Probably the quickest, least invasive method of estimating body composition is bioelectrical impedance analysis (BIA). It provides estimates of percent bodyfat (%BF) that are similar to those obtained through skinfold assessment.
As the name BIA implies, a low-level electrical current is passed between sending and receiving electrodes. The ability of the body's tissues to oppose the flow of that current is monitored and used to predict the person's fat-free mass (FFM).
'Impedance' is the term used when referring to the opposition to the current; mathematically, it is represented by the symbol Z.
The water within the human body helps to conduct the flow of the electrical current; another way to say this is to say that water does not impede the electrical current and that current will move quickly from the sending electrodes to the receiving electrodes.
Body fat, on the other hand, is anhydrous and, therefore, impedes the electrical current as it attempts to move through the body. Since the amount of water in the adult human body is assumed to be approximately 73%, we can estimate the FFM once we have a good idea of what the total body water amount is.
People with a large FFM and TBW provide less resistance to the electrical current. Conversely, people with a high fat mass and low TBW provide much more resistance to the same electrical current.
There are several varieties of bioelectrical impedance analyzers, all of which are heavily influenced by the participant's hydration level. Therefore, it is important to follow the standard BIA pretest guidelines:
The shoe and sock need to be removed from the right side foot. The client or patient should then lie supine on a non-conductive surface. The arms and legs should be abducted approximately 30-45° so that the thighs do not touch each other and the upper arms do not touch the torso.
Hands need to be pronated so that the dorsal surface of the hand and wrist is exposed. Then the test administrator needs to use an alcohol pad to clean the area between the styloid processes of the radius and ulna; using that as a reference point, measure a distance of at least 5 cm and then clean that area on the back of the right hand.
Similarly, the test administrator will use an alcohol pad to clean the area midpoint between the malleoli forming the ankle joint. Using the malleoli as the landmark, measure a distance that is at least 5 cm from there and clean that area on the dorsal surface of the foot.
Now place an electrode in each of the four spots that were cleaned. Start with the first area cleaned and proceed, in sequence, to the last so that the alcohol is dry before you position the electrode. Connect the red lead wires to the proximal electrodes on the wrist and ankle. Connect the black lead wires to the distal electrodes on the hand and foot.
Enter the proper height, weight, age, and gender into the analyzer. Visually re-check the abduction angle of the arms and legs as well as connection of the lead wires from the analyzer to the electrodes. Then start the analyzer. Be sure no one touches the client or patient while the analyzer is introducing the electrical signal. Once the analyzer has the information it needs, you need to record the data displayed.
For this type of bioelectrical impedance assessment, the client or patient stands upright and gently, but firmly, grasps the handles of the analyzer and holds it in front of them - like driving a car. But first, the test administrator needs to enter the height, weight, age, gender and activity level into the handheld analyzer. Then press "set" and have them get ready.
The test administrator needs to be sure that the client's feet are shoulder-width apart (shoes can be on) and the arms are parallel to the floor. The electrodes should be covered by the client's hands and thumbs to the greatest extent possible. Press the "start" button and stand back. In approximately 7 seconds, a result will be displayed on the analyzer for you to record.