You watt? Why I ran an ultramarathon measuring power instead of pace

We put the latest running metric to the test on a 100km multi stage ultra
2018 Race to the Stones by with Pic2Go 08:53:41
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Move over pace and heart rate, there’s a new running metric in town. Power has long been used by cyclists to help produce consistent performance and now companies such as Stryd, RunVi and Garmin want to kick off a Running Power Revolution too. But is power better than running on pace or heart rate?

I armed myself with a Stryd running sensor and went along to the Dixons Carphone Race to the Stones to see what lessons I could learn while powering my way through a 100km multi-stage ultra.

First a familiar tale of runner’s woe...

Earlier this year, after four months of hard training I tried to beat my marathon personal best of 2:57 at the Hamburg Marathon. To be successful, I needed to run an average pace of 6:45 minutes per mile for 26.2 miles. Like plenty of runners who toe marathon start lines, I didn’t really know for sure if I was fit enough to run that fast for long enough but I was up for giving it a go.

Essential reading: A guide to running power meters

It was a hotter than average day in Hamburg and I noticed my heart rate was elevated even before the gun had gone off. I ignored the booming BPM and pushed on, eventually managing to run 14 miles at my target pace before the wheels came off suddenly and painfully. Unable to sustain the pace, I blew up spectacularly and by the time I stumbled across the finish line I’d run 3:18, had a horrible time and I was a complete mess.

This is a tale many runners will recognise. I’d based my run on a target pace and made some classic marathon mistakes. Firstly, I’d clearly overestimated my potential, hoping that with a bit of luck and prevailing wind I could outrun my fitness. Secondly, at times during that first 14 miles I’d actually run a little faster than my target pace thanks to adrenaline-fuelled over-exuberance in the early miles. With the heat playing havoc with my heart rate that metric also became useless as guide to effort from the get-go.

Why is this relevant? Because these are exactly the errors running on power is trying to eliminate.

Why pace, heart rate and perceived effort can come up short

You watt? Why I ran an ultramarathon measuring power instead of pace

What Hamburg hammered home was that, when it comes to race day performance, running on feel (aka perceived effort), pace and heart rate have some major shortcomings.

You can be clever with your pacing strategy, perhaps running slower at the start to leave you more at the end, but regardless of approach, knowing how fast you’re running doesn’t tell you the price you’re paying for doing so.

Hills present a problem for pace too. Maintaining a target marathon pace up a steady incline takes a toll, easily pushing you over your threshold, and the impact it has on your whole race can be hard to judge. The smart thing to do is to ease off a little but that can make you nervous that you’re losing time.

Many people naturally turn to heart rate for a guide to how hard your body is working to maintain your pace. But your heart rate is subject to a huge range of external factors that includes things like mental fatigue, physical fatigue, caffeine, adrenaline, stress and heat. Go sit in a sauna with a chest strap on, and though you’re not moving a muscle, your heart rate will still rise sharply. So this doesn’t necessarily accurately reflect the work your body is doing.

There’s also a delay in your heart’s response to an increase in effort and that can make it hard to run to consistent heart rate, particularly over undulating terrain. And we’re not even going to get into the chest strap versus optical debate.

What about running on perceived effort? One theory here is that running to feel – free from numbers either telling you you can push harder or that you’re over cooking it when maybe your not – can produce a better performance.

While I love the idea of listening to your body so that it produces the performance its capable of, there’s a big problem with this. As I found out in Hamburg, it’s all too easy with adrenaline, freshly tapered legs and the excitement of the race to feel like the pace you’re currently running is sustainable. You feel invincible in those early miles, like you’re running way under the effort for the pace, so there’s no reason to slow down. The price you pay for this only becomes apparent deeper into the race by which time the damage is done and it’s too late to adjust.

The benefits of running on power

Power measures how much work you are doing and at what speed you're doing it - it's measured in watts. And it's tracked instantly in real-time. While pace tells you how fast you’re running but not the cost, and heart rate tells you how your body is responding to the work you’re currently doing, power actually measures the work itself.

Power is therefore affected by fewer physiological and external factors, and that, in theory, means power makes it easier to maintain consistent effort in most conditions, up and down hills. Crucially power won’t lie to you in those early miles so you can finally stop going out too fast.

There are still some conditions where power becomes less useful, for example heat, humidity and headwinds can’t be picked up by a foot pod. However, if you can predict those conditions it’s possible to adjust your power targets accordingly.

Putting power to the test

You watt? Why I ran an ultramarathon measuring power instead of pace

Intrigued by the new technology and eager to find out if I could avoid ever having to repeat my Hamburg Hell, I decided to put all of this power theory to the test.

I signed up to the Dixons Carphone Race to the Stones, a stunning 100km ultra run, split into two days of 50km. A mixture of trails and road with sections of up, down and flat, this gave me the perfect opportunity to see if I could indeed run smarter on power.

Would running on power instead of pace or heart rate improve my performance? What would it feel like to run with this new metric as a guide? Would it be easier to follow than pace?

For comparison, I also strapped on a Polar H10 chest strap to track my heart rate and paired it all up to a Garmin Vivoactive 3.

Setting a power target

You watt? Why I ran an ultramarathon measuring power instead of pace

Ahead of race day I needed to work out what my power target should be. In order to do this I had to conduct what Stryd calls a Critical Power test. The Stryd app provides four ways to do this.

Two are specific, timed, track-based interval drills, done every 4-6 weeks to provide an accurate assessment going into a race.

Alternatively you can drop in your most recent 5km and 10km times for an estimate, though the accuracy also depends on how recently you ran these times.

Stryd uses these numbers to give you a Critical Power Reading. This is defined as your “maximal average sustainable power over one hour of running, also referred to as ‘lactate threshold’ power.”

From here Stryd calculates your Power Zones. These operate much like heart rate zones, broken down into five watts-based ranges or zones: Easy, Moderate, Threshold, Interval and Repetition. These can be used to guide training intensity but when it comes to race day the aim is to run at a power you can sustain for the duration of your race, and that means running just under threshold.

Luckily you don’t have to suss this out yourself, Stryd also has a Race Power Calculator that caters for distances from 5k up to the marathon. Because I’d be running 31 miles on two consecutive days on trails with around 1,200m of climb and a good portion of descent, we adjusted my power targets manually.

Taking into account the elevation gain, the predicted temperatures and any fatigue or muscle damage I’d suffer from running two marathons on consecutive days, I was challenged to run on a power target of 279 watts for the climbs and the flats, and 245 watts for the descents.

This would give me a predicted finish time of 9 hours and 50 minutes, with a slightly quicker second day, due to there being fewer hills. So how did I get on?

What I discovered running on power not pace

You watt? Why I ran an ultramarathon measuring power instead of pace

Let’s start with the practicalities. Running with one number for the duration of a race definitely simplified things. It removed the need for complicated maths, the fears about loss of GPS for sections of the race and I had a watch customised to display just two stats: distance and power.

In a regular marathon where it’s possible to have two or three different paces to run to during the race, I can see how running to one number would be even more attractive and beneficial.

Provided you can run to that number. As a newcomer to the tech I found it hard to maintain consistent power even on the flat. My watch jumped sometimes by as much as 20-30 watts within a couple of strides.

On the hills this was more exaggerated, obviously it takes a lot more power to push up the inclines. Running the ups sent my output way over 300 watts while dropping to a walk often left me below my 279 watts target. What this suggests is that running at a consistent power is something that definitely requires practice.

During the first 13 miles of day one my mile splits show an average power range from 241 watts to 279 watts which equates to a pace range of 7:46 min/mile up to 10:46 min/mile. What’s really interesting is that there were three miles run at the same power (276 watts) and these produced a 7:47, an 8:14 and a 9:01 min/mile. I would have presumably paid a hefty price had I run on pace and stuck to a 7:47 pace for all three miles.

As I got into the race I was surprised to find I checked my watch less frequently. I used the kilometre markers much less than I do when I’m running on pace and worrying about splits. Instead, I got my head up to take in the surroundings more and subsequently felt freer mentally.

You watt? Why I ran an ultramarathon measuring power instead of pace

We’ve mentioned heat can have a big impact on our ability to sustain a power output and during the Race to the Stones temperatures hit as high as 29 degrees celsius, that’s 3-4 degrees higher than we’d anticipated when we calculated my target power. As a result, 30km into day one, I had to adjust my power target to account for the fact I was melting.

Over both days I’d end up running at a much lower power than the planned target of 279 watts. My average moving power on day one in the higher temperatures was 225 watts compared to 243 on day two where it was cooler. Day two was far more consistent and far more enjoyable too, backing up the idea that running an even power race can make running more fun.

If you look at my Strava readings, based on my heart rate tracking with a Polar H10, I spent a lot of time on both days in lower ‘effort’ zones which makes me wonder if I could have pushed harder. I didn’t feel like I could on the day but perhaps power has revealed that I might need to work on the psychology of running closer to my limits.

I finished the 100k in 10 hours and 19 minutes total, 39 minutes slower than Stryd’s predicted finish time but allowing for the adjustments for the heat – I spent at least 10 minutes sitting in the shade in one aid station – this feels like the Race Calculator provided a pretty accurate estimate.

On a slightly cooler day where I think I would have had a good chance of maintaining the 279 watts average, the original estimates would have been bang on. When the conditions change you still need to make smart decisions in the moment, just as you do with pace and heart rate.

Running on power isn’t a silver bullet that’ll let you run beyond your capability but what’s most exciting is that Stryd – and power – can help you set a more realistic race goal based on what you’re physically capable of. Then it can guide you with more certainty to achieving that goal by helping you avoid pacing mistakes. And lord knows we could all use some help with that.

And you know they say the proof is in the pudding. Well, running on power at the Race to the Stones, I managed to come third in my age group and sixth overall. I not only broke my 50k personal best on Saturday but I went back out smashed it by 25 minutes again on Sunday. So there might just be something in it.

TAGGED Running

How we test

Kieran Alger


Kieran is a world record-setting runner and one of the UK's most experienced running journalists.

A constant tester of the latest fitness technology, he's always hunting for innovations that can make him run faster, further and generally be in better shape.

Kieran is often found wearing four GPS running watches at once. And to date he's tracked more than 50 marathons, 13 ultras and countless half marathons - including the Marathon Des Sables.

In 2022, he became the first person to run the river Danube from sea to source, a measly 1,830 miles in 66 days. And still had time to test running gear.

Kieran regularly takes running tech to the extremes for Wareable and the likes of Runner's World, Mens Health and Wired.

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