How to Measure Your Energy Expenditure and Fitness in Detail

Just how fit are you? Most of us probably think we have a pretty good idea of our health based on how out of breath we are when we’re climbing the stairs or how we look in front of the mirror when our clothes come off.

But while these measures might give you a good hint as to your general strength and fitness, they won’t necessarily be particular accurate. If you’re a serious athlete and you want to know how you really measure up, then you need to get technical. That means doing math, running tests and getting nerdy. But at the end of it, you’ll have a much better idea of how you stack up and where you need to improve.

So let’s take a look…


The most basic place to start would be with your basic statistics, such as your:

  • Age
  • Height
  • Weight

This right away allows us to paint a basic picture of what someone looks like and whether they might be overweight.

Body Fat Percentage

We now need to try and work out body fat percentage. There is a very big difference between someone who is 176lbs of muscle, versus someone who is 176lbs of fat.

There are numerous ways to calculate body fat percentage but a good start would be to take a ruler, hang your arm out at your side and then find the halfway point between the should and elbow (the side of the tricep).

From here, you’re then going to pinch a fold of skin and fat away from the muscle and then measure the thickness of that skin. You can then refer to the following table to identify that body fat percentage:

Skin fold thickness (mm)

Body fat % Men

Body fat % Women
6 5-9 8-13
13 9-13 13-18
19 13-18 18-23
25 18-22 23-28
38 22-27


This isn’t an exact method and if you have access to a more accurate methodology, then you should use it.

Now we have an accurate indicator of your height, weight and size and we can visualize how much of that is likely to be muscle versus fat.

Basal Metabolic Rate

Using this information, we can now get a better idea of our basal metabolic rate (BMR). This is an idea of how many calories you burn in a day, before you move at all. In other words, this is how much energy in calories it takes to power your body’s basic functions like blinking, breathing, maintaining your immune system etc.

If we were being really fancy, then we could use a metabolic cart in order to calculate our BMR (1). But seeing as you don’t have that option, instead we’ll use an equation called the ‘Katch-McArdle Equation’. While there are various different types of equation, this is the best one because it takes body fat percentage into account. This is important because muscle is more ‘metabolically active’ than fat, meaning that it will burn more calories even when not in use.

First we need to calculate our lean body mass. This is simply the amount of body mass that isn’t fat (not counting fat in the brain etc.). So that would be our weight, minus the body fat percentage. If you weigh 179lbs and you have a body fat percentage of 10%, then your lean body mass is 161.1lb. Really easy!

Our BMR is now calculated as either:

BMR = 370 + (9.79759519 x LBM(lbs))


BMR = 370 + (21.6 x LBM(kg))

When I enter my details into the calculator, I have a BMR of around 1,921.

We can also use this to calculate how many calories we burn an hour or a minute when we’re not active. So if my BMR is 1921, then my calorie burn is 80 per hour and 1.33 per minute.

Active Metabolic Rate

From there, the usual way to calculate the active metabolic rate (AMR) – the amount you burn when you take movement into account, is simply to multiply your amount by how active you are.

You can do this by referring to the following table:

  • 1.2 if you’re sedentary (little or no exercise)
  • 1.375 if you’re lightly active (you exercise 1-3 times a week)
  • 1.55 if you’re moderately active (you exercise or work about average)
  • 1.725 if you’re very active (you train hard for 6-7 days a week)
  • 1.9 if you’re highly active (you’re a physical laborer or a professional athlete)

If you feel you don’t quite fit any of these descriptions, then you can feel free to pick something ‘in between’ using your best estimate. This is a rough calculation for sure but it’s also the best average you can take without using a lot of equipment. Mine is around 3,073.

Heart Rate

Now let’s take a look at our maximum heart rate (MHR) and resting heart rate (RHR). The easiest way to calculate these figures is to simply test our hearts. If you wear a fitness tracker fitted with a heart rate monitor, then you can measure your heart rate during sleep and it will usually indicate the lowest your heart rate drops. Otherwise, you can take a heart rate reading as soon as you wake up in the morning, either with a device or by monitoring your own pulse with a finger.

For maximum heart rate, you simply need to try pushing yourself as hard as you can with high intensity exercise and measuring the highest point – again, either with a device or with your own fingers. This will then tell you the highest BPM you can reach.

These numbers relate to your stroke volume – that’s the volume of blood you pump from the left ventricle. As you train with cardiovascular training, you will enlarge your left ventricle, thereby enabling it to pump more blood with fewer beats per minute.

VO2 Max

We can also use this information to calculate our VO2 max. This is how much oxygen (in litres) your body can use per minute. This correlates closely with athletic performance as it allows you to more effectively burn fat stores for energy during aerobic exercise. Meanwhile, a lower resting heart rate can also lower levels of stress and cortisol.

While this is again a very rough estimate and there are definitely better ways to calculate it if you have the necessary equipment, you can make a good estimate of VO2 max using the following equation:

VO2max = 15.3 x (MHR/RHR)

If we were to do this properly, we would look at the arteriovenous oxygen difference, which is the difference between the oxygen in the veins, versus the oxygen in the arteries.

According to this formula, mine is 54.849, which is a quite a lot higher than estimated by my Microsoft Band 2 (which puts it at 45.1).

Lactate Threshold

Another interesting factor that this relates to is your lactate threshold. While lactate is no longer blamed for muscle fatigue (it is now generally agreed that this is caused by a build-up of other metabolites), it is nevertheless true that muscle fatigue will generally correlate with higher levels of lactate in the blood. What’s more, is that a build-up in lactate sends a signal to the brain that can make us feel nauseous, thereby forcing us to resume a more sustainable level of intensity after just a couple of minutes.

Lactate itself is actually a good thing though because the body is able to recycle it for use as an energy source. For highly trained athletes, this energy source is actually preferred to glucose. An athlete with a higher lactate threshold can maintain a faster running speed (their ‘running speed at lactate threshold’ or RSLT) without depleting glycogen stores and without experiencing fatigue because they are able to reuse the lactate more efficiently.

On average, your lactate threshold should be around 85% of your MHR. So if your MHR is 196, that means your lactate threshold (also called the ‘lactate inflection point’) will be around 166. To calculate your RSLT, you can either use a fitness tracker and monitor your speed while trying to maintain that heart rate, or you can conduct the ’30 minute test’. Here, you simply run as fast as you can for 30 minutes, aiming to be completely fatigued once that time is up. You then divide your average speed by the distance covered (divide the meters by seconds). So if you complete 8,000 meters in 30 minutes, then your RSLT is somewhere around 4.5 meters per second. That’s a pretty good score though…

While you might not be able to quickly get your RSLT up to that speed, training at tempo will help you to improve the figure.

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