Ace Body Fat Measurement Methods

According to body mass index (ratio of height to weight), a muscular athlete may register as obese, while a lighter person could fall within the recommended range and still be "overfat." Body composition testing gives a more accurate picture of a client's body because it takes into account his proportion of fat to fat-free mass (muscles, bones, etc.). The American Council on Exercise (ACE) recommends a body fat percentage of between ten and 25 percent of total body weight for men, and between 18 and 33 percent for women.

Skinfold Measurements

Skinfold measurements estimate body fat percentage based measurements of a client's subcutaneous (below-the-skin) fat. The trainer uses calipers to measure the width of skinfolds at three points on the same side of the client's body. For men, measure a diagonal skinfold halfway between the top of his armpit and nipple, a vertical fold one inch to the left or right of his belly button and a vertical fold on the front of his thigh halfway between his hip and knee. For women, take a vertical fold on the back of her arm at the midpoint between the shoulder and elbow, a diagonal fold just inside her iliac crest (the point at the front of the hip bone) and a vertical measurement on the front of her thigh halfway between her hip and knee. ACE provides tables to determine body fat percentage based on the sum of these three measurements (see Resources). Skinfold measurements have a three percent margin of error when taken by an experienced personal trainer.

Bioelectrical Impedance

Bioelectrical impedance devices work on the principle that muscles are mostly water (which is conductive) and fat is mostly non-conductive. To measure body fat, electrodes send an imperceptible current from one body part to another. Based on the time it takes for the impulse to pass through your body, the machine estimates body composition. ACE's Personal Trainer Manuel recommends placing the electrodes on the client's hand and foot, because this method takes into account uneven fat distribution in the upper and lower body. Several alternative devices have gained popularity in recent years. Hand-held machines pass an impulse from the palm of one hand to the other. You can also buy impedance scales, on which the current is measured in the sole of each foot. The margin of error for the best devices is 3.5 percent, but the error on individual models may vary according to the equipment's quality. For bioelectrical impedance measurements to be accurate, the client should be fully hydrated.

Hydrostatic Weighing

Hydrostatic weighing is considered the "gold standard" for measuring body mass because it is accurate to within 2.7 percent. During hydrostatic weighing, a client is suspended in a tank of water, and her weight is compared to her dry-land body weight in order to determine body density, and from there body fat. The best hydrostatic weighing methods also take into account the air remaining in the client's lungs after she has exhaled fully.