Additional Math Pages & Resources

Thursday, January 13, 2011

How Fat Am I? Part IV

This week we've been talking about the math involved in measuring (or estimating) body fat.

Today we will consider a Bioelectrical Impedance Analysis scale. One simply stands on this scale in bare feet and the display shows weight and estimated body fat. You might ask, How does that work?
The body conducts electricity. The typical  $50-250 BIA home scale applies a small current through the foot pads which travels up one leg and down the other, back to the scale. The various components of your body (fluid, fat, muscle, bone, etc.) all have differing resistance to electrical current. Circuitry in the scale measures the signal it receives (as modified by your body) and estimates your body fat. This particular unit has handles. If you grip with both hands it can also estimate fat in your upper body.

NOTE - all of these scales do the math for you. No arithmetic or math skills are required. Sorry.

Here's a professional model that costs $1500. It has clever features such as subtracting the weight of your clothes, shoes, iPhone, etc. so you don't have to take the measurements in your birthday suit.
Lab quality testing equipment tends to have electrodes that connect to both arms and legs. It is capable of much more precise results. Here's a professional body composition measuring tool that costs $4000.



All of these scales come from a company called Tanita

NOTE: I have no connection with Tanita. I work for Excel Math and no, I haven't thought of a way to buy one of these from my expense account.

Getting accurate and repeatable readings with any of these neat devices requires good electrical contact at your skin, a consistent level of fluid in your body, constant temperature and time of day, and assumes you are not touching anything metal (or operating a short-wave radio, or flying Ben Franklin's kite, or ...). These scales should not be used if you have a pacemaker or other electronic implant.

BIA measurements don't usually tell you where the body fat is located, and are not accurate if you have extreme body fat levels (less than 10% or higher than 75%).

You can get lots of data from a scale like this. Here's a sample that could be saved in a spreadsheet:
 
Standard, Male, 30 years old, Height 67.0 in {0, 16, ~0, 1, ~1, 1, ~2, 1, MO, “SC-240”, SN, “00000003”, Bt, 0, GE, 1, AG, 30, Hi, 67.0, Pt, 0.0, Wp, 182.2, FW, 18.7, fW, 34.0, MW, 148.2, mW, 140.8, sW, 19, bW, 7.4, wW, 105.6, ww, 58.0, MI, 28.5, Sw, 140.4, OV, 29.8, IF, 6, rb, 8247, rB, 1971 ,rJ, 12, rA, 29, ZF, 372.8, CS, 2A 
 
(You'll have to read the manual to find out what all this means!)
 
The best use of a home-quality BIA scale is to compare your own readings from day to day (when you are on a diet, or workout regime). Home scales do not give you accurate figures for comparing yourself to others. Only the professional-level devices are used in clinics that test large populations.

If you want to do some math today, ask yourself how many times you will measure your body composition, and calculate the cost-per-measure figure for one of these three machines. For example, if I had made a New Year's resolution to track my body fat, I would expect my resolve to last about a month. I'd measure myself about 5-10 times. A $100 scale would cost me $10-20 per test. (But being a techie, I would rather have the $1500 professional model!)


If you are better at New Year's Resolution-keeping than I am, you might want to look here and get the full kit - body analyzer, food scale, jump rope, etc.