “profesionals” measure this variable in clinical hydostatic weighing?

On the other hand, the skinfold caliper measurements are highly variable depending on technique. And even with consistent technique by the same person the readings still vary by a few millimeters on each measurement which theoretically could compound to an varience of up to 2+ percentage points in the result. Errors in exactly how and where to measure could give errors in the result that are even more dramatic. It is my observation that the thicker the subcutaneous fat layer, the more difficult it is to get consistent measurements.

Given the above observations, I tend to believe that my hydrostatic calculated result is the more accurate. However, given the significantly lower result, I question whether it really will “almost certainly overestimate %BF”.

Thanks for the post. It was interesting to make the comparisons.

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