What I Learnt from Running 100km with a Power Meter (yes running).

At the end of 2016 I ran the inaugural Taupo Ultramarathon 100km event along the stunning Great Lake Trail which runs from the western to the eastern side of the lake. This was to be my first ultramarathon and being a sport scientist I wanted to get as much data as possible. To start off, I put on a “Stryd” running power meter and did a few tests in the lab a couple of weeks before I ran. Based off my data I made it my goal to run under 10hrs (under 6min/km). How did I come up with this?

I know from testing myself in the lab that when I’m running 6min/km it’s equivelant to 230 Watts and I am ultising oxygen at a rate of 35mL/min/Kg which is 45-50% of my VO2max (72mL/min/Kg). At this instensity my respiratory exchange ratio (RER) is 0.80 and this indicates what proportion of fat and carbohydrate I’m burning. An RER = 0.80 is equivalent to 40% CHO and 60% Fat. In efficiency terms this is not amazing but pretty good. If I wanted to burn more fat I could limit my carbohydrate intake during training but for me this ratio works and is sustainable. At this rate I’d be burning 800 kcal/hr or more specifically 90g/hr carbohydrate and 42g/hr fat. I won’t bore you with even more specifics but for me those numbers ticked all the boxes.

My secondary goal was to evenly pace myself using feedback from perceived effort, heart rate andpace, while keeping an eye on power. Being reasonably new to power I didn’t want to put all my eggs in that basket just yet. Each of these metrics are more useful at different times during an ultra-endurance event. At the beginning perceived effort can be low despite a high pace and heart rate and the opposite can occur near the end of the race. Power can be useful because it allows you to objectively assess your physiological effort regardless of how you are feeling.

A quick overview of my race in numbers;

As you can see from my power and heart rate data, my pacing strategy fell into the fatal trap of going out too hard and dying at the end. The key segments to draw from this graph are the first 50km and middle 50km (25 – 75km) and the final 50km.

Total 100km = 10hrs 36min, 217W, 155bpm, 6:22min/km
First 50km = 4hrs 50min, 233W, 159Bpm, 5:48min/km
Middle 50km (25-75km) = 5hrs 7min, 226W, 163bpm, 6:12min/km
Second 50km = 5hrs 46min, 206W, 153Bpm, 6:55min/km

Looking solely at the time and average heart rate, pace and power the data would suggest I wasn’t too far off what I planned to do. But if we look more closely at each 50km segment you can see I was either going too fast or too slow. I did think my heart rate was too high during the first part of the race but my pace was good and so was my power so I didn’t worry about it. However, I actually fell into two very common traps of pace and power. 1) Pace is affected by GPS signal and 2) power is affected by your weight. In reference to both of these I was running along a rocky coast which plays havoc with GPS and made it look like I was running slower than I was. Secondly I was wearing a camelback which added 2-3kg to my weight meaning I was pushing more power than my watch suggested. In this instance I should have paid more attention to heart rate.

When I was running approximately the right pace through the middle 50km you can see from my heart rate I was still pushing too hard. This was due to the undulating nature of course and again is where power can be helpful since it allows you to account for uphills; For example if you can run 5min/km on the flat at 250 watts and you try run uphill at 5min/km you might be at 400 watts. Therefore, if you run uphill at 250 Watts you know that’s an equivalent effort to 5min/km on the flat.

The final 50km is very interesting because it really shows you my struggle. Despite running over 1min/km slower my heart rate and power weren’t much lower than my first 50km. This is a result of the extreme amounts of fatigue my body was experiencing causing a massive deterioration of my running form.

What did I learn?

  1. Even on a perfect day I probably couldn’t have run under 10hrs.
  2. Update your weight on your running power meter if you’re going to be carrying added weight.
  3. Even at the best of times GPS data is ± 2% and this can be more like ±4% on the trails.
  4. Running power is an extremely useful tool for pacing.
  5. Running 100km hurts!

Will O’Connor PhD
Sport Scientist
Performance Advantage