The metabolic cost of bushwhacking: brush work, impedance work, and hazard work

[Ryan Jordan]

Companion forum thread to: The metabolic cost of bushwhacking: brush work, impedance work, and hazard work

The Metabolic Energy Mile (MEM) framework provides a method for quantifying the energetic cost of backcountry travel relative to treadmill walking. Off-trail conditions increase the Metabolic Difficulty Ratio (MDR) through three mechanisms: brush work (mechanical resistance from vegetation), impedance work (loss of locomotor efficiency due to disrupted stride), and hazard work (energy expended to maintain stability and avoid injury). Distinguishing these categories improves predictions of caloric demand, time requirements, and route planning accuracy.

[Terran Terran]

Good breakdown.

I think blowdowns can be equated with moving up a rocky river bed.

With vegetation, I’ve found walking between two different species such as buckthorn and manzanita to often be the easiest. As a preference, I often prefer the buckthorn. It doesn’t beat your legs up.

I’ll have to give it a study. Thank you.

[David D]

Interesting study!

I’ll share my method for doing the same with some field results.  Its simpler than the MDR method as the parameter list is reduced, but perhaps not as precise (I find it close enough).  My motivation was to drop as much food weight as possible without excess body weight loss, on 8-10 day trips.

The method:

• calculate energy miles accounting for elevation, using Petzoldt
• calculate BMR, your base metabolic calorie burn rate
  • based on Weight, Height, Waist, Neck, Age, BMR=13.397W + 4.799H – 5.677A + 88.362
• calculate “PAL” the Physical activity level parameter which is a multiplier applied to BMR,  similar to Ryan’s MDR.  I estimated this by curve fitting multiple PAL academic studies which factor in
  • energy miles
  • hiking speed
  • body weight
  • skin out weight

Comparing my model to my Garmin watch’s field estimates of caloric burn based on heart rate monitoring (these will be slightly underestimated as my watch’s setting doesn’t include pack weight), and to Go Ruck‘s improvement on the Pandolf equation:

My model predicts average trail calorie consumption almost spot on, but since it doesn’t factor in all trail difficulties, overestimates calories needed on easy trails by ~ 10% and underestimates calories needed on hard trails by ~ 10%

I adjust for this by multiplying the the PAL +/- 10% based on trail difficulty, and then it’s very close.

This keeps the calculation simple and accuracy is pretty good.

The hard trail was a nasty combination of brush work, impedance work and hazard work with some marsh trudging thrown in for sake of type 2 completeness.  For me, anything worse than this starts to venture into Type 3 terrain so is fairly close to representing an upper limit for 3 season below high alpine.

I double checked the model by measuring my weight loss/gain after several 5+ day trips and it’s almost dead on.  This calculation is tricky because not everyone is 3500 cals/lb but that estimate works well for me.

Given the accuracy and simplicity, for me, a higher level of precision quickly has diminishing returns on investment except in perhaps a couple cases where large differences show themselves:

• adjust this for high elevation.  10,000 ft consumes 25-40% more calories from what I’ve read
• adjust this for deep cold where the body burns calories to keep warm.  According to Skurka, this could double the calorie burn.  I do very long day snowshoe trips but doubt I’ll ever get the stones to test this over multiple days as sleeping multiple nights @ -20C sounds too type 3 for me!

[dueurt]

I don’t usually think of it like that, but ‘hiking’ for me is mostly ‘bushwacking’. I primarily use trails for transportation and occasionally while hiking with other people (and then I prefer the surface as unprepared as possible). I’ve gotten plenty of weird looks actively avoiding trails in our tiny patches of forest here in Denmark.

I find that experiencing nature from a trail feels akin to experiencing wild animals in the zoo, in that there is a distance/separation, and a lack of the sensory stimuli and visceral experience that you get off trail. Those stimuli are an essential part of why hiking is so important to my mental health.

 

This model helped me understand the royal asskicking I got on a hike two weeks ago, having to cut short a 3-day trip after 2 days. I was in Tresticklan national park in southern Sweden, mostly off trail, and it is the most strenuous hike I have ever experienced.

In preparation for a trip next summer, where I plan to hike somewhat similar terrain and consider bringning a packraft, I’m assessing realistic distances. My ambition on this trip was to test if I could do three successive 20km days offtrail in this kind of terrain, and I didn’t manage one! I intentionally chose difficult terrain, pushed harder and was more careless than I am on longer treks, but was never more than ~5km from my car. The challenge was real, but the consequences of pushing too hard were minor – the right kind of setting to test my limits imo.

Day one wasn’t very hard, interspersing on-trail, mostly easy off-trail and packrafting a lake. I could really have used a bivy though, ending up with the worst pitched tent I’ve ever had. Undoubtedly the somewhat subpar sleep I got influenced day two, strong winds and heavy rains didn’t help, and I wasn’t as mindful of eating and drinking as I should have been. But I’ve endured bad weather, bad sleep and bad hydration/nutrition before without a crash like this.

Day two started completely off trail, and only by pushing hard did I manage to keep a 1.6km/h (1mile/h) average, hitting my “morning break” spot by lunchtime completely exhausted. I spent almost 3 hours in my tent there. Some of that was getting dry and fed, but much of it was simply resting after hours of (unexpected) cognitive strain. After that, I decided to get to a trail and end the evening by my car (I also switched from paper map to smartphone navigation).
Interestingly, although it took several hours to get back on trail, as soon as I hit the trail again, I averaged 5-6.5km/h (3-4miles/h) for several hours. My ‘regular’ hiking stamina wasn’t significantly compromised, and the cognitive load almost disappeared.

I think what surprised me was two things:

1. How hard the terrain actually was. I know offtrail can be hard, but I definitely have to improve my understanding of the interplay of VD, MI and HP and how that translates to actual terrain BDR.
2. The mental exhaustion. A combination of constant vigilance at every step, constant assesment of routes to take and a change of terrain every 10-20 minutes burned cognitive energy much much faster than I expected.

The framework helps me break down the 1 and understand the 2.

• Brush work was often more intense than I usually encounter.
• Impedance work was very very intense.
• Hazard work was pretty much constant.

A few takeaways for me:

1. Combining brush, impedance and hazard work can increase dificulty exponentially.
2. Changing terrain presents an additional element of resistance. Terrain with significant impedance work still allows a kind of rythm after a while; in hazardous terrain you can get pretty focused on the relevant hazard etc. But regularly changing the type of obstacle or hazard ‘resets’ that.
3. The demands on your physiology get increasingly varied with higher BDR grade. This makes it more likely that your weaknesses are exposed, and you have to plan distances more with an eye to weaknesses than strengths.
4. Marsh presents a physical resistance that I find very distinct to the resistance from pushing through brush. The act of pulling your feet from the marsh (if you’re unfamiliar with it, think thick mud with treacherous vegetation cover) can be understood well within the model, but I think a valuable expansion of this article would be more varied examples of terrain.
5. Pack weight gets exponentially more important as BDR goes up. I had a 15kg backpack (packraft + a slew of gear for testing), and I could feel every kg of that pushing my feet into the marsh.

All that said, it was a fantastic experience – and while I spent some time recovering, it recharged me in ways that makes me crave it all the more. I’ll leave the packraft at home next time though.

[Philip Tschersich]

I periodically get inquiries about doing trips in the Kodiak archipelago by folks from outside Alaska. It can be hard to describe what they will be facing in terms of terrain and vegetation. They generally find it a bit tougher than they expected, and as a result the trip often does not go as planned. I found this honest and open-eyed take on Kodiak travel by a group of Danish hikers to be refreshing. They embraced the difficulty and persevered with aplomb.

 

[Megan W]

I know there is a cost to bushwhacking (scrub-bashing, bush-bashing)…but I often keep myself entertained in the middle of it contemplating how GOOD it is for me.

There is some research into the physical and cognitive benefits of things like dual task walking, complex purposeful movement (eg martial arts, dancing) and enriched environments. When I am threading myself over, around, under and through Tasmanian wilderness, I get fascinated by the thought of how many areas of my brain are lighting up and coordinating and refreshing themselves. All that sensory input and thought/movement output dancing together. The colours, the complex 3-dness of the world and me in it, the textures, the pressure of the branches, the shape of the ground (or roots or rocks) under my boots, the sounds, the proprioception, muscle effort, weird and wonderful limb shapes. I am a physio so perhaps that explains my fascination :).

I often see patients who go to the gym, lift weights etc (which is great!!), but so many of them only do exercise in straight-lines/one plane. Real life is more like scrub-bashing than ‘perfect form’.

People pay to go to Pilates (not denigrating Pilates at all , have trained in it :) )……maybe there is a place for specific bushwhacking exercise centres!!

[Jerry Adams]

Yeah, I’ve read that to delay dementia it’s good to go hiking.  Combination of exercise and figuring out where to go.

Just exercising on a treadmill isn’t as good

[Jerry Adams]

Regarding metabolic cost of bushwhacking, I was just walking around Mt Hood.

There are stretches of taking 100+ yards to cross a stream.  You have to climb over rocks, climb up and down hills, find way to get over creek,…

I find that it’s much more tiring than just walking on a trail.

[Dan]

I primarily use trails for transportation and occasionally while hiking with other people (and then I prefer the surface as unprepared as possible). I’ve gotten plenty of weird looks actively avoiding trails in our tiny patches of forest here in Denmark.

I do the same thing in my daily hikes in my backyard “wilderness.” I’m always exploring alternative routes and checking out the area between the trails. It’s amazing what you find.

[Brad W]

I swore I read an article with the same title in a 70’s men’s magazine.. Seriously though-loved this podcast. From the second it started I was saying to myself-what about the mental impact? Is Ryan going to figure this metric? Of course he did. I hike in the desert a lot. 90% off trail and dodging cholla cactus, agave spikes, etc. it very taxing, especially at the pace I enjoy moving. It’s exhausting to be hyper focused on obstacles that cause pain as well as scouting ahead for a path through them, looking for very faints signs of game trails or old native paths, anything to make my movement faster.

Again, love this topic and it was very well done.

[Author]

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