Wednesday, April 29, 2009

Energy Pacing your Ironman

One of the most difficult aspects, when it comes to pacing a triathlon is the fact that it is a multi-modal activity. It is very easy for an elite 10,000m track runner or 1500m swimmer to ascertain how ‘evenly’ he paced his event. He and his coach simply sit down and look at lap splits.

In the world of triathlon racing, however, it is a little more difficult. What’s the running equivalent of a 200W bike? For the pointy end of the field, is the all-too common 5:30 bike/4hr run an optimal way to race an Ironman? What if I’m a strong biker and a crappy runner? Does that give me carte blanche to take advantage of my ‘strengths’ on the bike, or vice versa for that matter, if I’m a 2:30 marathoner, how much will I slow down for my Ironman run split?

The guy pictured above, James Prescott Joule has some answers.

The first thing to realize is that, by and large, for both bike and run, the body is pulling energy from a single, finite energy pool. A fixed amount of calories or kilojoules, stored as fat, glycogen and protein.

The second thing to realize is that Ironman is an energy limited, not a fitness limited event. In other words, just because you can run a 40 minute 10K or bike 300W for an hour doesn’t mean that you have the ability to fuel this rate (or an arbitrary percentage of this rate) of performance in the context of an Ironman.

No, your best Ironman performance will come from a conscious, even, metering of your energy resources with only slight regard given to your personal strengths and weaknesses.

So, let’s get down to it. What is the run equivalent of a 200W bike split, or the bike equivalent of a 4hr marathon? The numbers may surprise you.

I have prepared a table below, comparing the energy equivalents of a 140-300W bike split for a 60, 70 and 80kg athlete.

The numbers are based on ‘average’ economy numbers of 21% gross economy on the bike and 210 ml/kg/km on the run. In other words, if you have extraordinary run economy due to superior technique &/or muscle composition, your optimal splits may be marginally different, but marginal is the key word.

So, assuming a flat run course, the above represent equal energy splits for bike and run. To put it bluntly, if you are a 75kg athlete, you have no business biking 200W on the bike unless you’ve proven your ability to run <4:00 off the bike. How do you ‘prove’ this? By exceeding these standards in your Ironman.

Truth be told, there is a speed advantage to a slight negative bike:run split (more so for bigger athletes!!) due to the energy on the bike that is ‘wasted’ overcoming aerodynamic drag. Put plainly you get more speed bang for your energy buck on the run where extra energy goes to increasing speed rather than overcoming additional aerodynamic drag. So, the athlete should seek to slightly exceed these run standards.

Looking at the table, it is clear that body type comes into play, with smaller athletes expecting a faster run for a given bike power split. Thus smaller athletes need a better speed reserve/run fitness than larger athletes.

So, what about the ‘strong bikers’ who make the argument, “Well, I’m not a fast runner so I need to make my gains on the bike?” 3 points to these guys:

1. You’re drawing from a common energy pool for both bike and run and if you’re a crappy runner, that is even more reason to leave some ‘gas in the tank’. I’ve seen good runners coast at <8:00/mi on fumes. Sub-par runners don’t have that luxury.

2. Speed benefits decrease as power rises on the bike, due to aerodynamic resistance, while you always get good speed benefit from increasing energy on the run.

3. If you’re a big, strong guy, you don’t have to be a ‘good runner’ in order to pace appropriately. An 80kg guy biking 200W need only pull off a 4:14 marathon (9:40/mi). In other words, enough energy to jog (not walk) the marathon.

And what about the flipside, a very strong runner, say a 65kg, 2:27 marathoner. Let’s name him, Kyle . Obviously, Kyle has the fitness to run a very fast 26mi. But what bike+run distribution will give him his best Ironman time? Or put another way, how close to his open marathon time should Kyle expect to run?

Let’s take a look at some of his test data for some more info we can use to structure some race simulations. ….

As I have previously stated, I generally find that top age-groupers can fuel 11-12kcal of carbohydrate per minute over the course of an Ironman. From Kyle’s previous FUEL test we see that, with his exemplary FUEL profile, this point occurs at 220W(NP) of power on the bike (see below).

Looking at the table, an even distribution of energy from a 220W bike split would result in a marathon time of ~3:10. More than 40 minutes slower than his open marathon time!! And this is a best case scenario assuming optimal fueling on the bike and a marginally better lactate curve than the last time we tested him.

Clearly, there are many athletes from a running background who have more run fitness than they have the energy to use (ditto for the swim, btw). This has big time implications on training – specifically, the amount of training devoted to improving run fitness.

When it comes down to it, if you are looking for your best possible race performance don’t listen to Mr. Hellriegel or Big Jurgen Zack and ‘make zem suffer on ze bike’. Nope, if your overall time matters to you, take the lead from my bearded friend at the head of this article. Use your energy wisely.

Race smart,



Thorsten said...


very intersting concept, and - as usual - a very well written post. One more thing (for clarity) can you add a similar table mapping the watts and body weights to IM bike splits (assuming these are the main areas of influence)? I know that this will be a gross simplification as there are quite some differnces in hills, aerodynamics etc. (I've seeen some rought tools that would allow to build such a table, but I'm hoping you've something like that already prepared.)
Such a table could be a great tool to provide a comparison between bike and run times - something better than "your run time should be 2 hours faster than your bike split" ..

Paul Fleuren said...

G’day Alan,

Great timing with this post. Just last week I have been discussing something similar with a few of my mates.
My question to you is it possible to out train your fuel system?
For example, I guy came to me with a 13:55 IM under his belt. 1year later and he did a 10:24. He developed is strength and endurance quite rapidly. He was a 80kg guy and a strong rider. At IM Germany he was able to swim around the 1hr mark, ride around 5:15 and run just over 4hrs.
He didn’t experience any GI problems in this race. However, move forward 8mths to IMNZ and he experience GI shutdown on the bike. His strengths are his ability to follow the training plan to a tee but more importantly is pacing in a race is exceptionally good. This makes it confusing as to why he experienced a GI shutdown on the bike.

That then gets me thinking did he actually develop his strength and endurance too fast for his fuel system? Looking at the fat/carb ratio on your blog that has me thinking along these sorts of lines. At IMNZ he was in a low 5hrs shape and he races the bike leg right at the top of zone 1 (ie 135bpm). His FTHR is 166bpm on the bike. Because he requires a lot of energy to be able to ride a 5hr bike split could it be possible that his fueling system (ie fat/carb ratio) needs more time to be able to fuel effectively a low 5hr bike split???? He is only into his 3rd year now of IM training.
Maybe because he might not have a well develop fueling system yet, he needs to consume more calories externally on the bike, which then puts him at a greater risk of GI Shutdown because he can’t get those calories yet from fat stores due to his young athletic age???


kevin boucher said...

Good beard on James. Thanks for the pic AC. It's inspirational.

Alan Couzens said...

I don't think it would be good for your swim times. Then again, if you used it for early season work like a drag chute and shaved down, you'd be an animal in the water :-)

Alan Couzens said...

Thanks Thorsten.

Good suggestion. I'll see if I can throw something together with normal CdA's for different bodyweights over the next couple of days.

Obviously, as you said, course elevation and wind will come into play but we could get a roundabout # that may make the table more practically applicable.



Alan Couzens said...

Hey Paul,

Great question.

My own (self-protective) opinion is that big guys get the raw end of the deal in Ironman racing.

As you point out, the power outputs required for a bigger rider are very challenging to fuel. The difference in ability to absorb CHO unfortunately does not scale with body size.

Potential fat oxidation on the other hand does seem to scale better with body-size. Our best fat-burners to date have been big guys. However, it takes a lot of work. Both nutritionally and in terms of changing training emphasis.

There are a lot of frustrated big guy IMers out there who are unable/unwilling to make these changes.



Paul Fleuren said...

"There are a lot of frustrated big guy IMers out there who are unable/unwilling to make these changes"

Thanks again Alan for your responses, much appreciated

Could you offer some suggestions on some of the changes you would recommend?

Myself and Eric, who I coach, we have both been working to improve our body compositions. For example, before IM Germany last year we were both aorund the 82-83kg mark and we are both 6ft 2inches tall. At IM Germany we raced right oon 80-81kg and now we are down to 78kg.

We are still having trouble trying to figure out how to improve our abilities to metabolise fat?


Alan Couzens said...

Hey Paul,

The two big changes that I would recommend:

- Institute a heart rate cap at LT/VT1 for a good chunk of the season.

- Only replace the carbohydrate you use. A 40/30/30 diet is a good starting point.

Hope this helps.



MuddyPuddle said...

quick question in estimating Kyles ~310 marathon time after the 220w bike: How did you estimate that?!


Unknown said...

Hello Alan,

Really interesting write-up! Is there any rule-of-thumb approach I could use to monitor my IM bike in this fashion if I do not have a powermeter? And... how did you determine relative power output for the run (maybe I missed that part of your write-up).

Thanks for your thoughts,

Alan Couzens said...

Hey Marian,

In the second post in this series, Energy Pacing your Ironman II, I look at some bike speed estimates for different courses based on wattage. Keeping the caveats that I mention in mind, you could use these numbers as a rough 'check in' on appropriate pacing for athletes of different bodyweight.

I calculated run 'powers' by looking at equivalent energy expenditures on the bike and transferring across. E.g. if 260W=3.5L/O2 on the bike, I looked at the equivalent run pace and attributed watts accordingly.

Thanks for the comment.