Trail running is becoming more and more popular—statistics say so. But I don’t need statistics to know that increasing numbers of runners are taking to the trails. I can tell by the emails I receive from advice-seeking athletes, a rising percentage of which are sent by trail runners.

The question that is most frequently asked by this cohort is a version of the following: “I do most of my training in the mountains and I find it difficult to keep my heart rate in Zone 2, especially on steep climbs. How do I obey the 80/20 Rule as a trail runner, or does it not apply to me?”

In case you are unaware, the 80/20 Rule is the idea that endurance athletes in all disciplines and of all ability levels gain the greatest amount of fitness when they do approximately 80 percent of their training at low intensity. On my 80/20 intensity scale, the top of Zone 2 corresponds to the upper limit of low intensity, so in practical terms, applying the 80/20 Rule means keeping your heart rate and/or pace and/or power below the top end of Zone 2 four-fifths of the time when running.

Due to the effect of gravity, runners must slow down to maintain the same physiological intensity when running uphill. Athletes with low to moderate levels of fitness may even have to dial all the way back to a walk to stay in Zone 2 on steeper climbs. Before I move on to talk about what these individuals should do to avoid falling into the all-too-common “moderate-intensity rut” as trail runners, let me first point out that runners at higher levels of fitness need not make any special modifications to their training as trail runners to stay in line with the 80/20 Rule.

I’ll use myself as an example. At my present level of fitness, my Zone 2 tops out at about 6:54 per mile. According to a certain online calculator, the effort level that is associated with running 6:54 per mile on level ground is equivalent to the effort level associated with running 9:32 per mile on a steep hill with a 10 percent gradient. So all I have to do to avoid creeping into moderate intensity in a hilly run that is intended to be done entirely at low intensity is keep my pace slower than 9:32 per mile on 10 percent inclines and make similar adjustments on hills with other degrees of slope. It’s just a matter of being aware and disciplined.

Now, I grant that most runners cannot ascend a 10 percent hill in Zone 2 without shifting to walking. So, then, what should you do if you’re in this group? My first suggestion is that you use a run power meter such as Stryd to monitor and control the intensity of your runs. This tool will give you a more reliable picture of how you are distributing the intensity of your training than will either pace or heart rate. Unlike your pace at the top end of Zone 2, your power at the top end of Zone 2 doesn’t change with topography. If your Zone 2 power tops out at, say, 220 watts, it does so regardless of whether you’re running uphill, downhill, or on level ground.

It’s true that your Zone 2 heart rate range also does not change with topography, but the trouble with heart rate is that it lags behind changes in intensity, so when you’re running on highly varied terrain your heart rate monitor is continually giving you yesterday’s news, so to speak. It can work, but not as well as a power meter.

My other bit of advice is that you match your workouts with your training venues so that you avoid spending more than 80 percent of your training time above Zone 2. One way to do this is to avoid challenging trail routes when doing runs that are intended to be done entirely at low intensity. A second, and complementary, way to achieve the same objective is to budget “unavoidable” time above Zone 2 into your weekly allowance of moderate- and high-intensity running. For example, suppose you like to run up a mountain and back down once a week and you’re above Zone 2 during the ascending portion of the run no matter how slow your pace is. Let’s supposed further that it takes you about one hour to get to the top and 35 minutes to come back down. There’s no reason you can’t include this workout in your weekly training schedule provided that the 60 minutes you spend above Zone, combined with any other moderate- to high-intensity running you do during the week, does not represent more than 20 percent of your total training time for the week.

There, I’ve taken away any and all excuses you might have had for falling into the moderate-intensity rut as a runner who trains primarily on trails.