On flat ground you run at 6:00 min/km, but on a climb with a grade of about 10% your watch shows 8:00–9:00 at a similar heart rate. This is a possible scenario, not a sign of lost fitness: every kilometre also involves lifting your body upward. The difference of two to three minutes here is a guideline, not a conversion rule. To understand the load in the mountains, it helps to look not only at regular pace but also at GAP, rate of elevation gain, and your own effort.
Why minutes per kilometre lose meaning on a climb
Regular pace answers a simple question: how much time it took to cover a kilometre of distance. It doesn't explain what happened within that kilometre. A flat path, a gentle switchback, and a steep trail can have the same length but require different work. A 10% grade means roughly 10 metres of elevation gain per 100 metres of horizontal movement. That's already a substantial addition to the load, even if the surface is even and running is technically easy.
On a climb, part of the energy goes into working against gravity. Your stride usually becomes shorter, your body position changes, and maintaining your usual speed costs more. Trying to hold a flat-ground 6:00 min/km turns an easy workout into an intense one. The watch isn't confused: it correctly shows your forward speed, but that speed no longer describes effort the same way as on flat road.
Even the same heart rate doesn't guarantee exactly the same load. It responds with a delay and changes due to heat, altitude, accumulated fatigue, and duration of work. On a short steep segment, breathing and muscles may already be working hard while heart rate is still rising. So the comparison "same heart rate but two to three minutes slower" makes sense primarily on sufficiently long, evenly graded sections and in similar conditions.
- Regular pace. Needed for estimating movement time and route completion, but not suitable as the sole measure of intensity in the mountains.
- Heart rate. Helps control prolonged effort if you account for response delay and external conditions.
- Perceived effort. Breathing, ability to speak in sentences, and leg tension complement the numbers, especially with abrupt terrain changes.
Heart rate zones provide a useful framework for training, but their boundaries should match your settings and method of determining zones. Don't speed up on a climb just to hit a pace number, and don't treat the colour on the screen as an exact description of the entire load.
GAP: an attempt to translate a climb into the language of flat ground
GAP stands for grade adjusted pace. It estimates what pace on a flat surface could correspond to moving on a slope at comparable energy expenditure. This is a calculated value, not a measurement of your efficiency and not a prediction of road race results. Usually, speed and elevation profile are needed for the calculation, not just heart rate.
One well-known model of the energy cost of running is the Minetti formula. In it, the cost C depends on the grade i, written as a decimal: for a 10% climb i = 0.10, for a 10% descent i = −0.10. The polynomial version for running looks like this:
C(i) = 155.4 × i⁵ − 30.4 × i⁴ − 43.3 × i³ + 46.3 × i² + 19.5 × i + 3.6
Here C is measured in joules per kilogram of body mass and metre of distance. On flat terrain C(0) = 3.6 J/(kg·m). This is a model parameter, not the universal cost of running for any person. The point of the formula is practical: on an uphill grade every metre usually costs more, on a moderate downhill it costs less, and on a very steep downhill the cost rises again because you have to brake.
In a simplified calculation, flat speed equals uphill speed multiplied by C(i) / C(0). For pace, which expresses time per unit of distance, the ratio is inverted: GAP = actual pace × C(0) / C(i). That is why on a climb the calculated GAP is faster than the actual pace. For example, at a 10% grade the model gives C of about 5.97 J/(kg·m): an actual 9:00 min/km corresponds to roughly 5:26 GAP.
This example illustrates the maths; it does not promise that you will run the flat at 5:26 with the same heart rate. The formula describes the energy cost under certain conditions, not your individual heart rate response. Services may use other models, elevation smoothing and their own constraints. That is why the GAP of one app does not have to match the GAP of another, and the average grade of a whole route cannot simply be plugged into the formula instead of calculating by segments.
Vertical pace and VAM: how much elevation you gain
On a long climb it is more convenient to change the unit of measurement: count not kilometres of trail but metres of gain. Vertical pace shows how many minutes it takes to climb 100 metres. VAM expresses the same process in metres per hour. In the first case, lower is faster; in the second, higher is faster. These metrics are especially clear on a continuous climb with no long flat sections or descents.
- VAM. Elevation gain in metres divided by time in hours. If you gain 300 metres in 30 minutes, that is 600 m/h.
- Vertical pace. Time in minutes divided by the number of hundreds of metres gained. The same 300 metres in 30 minutes is 10 minutes per 100 metres.
- Quick conversion. VAM = 6000 / vertical pace, if the pace is given in minutes per 100 metres. For the reverse, divide 6000 by VAM.
For an amateur on a sustained climb, a benchmark of 400–700 m/h is sometimes quoted, that is roughly 15:00–8:34 per 100 metres of gain. For elite athletes, a benchmark of 1000–1500 m/h corresponds to 6:00–4:00 per 100 metres. These are not standards or boundaries of fitness level. Such ranges only make sense together with the duration of the effort, the grade, the surface, the altitude above sea level and the mode of movement.
On a gentle slope you have to cover many horizontal metres to gain 100 metres. On a steeper one there are fewer, so VAM can be higher without better fitness. But beyond a certain steepness, your stepping technique and the state of the trail get in the way. Running and brisk walking also give a different ratio of speed to effort. Switching to a walk does not cancel the workout: on a steep section it can be more convenient at a similar rate of gain.
Do not compare the VAM of a short acceleration with that of an hour-long climb. Also clarify how time is counted: a metric without stops will be higher than one calculated on total time. For assessing a climb, it is useful to account for pauses, while for analysing continuous work, choose a segment without them. The main thing is that the rule stays the same across all the workouts you compare.
How to use these metrics in training
Choose a familiar climb with a clear start and finish, preferably long enough for the effort to stabilise. Record a separate lap manually or use the same segment. Comparing yourself with yourself on this trail is more useful than trying to derive mountain fitness from a flat personal best. On a single segment there are fewer unknowns: the grade, the turns and the nature of the surface are roughly the same.
- Before the race. Define the task: steady long effort or a set intense segment. Choose an effort reference, not a mandatory climbing speed.
- During the climb. Watch your breathing and how you feel, check your heart rate after it stabilizes. Use GAP and VAM as additional indicators, not as commands to speed up.
- After the workout. Compare time, elevation gain, average heart rate, GAP, and subjective difficulty. Note heat, wet trail, stops, backpack, and transitions to walking.
- After several repeats. Look for a stable trend under similar conditions. One fast climb doesn't yet prove progress, and one slow climb doesn't mean a setback.
For example, the same climb with 250 metres of elevation gain first took 30 minutes, then 28 minutes. VAM rose from 500 to about 536 m/h. If the effort, conditions, and timing method were similar, that's a reason to note the change and verify it in your next workouts. If the second time you worked noticeably harder, what improved first and foremost is the speed of that particular segment, and a conclusion about form is still premature.
For analysis, use the same elevation source and one app. GPS noise, barometric altimeter drift, and different profile correction methods change elevation gain, and therefore VAM and GAP. Instant values on the watch can jump; averages over the whole selected climb are usually more useful. Don't try to correct every fluctuation in the metric by speeding up or slowing down.
Where grade adjustment doesn't help
GAP sees the geometry of the slope, but not the full complexity of movement. Two trails with the same gradient can require completely different effort. On one you freely choose your stride, on the other you step over roots, go around rocks, and constantly change your trajectory. The algorithm may interpret this as slow running, even though the limitation comes from the surface and technique, not from aerobic fitness.
- Rocks, roots and mud. They require precise foot placement, disrupt rhythm and add work that a standard grade adjustment does not account for.
- Steep and technical descents. Speed is limited by grip, visibility, skill and readiness to brake, not just by the energy cost of movement.
- Altitude, wind and load. They change the effort, but a standard GAP based on a single elevation profile does not explain this.
Read the numbers after a descent with particular caution. A low heart rate and an attractive GAP don't mean a small load on the muscles: braking work can be significant. VAM doesn't solve the problem here either, since it describes elevation gain. For technical trail running, the overall picture comes from time on familiar sections, how you feel, the nature of the terrain, and how you handle the load, not from a single "corrected" speed.
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This material is for informational purposes and does not replace a doctor's consultation.
