Bioelectrical impedance analysis (BIA)

Bioelectrical impedance analysis (BIA) estimates body composition by passing a small, painless current between electrodes, often at the wrist and ankle.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 224.} Water and its electrolytes conduct well, so the device uses impedance to estimate total body water, then fat-free mass and body fat.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 224.} Fat tissue holds little water and impedes the current; muscle conducts it well.^{2Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 193.} Devices measure hand-to-hand, foot-to-foot or hand-to-foot, and inexpensive home units use the manufacturer’s own equations.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 224.}

When the protocol is followed, accuracy is similar to a skinfold assessment, with typical errors of 3% to 4% body fat.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 224.},^{2Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 193.} BIA also removes the dependence on the tester’s skinfold skills.^{3Hoffman, NSCA’s Guide to Program Design, p. 64.} It tends to overestimate fat in lean clients and underestimate it in clients with obesity, with individual errors as high as 5 kg (11 lb).^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 224.}

In practice

• Dehydration reduces conductivity and makes the body fat reading higher than it should be.^{4Hoffman, NSCA’s Guide to Program Design, p. 65.}
• The NSCA’s pretest protocol calls for no food or drink for 4 hours, no exercise for 12 hours, no alcohol for 48 hours and no diuretics for 7 days, plus voiding within 30 minutes of the test.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 224.} It also says not to test a woman who feels she’s retaining water during her menstrual cycle.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 224.}
• Match the equation to the client’s age, sex, ethnicity, activity and body fat; generalized equations are often inaccurate across ethnic groups.^{1Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 224.}