Anthropometric measures of body composition are based on the measurement of two compartments of the body, fat and fat-free mass. Fat-free mass consists of the water, protein, and minerals of the skeletal muscle, non-skeletal muscle, soft lean tissues, and the skeleton.
Fat is a variable component in the body. In women, fat comprises a higher percentage of total body weight (averaging 27%) than in men (averaging 15%).
Body fat is commonly assessed by skinfold thickness. This is an estimated measure of subcutaneous fat. There are various sites on the body where skinfold measurements are taken, and one or more are used to estimate body fat. Triceps skinfold is measured at the midpoint of the back of the left upper arm. Biceps skinfold measures the thickness of the vertical fold in the front upper left arm. Subscapular skinfold is taken below and lateral to the angle of the left shoulder blade. Suprailiac is measured at the midaxillary line superior to the iliac crest. Skinfold measures are taken with an instrument called a caliper that squeezes the skinfold with a pressure of 10 g/mm2 and an area of 20 to 40 g/mm2. There is no real consensus on what combination of skinfold measures best estimates body fat, though experts often recommend measuring skinfolds from the triceps, subscapular, suprailiac, and abdomen.
The waste-to-hip circumference ratio is a measure of the distribution of subcutaneous and intra-abdominal fat. The ratio tends to increase with age and excess weight.
Muscle is the major protein store in the body; a measure of muscle protein can be indicative of body protein reserves. Mid-upper-arm circumference is indicative of muscle mass and subcutaneous fat, and changes in arm circumference are relatively easy to detect. In less developed countries, where people tend to have little subcutaneous fat, the mid-upper-arm-circumference is a good measure of muscle mass and can be used in the diagnosis of protein-energy malnutrition. The measure is taken using a flexible tape, at the midpoint of the upper arm between the acromion process and the tip of the olecranon process.
The measure of the mid-upper-arm muscle circumference is calculated from the mid-upper-arm circumference and triceps skinfold thickness measurements. It is a measure of the circumference of the inner circle of muscle mass around the bone. The measure is used to assess protein-energy malnutrition.