Myth Busted
You will be slower. That part is true, and the physics behind it is not remotely subtle. What is not true is the conclusion everybody draws from it — because your watch is measuring the wrong thing entirely.
3.3%
Slower per 1%
of gradient
1.31
ml of oxygen per kg,
per metre climbed
10–20%
More calories at
the same pace
d=0.95
Trail group’s effect
on VO₂ max
Section 01
My first proper trail run humiliated me. I came home, opened the app, and found a pace I had not seen since my very first weeks of running. I had worked considerably harder than usual and produced a number that looked like I had been on holiday from the sport.
So let us deal with the true part first, because pretending otherwise helps nobody. Trail running is slower, and it is slower for reasons that are pure physics.
Climbing is expensive. Moving your body upward costs roughly 1.31 millilitres of oxygen per kilogram of bodyweight for every metre of ascent — energy spent going up rather than forward. The measured effect on pace is startling: for every 1% of gradient, even an elite runner slows by about 3.3%. A modest 5% climb takes a sixth of your speed before you factor in anything else.
Then add the ground itself. Every root, rock and rut forces a micro-adjustment, and each one costs a fraction of momentum. On uneven footing you brake constantly without noticing. That is why trail running burns 10 to 20% more calories at the same pace than road — you are doing more work to cover less ground.
Same Runner · Same Effort · Different Ground
| Terrain | What it costs you | Pace per mile |
|---|---|---|
| Flat road | Baseline | 9:00 |
| Rolling gravel path | Gentle climbing, softer surface | 9:30 – 10:00 |
| Undulating singletrack | Climbing, uneven footing | 10:00 – 10:30 |
| Technical trail | Rocks, roots, constant braking | 11:00 – 12:00 |
Typical slowdown is around 30 to 90 seconds per mile, and one to three minutes on genuinely technical ground. Effort is identical in every row.
Section 02
Pace is distance divided by time. It is not a measure of effort, of fitness, or of work done — it is a measure of how much flat ground you covered per minute. On flat road those things line up closely enough that we forget they are different. On a hill they come apart completely.
The sport has a fix for this, and if you take one practical thing from this page, take this. It is called Grade Adjusted Pace — GAP — and it answers a much better question: if you had run flat ground at this exact physiological effort, how fast would you have been going? Strava calculates it, most watches now show it, and almost nobody looks at it.
Start looking at it, and the picture inverts. That demoralising 11-minute mile on a climbing trail may have a grade adjusted pace of 9:15. You did not have a bad run. You had a hard one, and the ordinary number could not see it.
One honest limitation, because this page is not an advert for a metric. GAP adjusts for gradient only. It has no idea whether you were on smooth gravel or picking your way over wet roots, and technical ground is a large part of why trail is slow. So GAP still understates your effort on the roughest trails — it is a better instrument, not a perfect one.
A TWO HOUR TRAIL RUN
IS A TWO HOUR TRAIL RUN,
WHATEVER THE MILES SAY.
Trail runners measure time on feet. There is a reason for that.
Section 03
Somebody ran the experiment properly. Twenty previously sedentary people were randomly assigned to either a trail group or a road group, and both completed the same supervised, progressive, workload-matched eight-week programme. Researchers measured balance, gait, agility, leg strength and predicted VO₂ max before and after.
The trail group showed a large effect on predicted VO₂ max — d = 0.95. The road group showed a moderate one, d = 0.53. In plain terms, the aerobic improvement in the trail group was roughly twice the magnitude, despite every one of their runs being slower on paper.
Now the caveat, which matters and which I am not going to bury. With only twenty participants, the difference between the two groups was not statistically significant. The authors concluded there were tendencies slightly favouring trail, and that more research is needed. This is not proof that trail beats road.
But look at what it does establish. The myth claims trail running costs you fitness. In a controlled trial with matched workloads, the trail group's aerobic gains were, if anything, larger — while their pace numbers looked worse the entire time. Whatever the slower watch was recording, it was not a slower body.
Section 04
Load that is spread rather than repeated. Road running feeds an almost identical stride into the same tissues thousands of times an hour. On a trail your footstrike, stride length and joint angles change with every step, which is a large part of why overuse injuries appear to be less common off-road. The knee that dislikes ten thousand identical repetitions gets a different demand each time.
Stabilisers that road running never asks for. Every uneven landing recruits the small muscles around the ankle, foot and hip that hold you upright. The demand is so genuine that uneven-surface training is used clinically — there are rehabilitation programmes for patients with ankle instability built around exactly this kind of ground.
Descending strength, which is its own skill. Running downhill loads muscles while they lengthen, and that eccentric work is the reason your quads ache for days after a hilly run. Train it gradually and you build genuine resilience to it — which is precisely the quality that falls apart in the late miles of a hilly road race.
An hour where you cannot think about pace. On technical ground your attention goes entirely to the next three metres. Whatever you were turning over in your head does not survive it. That is not a training benefit you can measure, and it is the reason a lot of us keep going back.
Section 05 · Five Things Worth Knowing
The most common report from runners who spend a winter on trails is that their first road run afterwards feels oddly easy — the ground stops interrupting them and the effort suddenly all goes forward. It is not a promised outcome and I would not sell it as one. But it is a very common experience, and it makes mechanical sense once you have spent months paying for every step.
Nothing good comes from measuring a hill route against a flat one. Keep one or two regular trail loops and track your times on those, ideally alongside heart rate. Improvement on the same route at the same effort is real information. Comparing Tuesday's canal path to Sunday's moorland climb tells you about the terrain, not about you.
Your body has no established pattern for uneven ground yet, so the early sessions feel disproportionately hard and disproportionately slow. That improves quickly — a lot of the initial adaptation is coordination rather than fitness, which is why progress in the first month can be surprisingly steep.
Beyond roughly a 15% gradient, walking moves you at almost the same speed as running for far less energy — which is exactly why elite mountain ultrarunners power hike. Runners who refuse to walk climbs blow up early and produce worse times over the full route than the ones who hiked, and they enjoy it considerably less.
This is why experienced trail runners plan in hours rather than kilometres. Ninety minutes is ninety minutes of aerobic work whether it covered nine kilometres or fourteen. Switch your watch screen to elapsed time and heart rate, and put pace somewhere you have to scroll to find it. It changes the entire experience within about two runs.
The Bottom Line
Trail running will not wreck your pace. It will produce slower pace numbers, which is a different sentence entirely. Climbing costs oxygen, uneven ground costs momentum, and the figure your watch reports is distance over time — a measurement that was never designed to describe what a hill does to a body.
What To Track On Trail · Ranked
| Metric | Worth watching | |
|---|---|---|
| 01 | Time on feet | Yes — the primary one |
| 02 | Heart rate or perceived effort | Yes |
| 03 | Elevation gained | Yes |
| 04 | Grade adjusted pace | Occasionally, with a pinch of salt |
| 05 | Raw pace per kilometre | Almost never |
Meanwhile the trial evidence, thin as it currently is, points the opposite way to the myth: matched workloads, and the trail group's aerobic gains were the larger of the two. Add stabiliser strength, eccentric resilience, varied loading and fewer repetitive stress injuries, and the case for keeping trail in your week is strong whatever your road ambitions are.
So keep the trail runs. Move pace off your watch face. Compare hills to hills. And accept the slower number as the price of a harder run — because that is precisely what it is.
Sources & Research
Fröhlich M et al. (2023). Effects of trail running versus road running — effects on neuromuscular and endurance performance: a two-arm randomized controlled study. International Journal of Environmental Research and Public Health, 20(5), 4501. PMC10002259.
Minetti AE et al. (2002). Energy cost of walking and running at extreme uphill and downhill slopes. Journal of Applied Physiology — the basis of the oxygen cost of vertical ascent.
Vincent HK et al. (2022). Injury prevention, safe training techniques, rehabilitation and return to sport in trail runners. Arthroscopy, Sports Medicine and Rehabilitation. PMC8811510.
Physical Activity and Health (2023). Characteristics of lower limb running-related injuries in trail runners: a systematic review.
Journal of Experimental Biology (2015). Biomechanics and energetics of running on uneven terrain, 218(5), 711–722.
Besson T et al. (2022). Elite road versus trail runners: comparing economy, biomechanics, strength and power. PubMed 36515604.
ClinicalTrials.gov NCT04999904. A proprioceptive training program using an uneven terrain treadmill for patients with ankle instability.
Educational content, not medical advice. Research comparing trail and road running is still young and the sample sizes are small — figures here are reported with that limitation stated rather than hidden.
Frequently Asked
Two reasons, both physical. Climbing costs roughly 1.31 millilitres of oxygen per kilogram for every metre of ascent, and every 1% of gradient slows even an elite runner by about 3.3%. On top of that, uneven ground forces constant micro-adjustments that cost momentum. Typical slowdown is 30 to 90 seconds per mile, and one to three minutes on genuinely technical terrain.
There is no evidence that it does. In an eight-week randomised trial with matched workloads, the trail group showed a larger effect on predicted VO₂ max than the road group, though with only twenty participants the difference between groups was not statistically significant. Slower pace numbers are not the same thing as a slower body.
GAP estimates what your pace would have been on flat ground at the same physiological effort. Strava calculates it and most modern watches display it. It is a far better measure of trail effort than raw pace — though it adjusts only for gradient, so it still understates how hard technical ground is.
Heart rate or perceived effort, with time on feet as your primary metric. Raw pace per kilometre is close to meaningless off-road because it cannot see gradient or terrain. Many experienced trail runners move pace off their watch face entirely.
Keep one or two regular trail loops and compare your times on those, ideally alongside heart rate. Improvement on the same route at the same effort is real information. Comparing a hilly trail to a flat road tells you about the terrain, not about your fitness.
Yes — typically 10 to 20% more than road running at the same pace, because you are doing more work to cover less ground. It is one of the clearer signs that the slower number on your watch is not measuring effort.
MORE RUNNING MYTHS WORTH UNLEARNING.
More Miles Means Fitter →Heel Striking Is Bad →Trail Running Guide →