Think about the last time you did a long, hard run: Was there a point where you felt like you were going to die? If you’re reading this, then you obviously survived —and you probably finished the run, and maybe even walked another kilometre or so later that day to get beers or pizza.

That’s because exhaustion doesn’t come from physical limits (like glycogen depletion or dehydration), coach and author Matthew Fitzgerald explains in his book How Bad Do You Want It? Exhaustion is actually more of a psychological barrier, according to research by Samuele Marcora, a professor who studies the psychobiology of endurance performance. You hit the wall when you reach the maximum level of perceived effort you’re mentally willing to endure.

If you’ve ever had a treadmill class instructor yell at you about running at a 7 out of 10 effort, then you know what a rate of perceived effort or rate of perceived exertion (RPE) is. But is it something you proactively track? It should be.

RELATED: How to Train So You Don’t Hit the Wall So Damn Hard

What Exactly Is RPE?

Your rate of perceived exertion is a subjective assessment of how physically and mentally difficult an exercise is for you. “It’s not a number that you should be using in isolation to dictate a training plan, but it’s something that can better inform your training so you can be more efficient and optimize your workouts,” says Megan Roche, M.D., an athlete, endurance coach, and clinical researcher at Stanford University.

Since the 1960s, scientists and coaches have used a scale ranging from 0 to 10 to subjectively assess effort, with 0 being no exertion and 10 being the highest level. (“That was modified from a scale from 6 to 20; the idea was that when you added a zero to the number you chose, it should equate to your current working heart rate,” explains Polly de Mille, an exercise physiologist with the Hospital for Special Surgery in New York and a USAT-certified triathlon coach).

So, really, it’s just an arbitrary number—but one that gives a runner a way to track performance without fancy tech and while considering all the other variables that affect performance. Yes, activity trackers are incorporating fancy new technology to track training load, but “RPE clues you into your body’s actual response to what you’re doing,” says de Mille—which is so important because of the role perception of effort plays in running.

Training helps your body get fitter, but it also helps your brain become more comfortable with higher levels of perceived effort, Fitzgerald writes in his book. In other words, you have to train your brain to be uncomfortable just as much if not more than you train your body. “By training with RPE in mind, it extends your ability to withstand that hard effort,” says de Mille. When those higher levels of perceived effort become easier, that’s when you can really start to push your performance to the next level.

RELATED: Hard Workouts vs Overtraining?

How to Use RPE

Most runners use metrics such as pace and/or heart rate to determine effort ahead of time. Setting a specific RPE for a workout, though, can help you account for the effect that external stressors (think: dehydration, altitude, lack of sleep, or poor nutrition) have on your performance.

Any of those stressors will make your body work harder than normal to hit your goal RPE. In some cases, that could help you learn how to be comfortable at an uncomfortable certain pace; in other situations, you may have to adjust your workout to avoid overdoing it.

Assessing RPE, though, is highly subjective (it’s always hard to be honest with self-judgements). To help runners better track how intensely they worked out, Strava recently introduced a Perceived Exertion feature. The effort zones in the app break down like this:

• Easy (1-3): Could talk normally, breathing naturally, felt very comfortable
• Moderate (4-6): Could talk in short spurts, breathing more labored, within your comfort zone but working
• Hard (7-9): Could barely talk, breathing heavily, outside your comfort one
• Max effort (10): At your physical limit or past it, gasping for breath, couldn’t talk/could barely remember your name

If these descriptions sound familiar, it’s because they are often the same descriptions used to outline heart rate zones. So how do these RPE zones translate to running workouts? “Levels one to three should feel like a traditional recovery effort,” says Roche. “Four to six might be something a little bit more strenuous, like a tempo or threshold run, and then seven to 10 are really working into those high-intensity interval and speed efforts.”

But because the 0-10 rr scale is totally subjective, it’s important to figure out what feels “easy” and “hard” to you, not what someone else deems so.

RELATED: 5 Max Heart Rate Training Myths, Busted!

How RPE Helps You

While a coach likely won’t prescribe a training program based on RPEs, you should be paying attention to your RPE while you run. That’s because running workouts shouldn’t be one-intensity-fits-all. Slower, easier runs serve as aerobic conditioning or recovery, while harder speed work and intervals push your max heart rate and ability to sustain higher-intensities for longer.

RELATED: 10 Major Health Benefits Of Speed Training

The more you take note of your effort level, the better you’ll get at accurately gauging it, says de Mille. And the better you can gauge your intensity, the more you can push yourself and the less likely you are to push yourself too hard.

That has two benefits: First, it keeps you from falling into the moderate-intensity trap, that comfortably efficient pace that inevitably leads to a rut because you’re never doing low- or high-intensity, you’re just…running.

And if you’ve been following a training plan but the same workouts are actually starting to feel harder, “that pattern of increased exertion is actually a sign of overtraining,” says Roche. “If you’re tracking that, you can identify where it started and adjust your future training.”

Like pace and heart rate, RPE is just another tool in a runner’s arsenal—one that reminds you to trust your body and not just swear by high-tech devices. “Data, especially data in running, can be imperfect, which can negatively impact the course of a run,” says Roche. “I really like athletes to run by feel because I think it prevents judgements and provides a more holistic look at a run.” Because solid training is about the big picture—not just the data points on your activity tracker.

This article first appeared on runnersworld.com

READ MORE ON: RPE training training zones