Tanita BC-41 - Manual (Page 19)

Reliability of body composition measurements with an 8-electrode BIA
Introduction
This device calculates body fat percentage, fat mass, fat free mass, and predicted muscle mass on the basis of
data obtained by Dual Energy X-ray Absorptiometry (DXA) using Bioelectrical Impedance Analysis (BIA).
One of two settings must be selected when taking measurements:
1 Standard (for persons 7 ~ 99 years of age)
2 Athlete (sportsmen/women who exercise considerably more than non-athletes)
Making a distinction by body type in the measurement mode produces more reliable body composition
measurements for athletes, whose body compositions differ from those of average persons.
BIA is a means of measuring body composition – fat mass, predicted muscle mass, etc. – by measuring
bioelectrical impedance in the body. Fat within the body allows almost no electricity to pass through, while
electricity passes rather easily through water, much of which is found in muscles. The degree of difficulty
with which electricity passes through a substance is known as the electrical resistance, and the percentage of
fat and other body constituents can be inferred from measurements of this resistance.
The Tanita Body Composition Analyzer measures body composition using a constant current source with a high frequency
current (50kHz, 500 A). The 8 electrodes are positioned so that electric current is supplied from the electrodes on the tips of
the toes of both feet and the fingertips of both hands, and voltage is measured on the heel of both feet and the thenar side of
both hands. The current flows into the upper limbs or lower limbs, depending on the body part(s) to be measured.
a Principles of body composition measurement via 8-electrode BIA
a Body impedance measurement methods
In the conventional 4-electrode method, current is supplied from the electrodes at the tips of the toes on both feet, with the voltage
measured on the heel. This current flows from one lower limb through the lower abdomen and then into the other lower limb, and the
bioelectrical impedance (hereinafter, “impedance,”) thus measured is the impedance between the two feet. In the newly developed 8-
electrode method, however, current is supplied from the tips of the toes of both feet and the fingertips of both hands, and the voltage is
measured on the heel of both feet and the thenar side of both hands. This method allows five different impedance measurements to be
made – whole body, right leg, left leg, right arm, and left arm – by switching the part of the body in which the current is flowing and
the location where the voltage is measured. When measuring impedance in the right leg, for instance, the current flows between the
right hand and foot, and the voltage between both feet is measured.
For reference, the impedance measurement methods for the respective body parts are shown in the diagram below.
Technical Notes
Whole body Left leg Right leg Right arm Left arm
Current
Voltage
Impedance measurement methods for the respective body parts
In deriving the body fat percentage, fat mass, and fat free mass for the whole body, the Body Composition
Analyzer uses data acquired by DXA* from both Japanese and Western subjects as well as a regression
formula derived through repeated regression analysis using height, weight, age, and impedance between
right hand and foot as variables. Measurements of body fat percentage, fat free mass, fat mass, and
predicted muscle mass for specific body parts also use a regression formula for each body part derived from
repeated regression analysis using height, weight, age, and impedance for individual body parts (right arm,
left arm, right leg, left leg, trunk) as variables, based on data acquired through DXA.
A high degree of correlation has been found to exist between the figures for body fat percentage, fat mass,
and fat free mass for individual parts and for the entire body calculated with this predictive formula and the
figures obtained by DXA, and the results are thus highly reproducible.
Body composition measurements attained by the 8-electrode method are therefore extremely reliable.
Measurement of body composition using DXA
DXA was originally designed to measure bone mineral content, but in the full-body scan mode the body
fat percentage, fat mass, and fat free mass of individual body parts (arms, legs, trunk) can also be
measured. The image below shows one example of body composition measurement results obtained by
DXA.
Body composition measurement results obtained by DXA (Lunar Co., Ltd; DPX-L)
a Reliability of body composition analysis via 8-electrode BIA