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.
Home › Forums › The metabolic cost of bushwhacking: brush work, impedance work, and hazard work