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Body Mass Index (BMI)

February 12, 2018

Categories: Assessment, Exercise Science
Tags: , ,

This post is the first in a series intended to build an online fitness toolbox.  While it is expected that these tools will be helpful to fitness trainers, they are also accompanied by explanations and instructions for their use.  In our first edition we offer a description and tool for calculating Body Mass Index.  

The body mass index (BMI) is a value derived from the mass (weight) and height of an individual. The BMI is defined as the body mass divided by the square of the body height, and is universally expressed in units of kg/m2, resulting from mass in kilograms and height in metres.  The BMI is an attempt to quantify the amount of tissue mass (muscle, fat, and bone) in an individual, and then categorize that person as underweightnormal weightoverweight, or obese based on that value.  Commonly accepted BMI ranges are listed below:

BMI Standards
Underweight  <  18.5
Normal  18.5 – 24.9
Overweight  25.0 – 29.9
Obesity, Class I  30.0 – 34.9
Obesity, Class II  35.0 – 39.9
Extreme Obesity, Class III  >  40

 

Just fill out the form below and click on ‘Calculate BMI’ to find out your individual BMI score:
[ninja_form id=6]

Limitations of BMI

BMI offers a good guide as to overall physical wellness based on statistical averages alone.  However, the medical establishment and statistical community have both highlighted the limitations of BMI.  A few of these important limitations are provided below:

Scaling

The exponent in the denominator of the formula for BMI is arbitrary. The BMI depends upon weight and the square of height. Since mass increases to the 3rd power of linear dimensions, taller individuals with exactly the same body shape and relative composition have a larger BMI.  The mathematician Prof Nick Trefethen, who observed this said, “BMI divides the weight by too large a number for short people and too small a number for tall people. So short people are misled into thinking that they are thinner than they are, and tall people are misled into thinking they are fatter.”  In a nutshell, if you’re shorter or taller than average, the calculated BMI may not be a particularly good way to judge your overall fitness level. 

Ignores variation in physical characteristics

The BMI overestimates roughly 10% for a large (or tall) frame and underestimates roughly 10% for a smaller frame (short stature). In other words, persons with small frames would be carrying more fat than optimal, but their BMI reflects that they are normal. Conversely, large framed (or tall) individuals may be quite healthy, with a fairly low body fat percentage, but be classified as overweight by BMI.

Does not differentiate between muscle mass and fat mass

Assumptions about the distribution between muscle mass and fat mass are inexact. BMI generally overestimates adiposity on those with more lean body mass (e.g., athletes) and underestimates excess adiposity on those with less lean body mass.  BMI is particularly inaccurate for people who are very fit or athletic, as their high muscle mass can classify them in the overweight category by BMI, even though their body fat percentages frequently fall in the 10–15% category, which is below that of a more sedentary person of average build who has a normal BMI number.

Variation in definitions of categories

It is not clear where on the BMI scale the threshold for overweight and obese should be set. Because of this the standards have varied over the past few decades. Between 1980 and 2000 the U.S. Dietary Guidelines have defined overweight at a variety of levels ranging from a BMI of 24.9 to 27.1. In 1985 the National Institutes of Health (NIH) consensus conference recommended that overweight BMI be set at a BMI of 27.8 for men and 27.3 for women.

For additional reading you may want to start at Wikipedia’s introduction and sources for the subject  Body Mass Index.

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