Quantified Body: Measuring Body Fat and Lean Body Mass

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quantified self tools measuring body fat personal analytics tools tracking body fatIn this post, I will review different methods for measuring body fat, and discuss my own metrics that I use to track progress in getting six pack and gaining lean body mass.

While taking photos of yourself every two weeks or so seems to be the most popular method on the web, I decided to use more objective and quantifiable indicator: a weekly average of skinfold measures taken in several body locations. Having a single number makes it easier to track the progress and assess impact of different diets and exercises. I derived this metric while researching the easiest yet reliable and efficient ways to measure body fat percentage (BFP), which I initially intended to use as a measure of progress. In this post, I will briefly review these methods, highlighting their advantages and disadvantages, and reasons why I decided not to use them.

The US Navy formula method was the easiest of them all. You just need to plug your body height, and waist and wrist circumferences into anthropometric formula (see the calculator here), or calculate yourself in Excel (lg in the formula stands for logarithm with base 10):

BFP = 86.010 *lg(waist-neck) – 70.041*lg(height) + 30.30 (for men, in centimeters)
BFP = 163*lg(waist + hips – neck) – 97.684*lg(height) – 104.912 (for women, in centimeters)

This method, however, is considered to be the least accurate, as it does not take in consideration your body type and how active you are. Also, if you look at the components of the formula, the variability is based on the waist and neck size, and I suspect that this metric is not sensitive enough to capture weekly changes in BFP.

The Bioelectric Impedance Analysis (BIA) is another quick and easy method , because most of the electronic scales nowadays come with built-in BIA sensors. This method measures the strength of resistance of your body to electric current as it passes from one foot to another. Based on the density of your body (calculated using your height and weight), strength of impedance and the fact that fat is less conductive than muscles and other tissues, BIA scale can estimate your body fat percentage. The accuracy, however, will be affected by your hydration levels (the more water you have in your body, the more conductive it becomes) and other factors (like cleanness and roughness of your skin). My Tanita scale offers BFP estimate, but based on my own observations, the number quite often changes in both directions, often on the same day.

how to use skinfold calipers to measure body fat quantified self personal analyticsThe skinfold measurement method  is considered to be more accurate than the previous two methods. However, it requires use of caliper and some practice. You measure it by pinching the skin in certain parts of your body and measuring the “thickness” of the skinfold with calipers. Combined with your age, gender and body weight, these measurements then can be used to look up the BFP value via special tables or calculators like this. There are actually over 100 (!) different formulas to calculate the BFP based on caliper measurements, some requiring up to 9 skinfold locations. Since the thickness of your skinfold is partially affected by the underlying layer of fat, measuring it in several key locations of the body is a more direct estimate of the overall body fat amount. The problem, however, arises when it comes to using formulas to estimate BFP. Most of these formulas are derived using limited number of empirical observations. A researcher invites 50 or 100 people, measures BFP using one of the accurate methods (Bod Pod or underwater weighing), then takes skinfold measurements and uses statistical analysis to establish the relationship between the caliper numbers and actual BFP. The resulting formula, thus, is derived by averaging out the measurements taken from a relatively small number of people.

Finally, the most accurate methods: DEXA scan, BodPOd and hydrostatic weighing, require use of sophisticated equipment that can be found only at the hospitals and research facilities. Obtaining BFP estimates with these methods is somewhat expensive, especially if you plan to take measurements on a regular basis.  I found a couple of locations in New York City that provide such services:

DEXA SCAN
Chelsea Diagnostic Radiology
230 Wes 17th street
New YOrk, NY 100011
Tel. (212) 989-8999
Cost: $150

BOD POD
Brooklyn College
Tel. (718) 951-4125
E-mail: bcexetest@brooklyn.cuny.edu
Cost: $75

Despite the price, I definitely plan to use Bod Pod at least twice a year, for the validation purposes. In the meantime, I rely on skinfold calipers, but instead of calculating BFP, I use arithmetic mean of measurements taken in four key locations: pecs (a midpoint between armpit and nipple), abdominal (just one inch to the right from navel), suprailiac (an inch above the hipbone point), and thigh (a midpoint between the crease of the leg and knee). I refer to it as P.A.S.T. metric (for Pecs, Abdominal, Suprailac, Thigh). While this number does not equal to BF percentage, it is sensitive enough to capture changes in BF across the weeks:

P.A.S.T = (Pecs + Abdominal + Suprailac + Thigh)/4 .

The Accu-Measure caliper that I am using to take these measurements is extremely cheap and easy to use. The “click” feature is especially awesome: it tells you when to stop pressing and take the reading, and the sliding ruler “remembers” the reading. Just a few important points:

– take all measurements on the right side of the body, in the same locations. If you can, mark the spots with permanent marker and use these marks for a couple of weeks until you get used to the procedure;
– it is important to wait no more than 4 seconds before reading the caliper, before skin restores its tightness;
– wait at least a minute before repeating the measurement in the same location;
– take at least three measurements at each location, to make sure the readings are consistent;
– at each skinfold, make the pinch in exactly the same location: 1 centimeter away from finger and thumb, and halfway between the base of the fold and the crest. Make sure you hold caliper perpendicularly to the skinfold.

I guarantee, if you follow these instructions properly, after a week of regular practice you will be taking reliable measurements. I usually take measurement at the same time: on weekdays, in the morning, right after getingout of bed and weighing myself. I then average out readings across all 5 days, and use the result as that week’s BF indicator. So far this method lets me capture changes in my body composition every two or three weeks.

As for the BF percent, I still calculate it, using the Jackson-Pollock-3 formula and average weekly measurements, but only to monitor my weekly Lean Body Mass (LBM). I do so to make sure that my exercise and diet regimen do not cause me to lose muscle weight along with the fat. Since lean body mass is simply a function of your body weight and BFP, it can be calculated easily:

Lean Body Mass = (1- Body Fat Percent)*LeanBodyMass

For instance, if your average weight on a given week was 180 lbs, and your average BF percent that week was 15%, then your Lean Body Mass = (1- 0.15)*180 = 153 lbs.

Just like with BF, you want to have the “target” body weight: you weight when you achieve your target BF percentage. It can be easily computed using the following formula:

TargetBodyWeight = CurrentLeanBodyMass/(1- TargetBodyFat).

For example, currently I weight 165 lbs, and my body fat, according to the Jackson-Pollock 3 formula is 12%. My current Lean Body Mass, therefore is CurrentLeanBodyMass = 165*(1-.12) = 145 lbs. In order to get a six pack, I need my BF to drop below 10%, so let’s define my TargetBodyFat as 9%. Then my TargetBodyWeight would be 145/(1-.09) = 159 lbs.

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