Energy Expenditure

Recently I've been asked a question that proved the inspiration for this factsheet.  Lots of heart monitors, computers and power meters now display information relating to kilojoules and Calories.  Here I'll try to explain what they are, what they relate to and how you can use the information to improve your performance and results.  Although it appears a little heavy at first, this factsheet could give you the information you need to make or break one of your seasons key objectives.  Stick with it to the end and it will all fall in to place.

Watt's a kilojoule?
See my clever play on words there?  Well a kilojoule (kj) is a measure of energy expenditure or work performed.  A joule is defined as being the energy expended when one kilogram is moved one metre by a force of one Newton.  Wouldn't know a Newton if I fell over one, I'm still trying to get used to kilograms!  Anyway, a Newton is what it is.  

As you all know a watt is basically the calculation of work done divided by time.  One watt equals approximately a thousandth of a horsepower and one watt is also equal to one joule of work per second of time.  We now have a direct correlation between watts and joules.

Without getting in to too much detail there are approximately 4.2 joules in a calorie.  A joule and a calorie are very small units of measurement.  So, to make things more useable we measure in kilojoules and kilocalories.  To simplify (or complicate) matters further we almost always refer to kilocalories as Calories with a large "C".  When you see Calories spelt this way we are referring to a thousand calories.  One kilocalorie equals one Calorie.

Just a little more science and we're there.  Extrapolating our formula from above, there are 4.2kj in a Calorie.  When you return from your ride and you download your data you may get a reading reporting the work that you've done in kilojoules or Calories depending on the manufacturers prevalence.  At least you now have the information to relate them to each other.

Work done
In the Danguillaume Cyclo I cracked out 3160kj of work, as you can see from the power summary on the right.  It was 102 extremely hilly terrain miles, covered in a little over 5 hours at 19 mph.  (The duration time includes riding to and from the finish).  Normalised power output was 195 watts with a peak of 871 watts.  So how does this information help me?

I'll cover the other parameters in later factsheets, for this one we'll concentrate on the kilojoules.  We now know how many kilojoules were expended during the ride, 3160.  Under normal circumstances, using the formula above, you would just divide the kilojoules (3160kj) by 4 to arrive at a Calorie count.  As ever life, especially cycling life, isn't that simple.  This is the big mistake most people make and why they find themselves "under-fuelled" for big endurance events.  In the next section I'll explain the assumption that's made and what you need to take in to account.

Mechanical Efficiency
We all learnt at a very early age that perpetual motion is impossible.  You can't get something for nothing.  The days I attended engineering college, and later in motor racing, I learnt all about thermodynamics, frictional losses and mechanical efficiency.  Machine efficiency is very much different to human efficiency.  Here I'll give you a practical, condensed version.

Bicycles are extremely efficient.  Almost 97% of the energy put through the pedals gets to the ground to propel us forward.  Losses occur in the pedal and bottom bracket bearings, the chain, the freehub, the wheel bearings and the tyres gripping the road.  For now we'll ignore aerodynamics.  But for our purposes all most everything we put in we get out.

Work, no matter what its type, generates heat and heat burns energy.  That's why we sweat when we exercise.  Heat, unless you're specifically producing it to keep warm, is waste. Heat is energy that's not being directed to propelling you forward.  Heat is natures way of telling you your working hard.  Even though it's energy being effectively wasted it still has to be accounted for and replaced.

Humans are somewhat different to machines.  We are very, very inefficient.  At best our efficiency is around 25%.  Anyone who's been for one of my wVo2max tests will tell you how unbelievably hot they get in six minutes. 

When you're pushing out 250w in a test you will generate three times that in heat.  750watts, plus the 250 being generated, equals the heat of a 1 bar electric fire.  That takes some dissipating on a turbo!

Knowing what we know about human efficiency, we can relate this to our work calculations, energy expenditure and calorific intake.  Our 25% mechanical efficiency equals a 4 to 1 energy input/output ratio.  This means we need to consume four times the energy spent to replace it or to put it another way; for every four Calories we consume we can only use one as fuel

As I explained earlier 4.2kj equal a Calorie.  Using the information in the paragraph above, we need to consume 4 Calories to produce just under one kilojoule of work.  For our purposes 4.2 is close enough to 4 to equal a like for like comparison. We can now use these snippets of info to our advantage when preparing an energy replacement strategy.

How does this help you?
To recap; although the ratio of kilojoules to Calories isn't exactly one-to-one we can assume, for the purpose of our needs, that it is.  The difference is not worth worrying about in the big scheme of things.  So, in effect you need to provide one Calorie of energy to replace each kilojoule of work done.  This means 1000 kilojoules of work done requires 1000 Calories of energy to fuel it. 

Fuelling Strategy
Using my example above, you can see my work done equated to 3160kj, which we can "convert" to require approximately 3000 Calories of fuel to drive it.  It's science but we don't need to be exact; we're riding a bike not sending a man to the moon. 

If you've properly prepared for your event, the glycogen levels in your muscles and liver (the fuel tank) will be fully stocked.  The body can store around 350 grams of glycogen which will give you enough energy for around 90 minutes of hectic activity.  So in effect we've got 90 minutes of energy, or (my approximation) 900 Calories, in the bank.  For endurance events you need to make sure you keep yourself topped up!

Just as Michael Schumacher doesn't finish a Grand Prix with full fuel tanks, we don't normally need to finish an event with fully stocked glycogen levels.  The key word there is normally; I'll come back to this later. 

If we endeavour to keep our finishing reserves moderately stocked, we can pull 600 Calories from our equation to finish with our "tanks" a third full.  So 3000 required, minus 600 we don't have to replenish means we need 2400 Calories to fuel us to the end.  2,400 divided by the 5 hours it took me to complete the event requires a fuelling strategy of at least 480 Calories/hour.

The more enlightened of you will have something in your bottles that's stronger than water and more sophisticated than dilute Ribena, even if it is the "tooth-kind" variety.  Stocking up with two, 750 ml bottles of something like Viper or SIS, will give you 200 Calories per bottle. 

To come to the point, we should be looking to consume around another 200+ Calories an hour on the bike for optimal performance to be maintained.  This can be done with power bars, gels, dried fruits or just good old solid food.  I'm not going to dive in to the complexities of glycemic indexes and fast & slow foods here, but you should consider your options.  Especially in shorter or ultra long events.

Things you need to know
The body can only absorb 1 gram of carbohydrate a minute.  To process each gram of carbs we consume we need to drink 3 grams of water.  So when you eat you drink and you do everything in moderation.  Don't wait for an hour then eat a 100 gram energy bar and down a pint of water to help it on it's way!  Little and often is the cliché for the day.  Ten grams of carbs every ten minutes is ten times better than a 100 grams an hour. 

If you've read the other nutrition factsheets on this site you'll also know one gram of carbs supplies 4 Calories of energy.  So do your sums, work out how long you think your event might last and work out how much and how often you need to eat.  Don't prepare all year for a big event and let your excitement cloud your judgement on the details that matter.

For events lasting an hour or less you will not even need to eat.  You can get by with just an energy drink for hydration purposes rather than fuel.  Remember 6% carb solutions will be absorbed through the intestine quicker than plain water.  For events over an hour you will need to fuel your tanks.  If you ride at below threshold and have the correct nutritional strategy you can, in effect, ride all day.  How do you think the 24 Hour time triallists manage?

Don't follow the Pro's
When you see Tour de France riders throwing food down their throat in the last 20k of a stage they're not eating for today, they're eating for tomorrow.  They need to get glycogen in to their muscles during the twenty minute post exercise glycogen window.  The best way to do that is to eat before you finish.  Once you stop the digestion process is well under way and filling the tanks.  Unless you're in a stage race you don't need to so don't try it.  You'll only be sick come the sprint.

Final Note
Without going too overboard we must remember that these are figures to replace energy for sporting needs.  We have our own Basal Metabolic Rate (BMR), which is the amount of energy we need to consume just to support our normal body life support functions.

BMR is used to provide diet information for bed ridden hospital patients.  If we did nothing but stay in bed all day a 1.8 metre, male of 70 kilos, would "consume" 1760 Calories.  So you need this as a minimum to stay in balance.  Then there are lifestyle calories to be included; are you a sedentary office worker or do you work down a salt mine (another job I've had in the past).  One would obviously have a larger energy requirement than the other.  And it's not the office worker!  During a five hour event your body functions "naturally consume" 750 Calories.

The Message
In between starting this factsheet and finishing it, Dave Whitt, Steve Goziou and myself competed in the Paris Roubaix Cyclosportive.  During it Dave's Polar heart monitor clocked an energy expenditure of 5600 Calories.  Which, minus the 600 we didn't need to replace, for the seven and a half hours we rode, works out at 666 Calories per hour.  Which for a race dubbed the Hell of the North is quite appropriate.

Dave's count was a little higher than mine because his was "computed" from heat rate, speed, distance and time algorithm's against a pre-determined "average burn rate" formula that Polar use.  Mine was taken from my SRM which is taken directly from the power output of the cranks.  Remember the formulae above; one watt equals one joule per second and one kilojoule equals one Calorie?

Whatever the process we now have anecdotal evidence to prove that we need to consume big Calories for big rides.  At each of the five feed stations we stocked up with food and took on 3 pints of fluids.  That's fifteen pints, plus the three we started with.  We finished with empty pockets, empty bottles and empty stomachs.  But we finished and we finished well.

Eat little, eat often and mix your food between high and low glycemic index types to give an immediate and sustained burn.  Drink more than you think you need and eat, eat, eat.  Aim to consume at least 400 Calories an hour, around 200 of which should be coming from your drinks bottle.  The longer your event the closer to this figure you need to be.  I know it seams a lot but remember, for every 400 you eat only 100 gets to the muscles.

I know I've said it a thousand times but I'll say it again.  Don't try this, or anything else (including food types), for the first time on the day of your event.  Try a few dry runs during your preparation phase.

Happy endurance riding.