Body composition

Body composition describes how much of a client’s weight is fat mass and how much is fat-free mass, the two compartments used by most methods available to trainers.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 278.},^{2Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 176.} Four-component models separate fat, protein, mineral and water and are considered the gold standard for indirect assessment.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 67.}

Excess body fat, especially around the abdomen, is linked to hypertension, type 2 diabetes, stroke, cardiovascular disease and many cancers.^{2Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 176.} Aging tends to bring more fat, less bone mineral and less muscle, so body composition measurements can help monitor lean mass in older clients.^{2Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 176.}

In practice

• Choose a method with its cost, complexity and accuracy in mind.^{2Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 176.} With a trained tester, skinfold assessment is the most valid and reliable option most trainers can access, though DXA and underwater weighing are often called the gold standards.^{1Haff and Triplett (eds.), Essentials of Strength Training and Conditioning, 4th ed., p. 278.}
• The tester should be trained and have demonstrated reliable results before assessing clients.^{4Ozemek et al., ACSM’s Guidelines for Exercise Testing and Prescription, 12th ed., p. 177.} Follow the same protocol every time.^{5Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 217.}
• Don’t judge a program by BMI alone: it can’t distinguish fat from muscle, so it can misclassify muscular clients and miss risk in clients with little muscle.^{6Schoenfeld and Snarr (eds.), NSCA’s Essentials of Personal Training, 3rd ed., p. 219.}
• Read changes during a diet with care: models that assume a fixed water content in fat-free mass can report water lost from fat tissue as lost fat-free mass.^{3Schoenfeld, Science and Development of Muscle Hypertrophy, 2nd ed., p. 67.}