A variety of different techniques and calculations are available for personal trainers when
the appropriateness of a client’s body weight is being assessed. One of the
more popular and easy to calculate measurements is known as Body Mass Index, or
BMI as it is more commonly referred to in the fitness industry. One reason for
its popularity is its ease of use. The most commonly used BMI technique is
known as the Quetelet index. To calculate BMI using this technique all that is
necessary is an individual’s height and weight. BMI can be calculated using one
of two formulas. If the height and weight is available in meters and kilograms,
BMI is calculated by dividing a person’s weight in kilograms by their height in
meters squared. Or, if the height and weight are expressed in inches and
pounds, the alternate formula can be utilized for BMI calculation: the quotient
of weight in pounds divided by height in inches squared multiplied by 703.
Either of these formulas will give an accurate BMI value.
| Weight in kg. | Weight in kg. | ||
| --------------------- | --------------------- | x 703 | |
| Height meters2 | Height in inches2 |
Once the BMI has been calculated, the information needs to be properly interpreted. Before the BMI of your client is compared with the normative data, you must first understand what BMI is and what it is used for. Many people misinterpret BMI as a measure of body fatness. BMI is not a body composition measurement. BMI values do have a relationship with body fatness with a standard estimation of error of 5%. This means that an individual with a body fatness of 15% could be reported to have a body fatness of between 10% and 20%, not a very accurate assessment. BMI does not take into account specific tissue composition or the relationship between fat mass and fat-free mass.
BMI is actually much more reliable and accurate when used as a predictive value of relative risk for disease based upon normative data. Individuals who are heavy in weight for their height often have an elevated risk for disease due to body fatness. Certain populations such as athletes and body builders will have a high BMI, indicating an increased risk for disease (false positive), while it is fairly well received that these individuals are often in excellent physical fitness. The primary reason for a high BMI in these populations is their body weight is elevated due to larger quantities of lean mass. For example, a football player who is 6 ft. 3 in. and weighs 225 lbs. would have a BMI of 28, placing him in the overweight category and having an increased risk for disease. However, that same individual may have a healthy or even low body fat percentage.
When used within the correct populations, BMI can present a very accurate interpretation of the relationship between height and weight, much more so than traditional height/weight charts. The healthy, targeted range for BMI is between 18.5 and 24.9. BMI values between 25 and 29.9 classify individuals as overweight, with a BMI values above 30 categorized as obese. BMI has become more relevant to trainers in recent years as many doctors now use BMI to determine the appropriateness of their patient’s weight. As a trainer explaining or calculating BMI, it is very important to understand and have the ability to disseminate the information about BMI and body fat percentage correctly to clients. Many clients often think that their BMI and their body fat percentage are one in the same. This is incorrect. And as is true with many values, BMI is only useful if employed and explained properly.
Those individuals classified as moderate to high risk (27-30) should understand that these values suggest that they may prematurely develop metabolic or coronary heart disease and that appropriate steps to improve health should be considered. A qualifying test now used in conjunction with BMI is the waist or abdominal girth measurements. It has shown to be an excellent predictor of disease based on regional fat storage. Measured around the line of the umbilicus, values over 102 cm (40 inches) for males and 88 cm (35 inches) for females suggest elevated risk for metabolic disease. Additionally, Waist-to-Hip Ratios (WHR) can be utilized if a tape measure is available. WHR identify central adipocity, a primary risk factor for disease. To calculate WHR simply divide the waist measurement by the hip measurement (widest point). A WHR of greater than 1.0 for males and 0.8 females indicates an increased risk of diabetes, heart disease, and stroke. Individuals presenting high BMI values and elevated girth measures or WHR related to central adiposity should look to change their dietary behaviors, reduce their body fatness and increase participation in physical activities.
