How hard is it to run 21k on powder snow?

Nimble footwork handy. For purposes of damage dealt, the stream is lava liquid nitrogen.

 

I set out for a 21k course I know very well. There was fresh powder snow and it was not cold (-4C) with no wind. I did a cross-country the day before and a 12k run the day prior to it. In the end, I finished after 3hours totally exhausted. My previous attempt on the same course was 2h:08min with some snow on the trail and considerable time pressure. And I felt okay after the experience. So where can I attribute the differences in finishing time and the toll the run took?

My usual nutrition strategy for shorter (ca. two-hour-long) workouts centers around eating a 100 kcal gel after the first hour and an additional one 45-60 minutes afterwards if needed it. This was based on Jason Koop's 200-250 kcal recommendation for every hour after the first one in an ultramarathon. Granted, I aimed at the low end (or rather its 50%) reasoning that nutrition in a two-hour workout could be more frugal.

I tried to replicate the strategy today: I had the first gel at kilometre 11, where the snow forced me to hike anyway. Feeling somewhat tired, I ate the second (at the last) one at km 15.5. And when I was finishing, other than tiredness, I felt I was very low on energy. So what went wrong?

Before I look into this more, I had a rather faulty idea that the course could be completed in ca. 2 hours, and the pessimistic estimate I shared with my family was 2h:30. I never put too much thought into the estimate, preferring to be on the trail to modelling the estimated finishing time.

First, I clearly ate my first gel later than I normally do – I was almost 1h:30 into the trail. This was caused by my working the approximate location where I eat usually rather than watching the time. It is unclear if this was that bad in the grand scheme of things, had I followed my usual method I would have most likely finished my second gel after 2 hours, leaving me with an hour to finish (and no reserve food). It is unclear if this would have been better. As it was, my hunger proved a surprisingly good lead, since the second gel was eaten midway through the remaining 90 minutes.

I also ate much less than I would normally do. Given that it took me 3hours to finish, a better gauge than the two-gel two-hour run is a 3h:30 30k four-gel run. (Though I cannot recall if I finished three or four in the event). Now given that a runner's energy expenditure is known to be reasonably constant for a large interval of paces, I ate approximately half of what I normally would, based purely on the time spent on the trail. (Feel free to refer to Koop's Training Essentials for Ultrarunning for the surprising statement about the energy burnt being indepedent of a runner's speed.)

I ran all but a fraction of the run on (and in) powder snow. This must surely be more energy-consuming, but what is a good estimate of the factor? A fascinating article by Richmond, Potter et al. gives a good estimate for a walking person: if the sinkage of the boot is 5-10 cm, the multiplier of energy expenditure is 1.9-2.8. Take note that this should be taken as a lower bound for running. Why? It is known (Koop) that the efficiency of walking scales better in uphill conditions compared to running efficiency, and since some of the fundamentals, notably the ground reaction force remain the same, it is reasonable to expect a higher energy multiplier. So it is safe to take a multiplier in the range 2-3. (And I suspect that is still an underestimate.)

Combining the factors, it appears that I underestimated the energy consumption by a factor of 4-6. Now the exhaustion makes a lot of sense. And it is also clear that I need to factor the conditions much better in the future. After all, a runner usually errs on the side of being light, especially in short workouts. Realising that the short two-hour workout was equivalent to an eight-to-twelve hours one is an important lesson:)