running training plan

BMI (body mass index) calculator

First of all, what is BMI?

Simply, a person's BMI, or body mass index, is a numerical measure of a their weight compared to their height.
It's primary use is within the health professions as an objective assessment of a patient's weight.

Because of its simplicity it is often inappropriately used to determine a person's 'fatness' or 'thinness'. Importantly, it does not take into consideration the body's composition, for instance, a well-trained body-builder with large muscle bulk would be given a BMI that might suggest they are over-weight. Conversely, the very weak-limbed, huge tummy physique might return a healthy BMI index.

In most people's eyes, Dwayne 'The Rock' Johnson looks fit, exceedingly fit. Yet his BMI places him very firmly in the obese category.

BMI and runners

As far as runners are concerned (assuming they are not very muscle-bound), it does give a reasonable indication of where you fall on the scale of a 'healthy' weight, especially when combined with an honest look in the mirror.

To find your BMI, enter your measurements below and look at the scale to see where you're at.


What does your BMI mean?

Below 18
Underweight, although some elite runners fall into this category.


How to get more comprehensive body composition measurements

The BMI calculator above will provide you with your Body Mass Index, but if you want a more detailed information about your body composition then you can use weighing scales designed to do just that. There are various weighing scales available that provide BMI measurements as well as assessments of: skeletal muscle, Visceral Fat, resting metabolism rate and body fat.

However, the mirror and the way you feel are often all you need to gain an accurate idea of your body composition.

How does your BMI affect your running?

If BMI is a measure of your weight with regards to your height, then are there any guidelines to help you determine how much running you should do or how far your run?

Not surprisingly, a heavier load on joints and connective tissue will place greater strain on those parts of the body that are susceptible to running injury. Any biomechanical issues that make a runner more injury prone will increase as the load increases. This is true not just for weight but also for speed. As we run faster, greater loads are put through our joints, muscles, tendons and ligaments. Similarly, if these tissues are carrying heavier loads then they will be more prone to breakdown.

For example, hip alignment and strength have great influence on injury risk. Weak hips that allow too much collapse on impact are likely to allow too much movement in the other supporting structures of the body. If you stand on one leg and allow the hip to relax, you will notice how the pelvis twists, placing greater stress on the spine. But it's not just the obvious effect, the knee and the ankle will also be affected. Indeed, research shows that weak hip strength is a factor in the most common running injury: runner's knee. Naturally, hip strength becomes more vital as they weight they carry increases.

Runners with a high BMI, especially those new to running, should be particularly careful not to overload their training; start slow, over short distances and seek help if any injuries or niggles develop. It is far better to get these things diagnosed and sorted out before they sideline you for weeks. This is specifically important for those who have quite well developed fitness, perhaps from another sport. For example, a rugby player might have a very high BMI and also be extremely fit. A rugby player's body is likely to be well developed and strong, and not unhealthy at all. However, if they have not done much endurance running then they are asking a lot from some connective tissues that get a tough workout from running, but are not necessarily worked much during their rugby training. Always be mindful of not doing too much too soon. This is vital for people who are very fit and able to run fast and perhaps even for long distances. The body needs time to strengthen the running-specific structural supports.

How does running affect BMI?

Running will not necessarily reduce your BMI. Assuming satisfactory and sensible nutrition, regular running will develop muscle and burn calories. You are likely to exchange fat for muscle. This is rather an over-simplification, but if you exchange fat for muscle your weight will increase (muscle is the denser tissue), and so too will your BMI (assuming you don't get any taller). However, for many runners, what happens is that as they become fitter and more durable runners, they run further and burn more energy. If they don't over-compensate by eating too many cakes then they lose weight, and hence lower their BMI.

Is there an amount of running you should do according to your BMI?

No, we are all different, and there is no set amount of running you should do according to your BMI. The sensible guidance for those with a high BMI is to get checked out by a health professional before undertaking any exercise programme. Start slowly and carefully, walk if you need to, and increase distance and frequency gradually, all the time monitoring what's going on with your body. If you've not been exercising at all, then do some strength training too; 15 minutes of leg exercises will almost certainly do you more good than 15 minutes more running.

In summary, as mentioned above, the mirror (or a very good and tactful friend) is probably a better judge of your body composition than BMI alone.

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