Becoming Fit and Measuring It

November 16th, 2009

Getting in shape can be a herculean task. Trying to decipher the terminology of fitness is a big part of what makes it so daunting. BMI, BF%, creatine, l-glutamine, l-argine… what do those things even mean? In this article I will tackle the two most often metrics used to gauge fitness: body mass index (BMI), and body fat percentage (BF%). In simple terms, you’ll discover what they mean, and find out why understanding them is so critical to success.

Body Mass Index (BMI) Fact Sheet

What BMI measures: BMI gives a quick-and-dirty numeric representation of how heavy you are for your height. “Normal” is between 18.5 and 25 on the BMI scale. Over 25 is considered overweight. Over 30 is considered obese.

Click to enlarge.

How to measure BMI: The calculation is simple: weight divided by height squared (using metric units). Easier still, locate your dimensions on the chart shown.

Pros of BMI:

• Quick and easy to measure
• Provides a comparison against statistical norms

Cons of BMI:

• Doesn’t consider if weight comes from muscle, fat, bone or water
• Can be particularly misleading when measuring athletes, children or the elderly
• Often misused as a “fatness” gauge

In short: BMI gives a number that represents how “normal” your weight is for people of your height. While the results are reasonably consistent, there are several exceptions — and misconceptions — that reduce BMI’s usefulness.

Body Fat Percentage (BF%) Fact Sheet

What BF% measures: Body fat percentage represents how much of your total weight is due to fat. “Normal” ranges are different for men and women. Men usually fall between 14 and 22% body fat, and women between 21 and 31%. A percentage of body fat is “essential” (2-12% depending on genetics), meaning it cannot be shed through exercise without serious health risks.

How to measure BF%:

• Measurement and calculation: By simply collecting bodily measurements and performing calculations, you can determine your BF%. These calculations can give inaccurate results in certain cases, and the required measurements are tedious to perform. Nevertheless, this method doesn’t require special equipment, other than a measuring tape.
  • Accuracy grade: C
  • Daily usefulness grade: C
• Calipers: Pinching a fold of skin and measuring the resulting bulge of fat can provide useful body fat information. In order to get a final BF% you would need to measure in many different locations and perform a calculation to combine the results. Even with experience there is room for error, and different calculations produce different results. Many skip the calculations altogether and simply track the individual measurements of fat. Calipers are inexpensive and easy to find.
  • Accuracy grade: C-D (depending on caliper quality and proper use)
  • Daily usefulness grade: B
• Body fat measuring scales: Using a principal called bioelectrical impedance analysis (BIA), these scales send a small, painless electrical charge through your body and measure the result. Body fat resists the charge more than muscle, helping to determine your BF%. Results are ready in seconds with a simple push of a button. These scales are more expensive than calipers, trading price for speed and ease of use.
  • Accuracy grade: B-C (depending on scale model and proper use)
  • Daily usefulness grade: A
• Impractical alternatives: Other methods exist, although their availability is limited to experts in well-funded organizations. These methods include hydrostatic weighing, air displacement measurement, near infrared interactance (NIR), and even dual energy x-ray absorptiometry (DEXA). These systems exchange convenience and price for superior accuracy and are not designed for home use.
  • Accuracy grade: A
  • Daily usefulness grade: F

Pros of BF%:

Many measurement options, including quick and easy methods
• Focuses on body fat composition, not just weight vs. height

Cons of BF%:

• Requires equipment for practical daily use
• Varying accuracy
• Not appropriate to compare BF% with others

In short: BF% gives a number that represents how much of your weight is made up of fat. The measurement is non-transferrable as certain body types require different body fat percentages. Attempting to match someone else’s BF% measurement may be unhealthy.

Visualization Tools

BMI

Generally, BMI categorizes people reasonably well. However, there are notable exceptions.

Underweight (BMI < 18.5)	Overweight (BMI > 25)	Obese? (BMI > 30)
Flickr: S. Witherden	Flickr: mikebaird	Obese?

At the peak of his bodybuilding career, Arnold Schwarzenegger was 235lbs and 6′2″. This made his BMI 30.2; obese, according to the World Health Organization’s classification system.

BMI is best used to gauge weight vs. height. Don’t think of it as a fatness vs. fitness measurement.

BF%

Here I am. On the left, ~175lbs, 18% body fat. On the right, ~155lbs, 10% body fat. Both measurements put my BMI in the “normal” range, but the grimace I’m making on the left speaks volumes to my self-satisfaction at the time.

The Best Metric for Fitness?

While BMI requires no equipment other than a scale, it’s not suitable for some people, notably athletes. It’s a good method to gauge an average, inactive person’s weight, but only if it is interpreted with a grain of salt. When you’re starting out, there’s something motivating about knowing you qualify as “overweight” or “obese” on a widely used scale. Nevertheless, as fitness level increases, the usefulness of BMI tends to decline.

I believe BF% is the better metric to focus on when serious about getting in shape, provided you set healthy, personalized goals. Why? According to the BMI scale I was “normal” in the photo on the left, but I felt just terrible. Although I’m approaching “underweight” on the BMI scale in the photo on the right, I’m definitely in much better shape, and feeling much better too.

I also found body fat percentage more relatable. What does a BMI of 30 mean? Nothing really; it’s an arbitrary grading number. 30% body fat, however, is tangible. It means your body is approximately one-third comprised of fat, much of which is the globulous, gelatinous variety that looks bad and shortens your lifespan. Having reached as high as 29% body fat and just short of an “obese” BMI rating, I can attest to the motivational power of these metrics. According to BMI and BF%, I was in rough shape. However, as BMI’s usefulness declined, my target BF% gave me a unchanging goal to work towards.

As you can see, BMI and BF% are nuanced tools for representing fitness level. Very often they’re reported side by side since they both have their own individual pros and cons. If you truly want to get in shape, taking the time to learn their differences can prove very useful. For the majority of the population, BMI is an easy way to guide yourself from overweight to normal, however as you become more athletic, BF% becomes a more useful consideration. Whatever your fitness level, understanding these measurements can help you achieve your fitness goals, but not as much as exercise. So get up, and get working on your numbers!