Fat: Who Says? (Measuring Obesity) What’s Your Resistance?
Measuring Resistance in the Body (A)
Student Data Page 3C Part 3
Problem Statement:How can you measure the resistance of a human body?
Activity Background:Human body composition is an interesting topic to consider. No two people have the samebody composition. Drs. Heymsfield, Lohman, Wang and Going (2005) explained that thebreakdown of a lean male is approximately 19% fat, 77% soft tissue and 4 % bone. Butnobody is perfect and variations in bone density, gender, and ethnicity can make predictionsabout total body fat inaccurate. Fat distribution refers to the relative amounts of fat in theprimary parts of the body where adipose tissue (specialized connective tissue that functionsas the major storage site for fat) is stored.
Electrical resistance is a measure of how much an object opposes the flow of an electric current. The SI unit of electrical resistance is the ohm (Ω). Its reciprocal quantity is electricalconductance (1/resistance = conductance). In turn this is a measure of how easily an objectallows the flow of an electric current.
Impedance (symbol Z) is a measure of how much a circuit blocks (obstructs) the flow of current. Like resistance, it also takes into account the effects of capacitance and inductance.Impedance and resistance are both characteristics of alternating current which has a frequen-cy component and is measured in ohms. Impedance is more complex than resistancebecause it varies with the frequency of the alternating current passing through the circuit and this means impedance varies with frequency. The effect of resistance is constantregardless of frequency. Since this activity uses a direct current with a constant voltage (not alternating current) for this activity, Resistance = Impedance.
Bioimpedance, a medical engineering term, is used to describe the measure of changes in theelectrical conductivity of various body parts. More specifically, this concept has been used todevelop devices that measure the amount of fat in the human body. Commercial devices arenow available at very affordable prices that claim to accurately determine the human bodyfat content. Simple hand held devices as well as more sophisticated (and expensive) toolssuch as the machines manufactured by TANITA™ all claim to give good results. However,when one looks closely at their product manuals there are disclaimers that indicate these machines should not be used for various groups of people or at different times in the day. All of which makes it very confusing as to what they are actually measuring.
How Do We Measure Bioelectric Impedance to Determine Body Fat?There are many devices on the market today that range in price from $2000 to as low as $30 for the consumer. These devices use small alternating electrical current (AC) applied to the body by electrodes. They also contain high frequency meters, microprocessors, and some sort of way to view the results (LCD panel) or a printout. The meter measures impedance and reactance. The microcomputer computes the size of fat-free mass and body cell mass. The reactance is part of impedance related to the storage of energy in material that conducts electricity. This value comes into play with AC current that has frequency values associatedwith the production of current.
The total body mass of a human can be split into two categories for this type of testing, the fat mass and the fat-free mass. Fat-free mass is the sum of the mass of the bones, blood, muscle and water. Where there is no water content, there is usually no conductance of electricity. Since muscles, blood vessels and bones are body tissues with high water contentthey conduct electricity well. Fat on the other hand has little or no conductivity value andtherefore has a very high resistance to current flow.
Body Fat Percentage = Body Fat Mass (lbs)/Total Body Weight (lbs) X 100
Activity Materials: (per group)• 1 Multimeter• 1 Bottle Electrode Gel*• 1 Metric Tap measure• 1 Copy Student Data Page per student * Recipe included in Teacher Information page.
Activity Instructions:1. Set your multimeter to measure resistance. If you have difficulty remembering how to do
this refer to Instruction Card 2 to review this procedure.
2. Using the procedure you learned in the previous activity, make sure the meter is functioning properly by touching the two probes together. Record your reading.
3. Create a data table in the space provided below that indicates the location of each electrode, the distance between those electrodes (as measured in cm), and the resistancemeasurement (measured in Ω) as indicated by the multimeter.
Resistance in the Human Body (Create your data table here)
4. Apply the electrode gel to two locations on your body. Place the black electrode of the multimeter on one location and the red electrode on the second location making sure that the metal part of both electrodes are touching the gel and in contact with your body. Record this trial in your chart.
5. Repeat step 4 of this procedure making sure you change locations around your body. Make a total of 10 measurements.
Activity Results:Resistance in the Human Body (Create your data table here)
Processing Out:1. Look at the results of your measurements. What general statements can
you make about the relationship between location and/or distance and resistance?____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
2. For each of the locations on your body that you chose to take measurements, is there an equal distribution between bone, muscle and fat?____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
3. List two areas you chose to measure. How did this distribution differ?____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
4. Why might an uneven distribution of bone, muscles and fat produce inconsistent results?____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
5. How could you choose measurement points so the results would be more consistent? Explain your answer.____________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________
