Working out too hard and not working out hard enough can both sabotage your fitness progress. The RPE scale can help you exercise at the right intensity.

Fitness progress is all about intensity. You'll only get faster if you practice running faster, and you'll only get stronger if you keep adding more weight. It's the basic rule of progressive overload: You must gradually increase the physical demands of your body to reach your fitness goals.

When working out, you should monitor your intensity to make sure you're exercising at a level that pushes you toward your goals, but not at a level that might injure you. You can do that with metrics like heart rate and VO2 max, but there's something much simpler you can use: perceived exertion.

Related:7 ways to recover from a tough workout

In the world of fitness, there's a nifty scale called Rate of Perceived Exertion (RPE) that goes from 1-10. Personal trainers often use RPE to gauge their clients' efforts during or after a workout. It's an incredibly simple yet powerful way of tracking fitness, but it hasn't gained much traction outside of the realm of personal training.

RPE basically involves asking yourself, "on a scale of one to 10, how hard is this activity right now?" Because of its simplicity, RPE serves as an easy, accurate way to track fitness progress for beginners.

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If you do any internet research on this topic, you'll notice that there are two RPE scales. The original RPE scale was called the Borg Scale of Perceived Exertion, developed by Dr. Gunnar Borg, who introduced it in the 1950s.

This older model runs from 6 to 20, with 6 correlating to no exertion at all (like watching TV) and 20 correlating to maximum effort (like sprinting at a pace you can't maintain for more than a few seconds).

The Borg scale's range of 6 to 20 might seem odd, but Borg designed it that way to correspond to healthy heart rates. The starting point, 6, corresponds to the healthy resting heart rate of 60 beats per minute (bpm); 8 corresponds to 80 bpm; and so forth.

Over time, a modified version of the Borg scale worked its way to the forefront. It's a typical 1-10 scale, and it's based on overall exertion, not heart rate. This is the scale known today as "RPE" and it's the one most fitness professionals recommend for beginners.

The RPE scale goes as follows:

Level 1: no effort at all (watching TV)

Level 2: barely noticeable effort (tying shoes)

Level 3: very light effort (washing dishes, folding clothes)

Level 4: light effort (leisurely walking)

Level 5: moderate effort (brisk walking)

Level 6: moderately difficult effort (jogging, easy hike)

Level 7: difficult effort (running, lifting moderately heavy weights)

Level 8: very difficult effort (lifting very heavy weights, difficult hike)

Level 9: extremely difficult effort (mountain biking on an incline)

Level 10: maximum effort (sprint at the end of a race)

RPE is based on how you feel during a specific workout, and everyone has different RPEs for different types of workouts.

You can also think of the RPE scale in this way, which is more fun and applicable to any activity:

Level 1: I'm doing nothing and I feel great.

Level 2: Still not really doing much, I could do this all day.

Level 3: I'm breathing a bit harder now, but I can maintain this pace.

Level 4: I'm starting to sweat, but I can still hold a conversation.

Level 5: I just pushed past the point of comfortable, but I can still talk easily.

Level 6: I'm feeling breathless and it's getting harder to talk. I'm sweating more now.

Level 7: I'm breathing very hard and my muscles burn. No talking, just grunting.

Level 8: Don't ask me a question -- I can't answer. I'm dripping sweat.

Level 9: OK, this really hurts. Can we stop now?

Level 10: I think I might die.

Someone who does yoga often might rate a basic yoga class as a level 3. Someone who's never done yoga, even if they are very fit, might rate the same yoga class as a level 6.

Professional runner Megan Roche told CNET that for most people, RPE is event-specific. For example, a runner and a powerlifter will rate a 3-mile run very differently. The runner might log it as a level 4, while the powerlifter might log it as a level 8. If they go on a hike together, they might both rate the hike as a level 6.

"The key is tuning into relative efforts for the activity, which should correlate with fitness over time," Roche said. "However, it's a constant recalibration process. If that 'easy' run makes you sore, it's probably not easy and you need to adjust how you think of your RPE."

Keep in mind, though, that effort isn't everything. You shouldn't misconstrue RPE to mean that you always need to workout at level 7 or higher. Exercising at too great an intensity too often can lead to injury and fatigue, among other symptoms of overtraining.

In fact, as a CrossFit coach, I encourage all of my athletes to take at least one full rest day (do nothing day) and one active recovery day each week. On an active recovery day, you should exercise at an RPE of 2 to 4. This easy level of exercise encourages blood flow, flushes out lactic acid, loosens up stiff muscles and counteracts inflammation.

Related: 6 health stats that matter more than your body weight

Effectively using RPE requires diligent logging, as does any other method of tracking your fitness.

You could always go the old-fashioned route of logging RPE by hand, much like the way you'd keep a pen-and-paper food journal. Make sure to write the full workout and any details that affect your effort level. For a running workout, you'd want to log pace and distance at a minimum. You could also include incline, temperature and terrain.

For a weightlifting workout, write down your sets and reps, weight loads and rest intervals. Don't just write "3x10 squat." Write "Back squat, 3x10, 150 pounds, 60 seconds rest between sets." Then log the RPE.

Here's an example of what might go in my RPE notebook:

CrossFit benchmark workout "Grace": 30 clean-and-jerks for time at 95 pounds. Time: 4 minutes, 51 seconds. RPE: 8.

If I decided to complete "Grace" again three months later, the entry might look like this:

CrossFit benchmark workout "Grace": 30 clean-and-jerks for time at 95 pounds. Time: 4 minutes, 25 seconds. RPE: 8.

According to those entries, I completed more work in less time, but felt the same. That shows I got more efficient (stronger, faster or both) at the clean-and-jerk.

You should also note other variables that play into RPE, such as fatigue level or soreness when starting the workout. You could do the same exact workout twice, but rate the second go-around as more difficult because you were tired or sore before even beginning.

Strava's new features, Perceived Exertion and Fitness, allow you to see long-term RPE trends across your workouts.

There aren't many ways to track RPE digitally, but there are some. Strava, the endurance sport-tracking app, just released a new RPE feature called Perceived Exertion, along with a feature called Fitness. These two features work together to allow app users to manually log workout intensity and view how workouts stack up over time.

Intensity, a workout app for strength training, has a nice setup based on progression tracking, which is what RPE is all about. Its many features collectively allow you to identify strengths and weaknesses among your workouts.

Loaded - Workout Journal also supports RPE tracking, along with a number of other functions.

With so many high-tech and in-depth ways to track health and fitness, you might wonder why you should use RPE. True, RPE might seem old-school, but exercising based on how you feel can offer valuable insights.

As Roche notes, "RPE is valuable because the brain incorporates countless variables to determine exertion. Heart rate is one thing, aerobic capacity another, but also everything from temperature to stress levels to sleep to energy availability."

So on the surface, RPE seems simple, but it actually meshes many fitness metrics into one.

"We have a computer in our head doing all these complex calculations every second, so if we learn to tune into those signals, it can be more powerful than any tracker," Roche said. "Plus, avoiding strict quantification of performance is usually better for love of the process."

The information contained in this article is for educational and informational purposes only and is not intended as health or medical advice. Always consult a physician or other qualified health provider regarding any questions you may have about a medical condition or health objectives.

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