Body Mass Index BMI

The World Health Organisation (WHO) determines obesity and being overweight by using the Body Mass Index (BMI) scales.

BMI is an effective measure of weight status at a population level but can be less accurate for assessing healthy weight in individuals, especially for certain groups (e.g. athletes, the elderly) where a slightly higher BMI is not necessarily unhealthy.

For children the situation is more complicated. There is no fixed BMI to define being obese or overweight since this varies with gender and with growth and development. Parents can get an indication of their child’s weight status by checking their position on a standard growth chart but should seek expert guidance before acting on such information.

BMI is measured by dividing a person’s weight (in kilograms) by the square of their height (in metres).

Formula

Weight (KG) / Height (Meters²)

E.g. for a person who weight 70kg and is 1.78 meters tall

70/ (1.78²)

BMI = 22.09

The calculation produces a figure that can be compared to various thresholds that define whether a person is overweight or obese.
For adults these thresholds are:

Hip to Waist Ratio

BMI is often supplemented by measuring waist circumference and Hip circumference and by considering individual circumstances. The hip to waist ratio gives an indication of where fatty deposits are located. Indications suggest a larger amount of fat located in the upper body when compared to the lower body will predispose an individual to a greater risk of disease. High hip to waist ratios have been associated with heart disease, hypertension and stroke.

It is calculated by measuring the circumference of the natural waist which is located two fingers above the Iliac crest or top of the hip. Divide this measurement by the hip circumference at the greater trochanter of the femur which is generally the widest part of the hip. The ratio is applied both to women and men.

Formula

Hip circumference / Waist circumference

Female

Male

Heart Rate Zones

The Heart rate zones are a critical component of any training program, these zones will allow for different physiological responses from your body. The Karvonen formula uses the heart rate reserve to calculate training zones based on both maximum and resting heart rate. Heart rate reserve is simply the difference between your maximum heart rate and your resting heart rate.

Formula (males)

220 – age = maximum heart rate
Maximum Heart rate – Resting Heart Rate = heart rate reserve
(Heart rate reserve x training %) + resting heart rate = heart rate target

Formula (Female)

226 – age = maximum heart rate
Maximum Heart rate – Resting Heart Rate = heart rate reserve
(Heart rate reserve x training %) + resting heart rate = heart rate target

Resting heart rate is take when the body is at rest, which is generally in the morning just as you have woken up.

Here is an example for a 50year old male with a resting heart rate of 65bpm who wants to train at 70% maximum

• 220 - 50 = 170bpm (maximum heart rate)
• 170 - 65 = 105bpm (heart rate reserve)
• (105 x 0.7) + 65 = 139bpm