Metabolic Energy Mile (MEM) theory provides a framework for expressing the actual energy cost of walking a mile on variable backcountry terrain. On a treadmill (no incline), one MEM equals one mile. Step onto a backcountry trail in the mountains, though, and the math changes quickly. Off-trail, even more so. We use the Metabolic Difficulty Factor (MDR) to define the caloric energy required to walk a backcountry mile relative to the caloric energy required to walk a mile on a flat treadmill. In other words, a route where MDR = 2 suggests that twice as many calories are required to walk a mile along that route as are required to walk on a non-inclined treadmill. Many factors (environmental and physiological) can increase MDR, but terrain, vegetation, and obstacles are important considerations while bushwhacking.
So what drives the multipliers that increase MDR while bushwhacking? Why does one bushwhack feel like sweaty resistance training while another feels like a frustrating stop-and-go crawl? That’s where three categories of bushwhacking work come in: brush work, impedance work, and hazard work.
Brush Work: Wrestling with Plants
Brush work is the mechanical effort of forcing your body through vegetation.
Examples include bushwhacking through willow flats in a river valley or krummholz growing out of sand.
Here’s a bushwhack through krummholz in Rocky Mountain National Park, where foot placement is easy – we’re following game trails here (BDR Grade I/II: VD < 0.5m, MI = 0.2, HP = 0.1):
As you step, stems hit your arms, branches catch on your pack and clothes, and your arms and shoulders are constantly in a state of rotational motion in an effort to maintain the momentum required to push through the brush and keep it from slapping your face and neck. You still have a walking rhythm that drives forward progress (foot placement is not impeded), but your heart rate spikes from the muscular effort required to drive your torso and legs forward through the brush. That’s brush work.
Brush work is a function of vegetation density (VD), i.e., the degree to which vegetation physically occupies the space at body height (e.g., thighs to shoulders) and resists passage. VD reflects how often a hiker must make physical contact with stems, branches, or foliage while moving forward. High VD environments require continuous pushing, bending, or squeezing through vegetation; low VD environments allow largely unobstructed movement.
In MEM terms, brush work raises your MDR because you’re adding resistance with every step – it’s kind of like swimming while pulling a drag chute behind you. You can still move with rhythm, but every stride costs more energy.
Impedance Work: Losing Your Stride
Impedance work is the energy cost of broken rhythm.
Humans move efficiently when we can settle into a cadence. But that cadence collapses off-trail when obstacles force us to stop, reroute, or scramble.
Imagine a burned forest tangled with blowdowns. There’s little or no brush – the air is wide open – but you’re climbing over trunks, crawling under others, and zig-zagging constantly to avoid obstacles that you can’t just push through as if it were brush work. You’re not tired from fighting vegetation; you’re tired because your stride never gets going. Every restart and change of direction, rotation of the body, side-step, high-step – this all wastes energy.
Blowdowns in SE Wyoming (BDR Grade III: VD > 3m, MI = 0.1, HP = 0.3):
Mobility impedance (MI) is the reduction in locomotor efficiency caused by obstacles or terrain features that disrupt stride rhythm and force detours, stops, or non-walking movements. MI is measured relative to baseline trail speed. High MI environments require directional changes that disrupt forward momentum. Low MI environments allow steady cadence with minimal deviation in forward progress.
In MEM terms, impedance work raises your MDR by destroying locomotor economy. Instead of recycling momentum from step to step, you’re burning fuel on inefficient starts, stops, detours, and moves that feel less like walking and more like scrambling.
Hazard Work: Staying in One Piece
The third category is hazard work – the effort of negotiating terrain and vegetation that can injure you.
Scrambling across loose talus, crawling under thorny blowdowns, or balancing on slick logs all require awkward body positions, stabilizing muscles, and mental focus. Hazard work isn’t just about going slower; it’s about protecting yourself from falls, cuts, and punctures.
Uneven, rocky terrain underfoot creates hazardous bushwhacking, like in this copse of subalpine fir in Rocky Mountain National Park, especially when the underfoot terrain is obscured by brush (BDR Grade II/III: VD < 1 to 2m, MI = 0.5, HP = 0.5):
Hazard potential (HP) defines the risk and energetic cost associated with terrain or vegetation that can cause injury or require protective or stabilizing maneuvers. HP encompasses exposure to falls, slips, cuts, punctures, and unstable footing. High HP environments demand crawling, scrambling, or balancing on precarious surfaces; low HP environments allow safe foot placement without unusual movements.
In MEM terms, hazard work is the tax you pay when safety, not just speed or sweat, becomes the limiting factor. MDR goes up because you are using more muscles – and firing those muscle fibers – just to remain stable in hazardous terrain.
Brush vs. Impedance vs. Hazard Work
These three types of work explain why different bushwhacks inflate your MDR in different ways.
- High brush, low impedance: Willow flats. Game trails through Krummholz. Constant contact, steady pace. The presence of brush doesn’t impede forward progress, and you can push through it.
- Low brush, high impedance: Blowdowns. Stride efficiency is disrupted by vegetation you can’t push through and have to go around.
- High brush, high impedance: Alder thickets. Brute-force bushwhacking through krummholz. Slow and exhausting.
- Hazard work dominant: Krummholz in a talus field. Thorn jungle. Brush may not be a significant issue, but the presence of brush (and impedance) complicates visibility and significantly raises energy costs.
In short, brush work inflates MDR by adding resistance, impedance work inflates MDR by wasting motion, and hazard work inflates MDR by forcing protective maneuvers.
Each one leaves you tired in a different way – sweat and muscle fatigue in the first, frustrated rhythm in the second, and mental or physical strain in the third.
Energy Multipliers
I’ve been surveying my data from the last several years, investigating the impact of various types of bushwhacking on energy cost. Here’s a quick summary:
The first number in the range represents non-inclined terrain, the second number in the range represents steep uphill terrain.
- Baseline (treadmill): MDR = 1.0 to 1.5
- Dirt trails: MDR = 1.3 to 2.5
- Willows (brush work): MDR = 1.5 to 3.5
- Blowdowns (micro-scale impedance work): MDR = 2.5 to 4.5
- Slide alder (brush + micro-scale impedance): MDR = 4.0 to 6.0
- Krummholz, broken by game trails and copse edges (macro-scale impedance): MDR = 2.0 to 3.5
- Krummholz, unbroken (brush + impedance): MDR = 5.0 to 8.0
- Krummholz, unbroken, underfoot talus (brush + impedance + hazard): MDR = 7.5 to 12.5
Bushwhacking Difficulty Ratings
As part of ongoing development with the Metabolic Energy Mile Model, we’re incorporating quantifiable bushwhacking difficulty ratings that define the metabolic cost of bushwhacking. These can be correlated to the physical characteristics of the bushwhacking environment (VD, MI, and HP) as follows:
Bushwhacking Difficulty Ratings (Metabolic Energy Mile Framework)
BDR Grade represents the overall grade based on VD (vegetation density), MI (mobility impedance, quantified as the relative reduction in pace as a result of brush and hazards), and HP (the % of steps with disrupted foot placement as a result of hazards). VD, MI, and HP should all be considered when grading a bushwhack. For example, for a route with VD < 0.5m, MI > 0.5, and HP < 0.1, the actual measured energy expenditure (MDR) is a more useful metric for assigning a BDR grade than any one factor alone. MDR values represent typical ranges for each grade.| BDR Grade | VD | MI | HP | MDR |
|---|---|---|---|---|
| I - Minimal | > 3m spacing (incidental contact) | > 0.75 | < 0.1 | < 2 |
| II - Light | 2 to 3m spacing (occasional contact) | 0.5 to 0.75 | 0.1 to 0.2 | 2 to 3 |
| III - Moderate | 1 to 2m spacing (frequent contact) | 0.25 to 0.5 | 0.2 to 0.4 | 4 to 6 |
| IV - Difficult | 0.5 to 1m spacing (constant contact) | 0.1 to 0.25 | 0.4 to 0.7 | 7 to 9 |
| V - Severe | < 0.5m spacing (dense brush, constant contact) | < 0.1 | > 0.7 | 10+ |
Summary
Every bushwhacker knows the difference between a day spent sweating in willows, a day spent clambering over blowdowns, and a day spent picking through krummholz growing in a talus field. Those are brush work, impedance work, and hazard work in action.
When we frame bushwhacking in MEM terms, brush work, impedance work, and hazard work are quantifiable drivers of energy cost. Not just “that alder patch was brutal,” but: “High brush, low impedance – tiring, but steady.” Or: “Low brush, high impedance – frustratingly slow.” Or: “Hazard-heavy – safe pace mattered more than speed.”
Quantifying the energy cost of bushwhacking enables us to more accurately predict energy (in calories) and time requirements (for route planning), and to share accurate expectations with others.
Discussion
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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.
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.
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:
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:
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:
The framework helps me break down the 1 and understand the 2.
A few takeaways for me:
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.
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.
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!!
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
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.
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.
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.
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