**Overview**

This Blog-post is about how you
can use a power-meter to estimate energy expenditure (a.k.a. calories burnt) during
a bike ride. The main aim is to debunk some misconceptions and to shed light on
the link between mechanical work and metabolic energy expenditure.

The core of this post will be
broken down in three sections.

1. The
link between mechanical work and metabolic energy expenditure.

**(THEORY)**
2. The
use of power-meter data to estimate energy expenditure, differentiating between
kilojoules of work and kilocalories of energy expenditure.

**(PRACTICE)**
3. How
your Garmin cycling-computer might be calculating your energy expenditure.

**(ANALYSIS)****Introduction**

One of the several uses of a
power-meter is to estimate energy expenditure. While the information derived
from power meters makes easy to measure the amount of mechanical work, the
relationship between mechanical work and the energy expenditure is not obvious.

This information is particularly
important when integrating power-meter derived information onto different
nutritional (refuelling or weight loss/control) practices.

The aim of this post is to debunk
some misconceptions and define some terms and concepts necessary to better
understand energy expenditure. This post will be followed by a post on weight
loss, performance and health.

**1.**

**The link between mechanical work and metabolic energy expenditure**

Turning over the pedals of the
bike (mechanical work) is possible thanks to the biochemical events that happen
inside the skeletal muscle to generate energy (metabolic energy). Consequently,
metabolic energy is ultimately turned into mechanical work, but a part of the
metabolic energy is turned into heat (‘wasted energy’).

**Efficiency**is an important concept that is used to indicate what percentage of the metabolic energy is turned into actual work and what percentage is turn into heat.

A process that is 100% efficient generates
no heat. To understand this better I like to think about lamps and cyclists
(Figure 1).

Figure 1. A schematic
representation of 3 different systems with different efficiencies. A tungsten
lamp (2.5 % efficiency), a LED lamp (15% efficiency) and a cyclist (21%
efficiency).

The input of energy for lamps is
electricity, tungsten lamps are probably the least efficient lamps in the
market at the moment and they generate a lot of heat (I dare you to hold a tungsten
light-bulb with your bare hand to test this). LED lamps instead are more
efficient (this varies hugely depending on the type of LED lamp) and generate
less heat (you can touch them without getting burnt).

In a way, a cyclist can be
interpreted as the light-bulbs. To some extent we can say that the metabolic
energy generated is analogous to electricity, turning over the pedals is
analogous to the light generated by the light-bulbs and heat is heat.*

Power-meters measure mechanical work
and not metabolic energy expenditure, therefore to calculate energy expenditure
during a ride, it is necessary to know how efficient a cyclist is.

**2.**

**Using a power-meter to estimate metabolic energy expenditure**

Now that you understand the relationship between metabolic
energy, heat and mechanical work, and the concept of efficiency it is time to
jump into the practical application of this.

As you well know power meters
measure power, mechanical power, but what we want to know is the

**metabolic energy**to generate**mechanical work**. Power is the amount of work done (or energy used) in a unit of time. By convention it is measured in Watts (W) which is Joules per second (1 W = 1 Joule/s). So, by knowing the amount of power held for a given amount of time you can know the amount of**mechanical work**completed.**Example:**Frank Lampard rides his bike for 1 h (or 3600 seconds) at an average power of 150 W (or 150 joules/s). Frank has done 540000 Joules (or 540 kilojoules) of

**mechanical work**.

However, as explained above, mechanical
work is not equivalent to metabolic energy. To know how much metabolic energy
(derived from fat and carbohydrates metabolism) has been used to do this amount
of mechanical work it is necessary to know the efficiency of the process. Efficiency
varies between people. Therefore it needs to be measured for each cyclist, and
it requires specialized equipment (an ergometer and indirect calorimetry
metabolic cart). Thankfully efficiency is fairly constant in standard
conditions and it has been shown to be in average 20.7% (range 18.3-22.6%) for well-trained
males [1] and 19 % for very
well-trained females [2]. Therefore, despite measuring
is the best way to accurately make any estimations, it can be assumed that you
fall somewhere close to these average values. This makes things easy for estimating
the actual

**metabolic energy**spent on a ride.**Example:**Frank Lampard did 540 kJ of mechanical work during his 1 h ride. How much metabolic energy did Frank have to generate for his ride? Assuming Frank’s efficiency is 20.7%, he used around 2609 kJ of metabolic energy (540 x [100/20.7]).

In short, assuming an efficiency of 20.7% you can estimatemetabolic energy expenditure by multiplying mechanical work by 4.8 (100/20.7).

The seemingly simple outcome of this simple calculation
can become very confusing when taken into consideration that different units,
calories and joules (or kilocalories and kilojoules), are used to refer to energy
expenditure.

**A problem with terminology and units, Joules and calories:**

The calculations above have been
done using Joules. Now what is the role of the calories here? Calories are
another unit to measure energy. So joules (J) or calories are interchangeable, 1
calorie is 4.184 J. It is analogous to comparing kilometres to miles. Joules
(like meters and Watts), belong to the International System of Units (SI), and
calories don’t.

On the example above Frank’s
metabolic energy expenditure is 2609 kJ. Therefore, this amount is equivalent
to 623 kcal. However this can be confusing as this amount is remarkably close
to the 540 kJ of mechanical work that Frank did (Figure 2, top panel). This can
so confusing that some manufacturers of cycling computers seem to not have
quite understood it (see below).

Many people think or assume that
the value obtained in kJ from the power-meter computer is the amount of ‘calories
burnt’. It is easy to say that the value of metabolic energy expenditure can be
calculated by changing the unit of the value of mechanical work from kJ to
kcal, but this leads to incorrect results and it is not correct conceptually
either (Figure 2).

Figure 2. Two ways of estimating
metabolic energy expenditure, the right way and the wrong way.

**3.**

**How your Garmin cycling-computer might be calculating your energy expenditure**

Most cycling computers provide an
estimate of energy expenditure based on different measures. As discussed above,
using information coming from the power-meter meter is one way to do it and a fairly
straightforward one. However this is not the only way in which cycling computers
estimate energy expenditure. Moreover, cycling computers can be wrong and provide
misleading information.

Before I proceed I must make
clear that the information of this section applies to Garmin Edge 800 units
(the one I own and used to get the information for this post). You should check
how your brand and unit model works before you generalize what I describe here.
I do not hold anything against Garmin or firstbeat companies and it is not the
point to denigrate them but to share my findings about how their products work
according to my findings. Any comments are expected to be taken as constructive
criticism.

On my garmin unit I can set the
screen to display Calories and/or kilojoules. While ‘kilojoules’ indicates the
amount of

**mechanical work**, ‘Calories’ is supposed to indicate the**metabolic energy expenditure**. Let’s jump straight into the problem and see what the computer shows after two different rides (Figure 3).
Figure 3. Mechanical work (Kilojoules)
–green boxes- and Energy expenditure (Calories) –red boxes- displayed on my
Garmin Edge 800 unit when used after a short ride with (left) and a long ride
without (right) a heart rate (HR) strap.

So we can see that there are two
problems here:

1) When
used with a HR strap it underestimates energy expenditure (my estimation says
it should be somewhere around 1732 kcal, 44% higher).

2) When
used without a HR strap it also underestimates energy expenditure (my
estimation says it should be somewhere around 4209 kcal, 13% higher).

While it is clear what it is
doing wrong in scenario #2 (it simply converts the value of mechanical work (in
kilojoules) into energy expenditure (Calories) by just changing the unit –as I
what I say in Figure 2 is the wrong way of doing it-), it is not so clear what
it is doing in scenario #1.

To unveil this it is necessary to look
under the hood and see how the computer operates (Figure 4).

Figure 4. Schematic
figure of how my Garmin Edge 800 seems to estimate energy expenditure.

To estimate energy expenditure
the computer prioritizes the information coming from the heart rate strap, when
available, over the information coming from the power-meter. The computer uses an
estimation method developed by a company from Finland called firstbeat (http://www.firstbeat.com/). This method
integrates different variables that are entered in the computer (age, gender,
activity level, etc) and using some sort of algorithm that integrates heart
rate and heart rate variability, it estimates what is your actual energy
expenditure. In my case the value that comes from the heart-rate based
estimation is about 44% off from my power-meter and efficiency based estimation.
This algorithm has not seem to be properly scientifically validated through the
peer review process. There is some information about how this algorithm works
on firstbeat’s webpage ‘white
papers’, but these are not scientific articles, therefore it is not
science.

In my opinion these head units
would provide a much more accurate estimation of energy expenditure if they

a) Prioritized
the information coming from the power meter over that of the heart rate strap.

b) Assumed
a standard value of efficiency or provided the chance of entering the rider’s
efficiency into the computer.

c) Properly
reported the energy expenditure based on that efficiency value.

**Conclusion**

Estimating metabolic energy expenditure using a power meter requires
understanding the concept of efficiency (Figure 1).

Provided the common use of joules and calories to measure
energy, the output of some calculations can be confusing and it is important to
follow the right logical steps to make this calculation (Figure 2).

Some cycling computers on the market provide misleading
information regarding metabolic energy expenditure (Figures 3 and 4).

I hope this post has made this
topic clearer to you and now when you look at your cycling computer in search
for the kilojoules or the Calories display you ask yourself:

- What is this value on the screen, mechanical work or metabolic energy expenditure?
- Is it derived from my power-meter or my heart-rate?
- If derived from my power-meter, is it assuming any specific efficiency value for me or is it just mistakenly changing the units from joules to calories?
- If derived from my heart-rate, is it accurate? Has it been validated?

**References:**

1. Coyle, E.F., L.S. Sidossis, J.F.
Horowitz, and J.D. Beltz, Cycling efficiency is related to the percentage of
Type I muscle fibers. Medicine & Science in Sports & Exercise, 1992.
24(7):782.

2. Haakonssen,
E.C., D.T. Martin, L.M. Burke, and D.G. Jenkins, Energy expenditure of
constant-and variable-intensity cycling: power meter estimates. Medicine and
science in sports and exercise, 2013. 45(9):1833.

**Further reading:**

- Ettema G, Loras HW. Efficiency in cycling: a review. Eur J Appl Physiol. 2009;106(1):1–14

* This
is a simplification because for the lamp what it generates the energy (the
power station) is not part of what uses that energy. In case of a human the
‘power station’ (mainly the mitochondria) is on the same place that uses that
energy, the skeletal muscle.

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