Body fat testing....

How do Bio-electrical Impedance and skin calipers work? 


BIA is one of the quickest and easiest methods for predicting body fat
It is measured when a very small electrical signal carried by water and fluids is passed through the body. Impedance is greatest in fat tissue, which contains only 10-20% water, while fat-free mass, which contains 70-75% water, allows the signal to pass much more easily. By using the impedance measurements along with a person's height and weight, and body type (gender, age, fitness level), it is possible to calculate the percentage of body fat, fat-free mass, hydration level, and other body composition values.

Why they can be inaccurate.

Using BIA to estimate person's body fat assumes that the body is within normal hydration ranges. When a person is dehydrated, the amount of fat tissue can be overestimated. Factors that can affect hydration include not drinking enough fluids, drinking too much caffeine or alcohol, exercising or eating just before measuring, certain prescription drugs or diuretics, illness, or a woman's menstrual cycle. Measuring under consistent conditions (proper hydration and same time of day) will yield best results with this method.

Why do I get "fatter" as I age?
The reason our body fat is higher on the testing as we get older is because all Body Fat testing methods are based on averages for certain ages and sex. The scales/charts are age adjusted due to inherent bone density loss and additional fat around the internal organs. The reason we do testing isn't so much to give you that first "rating or number" but more to show your progress throughout the training and make sure that you are headed in the direction that meets your goals.
The following BIA testing guidelines are important in order to keep the prediction error of the BIA method at no more than 4% (Heyward, 1991).
1. No eating or drinking within 4 hours of the test
2. No exercise within 12 hours of the test
3. Urinate within 30 minutes of the test
4. No alcohol consumption within 48 hours of the test
5. No diuretics within 7 days of the test

It appears that the BIA method is more accurate for estimating body fat of persons within the optimal health category (18-30% women; 10-25% men). There is a tendency for BIA to overestimate percent body fat in very lean clients and underestimate body fat in obese clients. All in all, if the guidelines for testing are followed, the BIA method is a satisfactory method for assessing body composition of most people. 

Skinfold Method
The skinfold method of measuring body fat is a practical, economical, and administratively feasible field technique for body composition analysis. It involves measuring the skinfold (subcutaneous fat) thickness at specific sites of the body. Most equations use the sum of at least three skinfolds to estimate body density from which body fat may be calculated. Skinfold measurement does not require expensive equipment and it can be routinely incorporated into many health promotion settings. Skinfold technicians can be trained rather easily, but must practice on at least 50-100 clients before the skinfold technique is mastered.

When using the skinfold method, it is assumed that the distribution of subcutaneous fat and internal fat is similar for all individuals. This assumption is not fully supported. It is now known that older subjects of the same body density and gender have proportionately less subcutaneous fat than their younger counterparts. There is considerable biological variation in the distribution of subcutaneous, intermuscular, intramuscular, and internal organ fat due to age, gender, and degree of fatness (Heyward, 1991). However, generalized skinfold equations have been developed to estimate the body fat of men and women varying greatly in age (18 to 61 yrs) and degree of body fatness (4 to 44% fat).
Accuracy of Skinfold Measurements
The accuracy of the skinfold method is dependent on the technician's skill as well as the type of caliper and the skinfold prediction equation used. When choosing a skinfold caliper for a health/fitness setting, the cost, durability, and degree of precision of the caliper are important considerations. Reasonably priced plastic calipers have a less precise measuring scale, and often provide variable pressure and a smaller range of measurement. Despite this, a number of researchers have reported only small differences between skinfolds measured with high quality calipers and plastic calipers for highly skilled technicians (refer to Guide to Skinfold Caliper for more information on where to purchase calipers). However, plastic calipers are not recommended for use by untrained technicians.

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