Understanding the Calculation of Body Composition Metrics

Fitness is about more than just burning calories and building muscle. It's about understanding your body and how it changes over time. Gymfile provides several essential measurements that give you a fuller picture of your body's composition: body fat percentage, Fat-Free Mass Index (FFMI), Body Mass Index (BMI), and lean body mass. This article will explain how the app calculates each of these metrics, providing a more accessible understanding of these seemingly complex calculations.

Body Fat Percentage

Body fat percentage is a crucial fitness metric that represents the proportion of your total body weight that is composed of fat. It's a more accurate indicator of your fitness than weight alone because it accounts for variations in muscle mass. A healthy body fat percentage can reduce the risk of many health problems, including heart disease and type 2 diabetes.

In Gymfile, body fat percentage is calculated using a method based on the Jackson & Pollock 3-Site skinfold test. This method is widely recognized and validated in the scientific community for its accuracy and reliability. Here's a more detailed look at how it's calculated:

• For men, Gymfile uses a self-tuned algorithm that incorporates measurements from four different skinfold sites: the chest, abdominal, thigh, and suprailiac (just above the hip bone). The measurements are combined and adjusted using specific multipliers before being squared. The resulting value is then plugged into a formula that considers age as a factor, reflecting the fact that body composition tends to change as we age. Finally, a correction is applied to account for potential errors in the skinfold measurements.

• For women, the formula uses measurements from three skinfold sites: the tricep, suprailiac, and thigh. Just like in the men's formula, these measurements are combined, squared, and adjusted for age. The correction factor is also applied in the same way.

The Jackson & Pollock 3-Site formula is highly regarded for its accuracy in estimating body fat percentage. Research published in the Journal of Sports Medicine (1) found that this method provides a valid estimate of body fat for both men and women when compared to more direct methods of measurement.

However, it's important to note that like all estimation methods, the Jackson & Pollock 3-Site formula may not be 100% accurate for all individuals, especially those with unique body composition characteristics. It's always a good idea to consult with a healthcare or fitness professional for the most accurate body fat analysis.

Body Mass Index (BMI)

BMI is a well-known and widely used index that classifies an individual's weight status. It's calculated by dividing a person's weight in kilograms by their height in meters squared. The resulting number can indicate whether a person is underweight, normal weight, overweight, or obese.

In Gymfile, your BMI is calculated whenever your height or weight measurements are updated, ensuring that it always reflects your current status. This calculation is simple and widely recognized by health organizations worldwide (2).

Lean Body Mass

Lean body mass is the weight of everything in your body that isn't fat: bones, muscles, organs, and body water. Knowing your lean body mass can help you understand how much of your weight is muscle, which can be beneficial if you're trying to build muscle or lose fat.

In Gymfile, your lean body mass is calculated by taking your total body mass and subtracting your total body fat, which is your body mass multiplied by your body fat percentage. This gives you an estimate of the weight of all non-fat tissues in your body (3).

Fat-Free Mass Index (FFMI)

FFMI is a measure of how much muscle mass you have relative to your height. It takes into account the fat-free mass (lean muscle) in your body and compares this to your height, providing a more accurate measure of muscularity than BMI for those with a high muscle mass, such as athletes.

Here's how Gymfile calculates FFMI:

1. It calculates the total lean mass by subtracting the total body fat from the total body mass. This is done by taking the percentage of body fat and dividing it by 100, then multiplying it by the total body mass. The result is then subtracted from the total body mass.

2. Next, the app calculates the FFMI using the total lean mass and your height. It divides the total lean mass by the square of your height (in meters). However, unlike the simple BMI calculation, the FFMI formula includes a normalization factor to account for the difference in muscularity for varying heights. This normalization factor is calculated as 6.1 * (1.8 - height in meters).

The resulting FFMI value gives you a good indication of your muscle mass relative to your height.

These calculations are based on an approach in a study "Body composition interpretation. Contributions of the fat-free mass index and the body fat mass index" (2003) (4). This study demonstrated that FFMI is a more accurate measure of muscle mass than BMI in both athletic and non-athletic populations. And also on clinical guidelines on the identification, evaluation, and treatment of overweight and obesity in adults: The evidence report. (1998). National Institutes of Health (5).