Using Rate of Perceived Exertion To Pace Your Distance Runs
by Eric Oliver, PT, USATF1
The rate of perceived exertion method (RPE) of gauging your body’s effort level during endurance exercise will help you utilize the appropriate energy system to power your endurance effort.
QUICK PHYSIOLOGY
Where Does Your Body’s Fuel Come From
The energy that powers our muscles during exercise is called ATP (adenosine triphosphate).
ATP is created when various energy systems break down a fuel source.
These fuel sources are called substrates.
The key substrates used by an energy system to create ATP include: glucose, glycogen, and fatty acids
Glucose is a form of sugar found in the bloodstream.
Glycogen is a sugar stored in muscle tissue and the liver. It is converted to glucose before entering the bloodstream to be used as a fuel source.
Fatty acids are found in the bloodstream and are sourced from the body’s stored fat.
How Are Substrates Converted To Energy?
The two energy systems that create ATP (energy) are the anaerobic energy system and the aerobic energy system.
The anaerobic energy system can create ATP WITHOUT the presence of oxygen. This system has a shorter duration during which it can create energy. There are two types of anaerobic pathways—one in which lactate is produced as a byproduct (lactic anaerobic), and the other in which no lactate is produced (alactic anaerobic). The alactic anaerobic system produces energy for explosive movements and sprints. It can only supply energy for this type of work effort for 10-15 seconds. The lactic anaerobic system is utilized for intense work efforts that can last up to 60-90 seconds.
The aerobic energy system creates ATP WITH the presence and consumption of oxygen. This system can create energy for an indefinite period of time. If you are sitting while reading this, you are using your aerobic system. You also will use your aerobic system during non-explosive and less intense forms of exercise.
The Effort In Which You Exercise Determines The Energy System Being Utilized
The reliance on a particular energy system is dependent on the effort of your activity. In other words, once your running effort reaches a certain difficulty threshold, the more you start to rely on the anaerobic system to help fuel your body. This is because a higher effort run requires more muscle recruitment, strength, and power. This requires more energy and quick production of it—both of which can only happen through the anaerobic pathway. Unfortunately, the anaerobic system can only supply energy for a finite period of time; the harder you work, the shorter the duration in which this system can provide energy.
Download RPE scale here.
What is Rate of Perceived exertion (RPE)?
The rate of perceived exertion is a grading system to gauge your effort level during cardiovascular exercise. The RPE scale has been backed by research to be a reliable estimate of how hard you are working and which energy system you are predominately relying on to create ATP to fuel your activity.
Knowing which energy system you are primarily operating within is important because it guides two key things:
duration that you are able to perform your activity
progress in your overall fitness (ie. your ability to run faster and longer before fatiguing)
As you can see with the RPE Scale to the right, different RPE grades (first column, 0-10) correlate to different efforts of running.
The higher the RPE number (0-10 scale) in which you are exercising, the higher your heart rate and respiratory rate.
As your heart/respiratory rate increase, the closer you get to needing your anaerobic energy system to function to help create ATP (energy). The moment your anaerobic system turns on is when the energy needed to run exceeds the ability of the aerobic system to create it. This moment is called the aerobic threshold.
You can hit your the aerobic threshold by increasing your run pace, but other factors that will increase your heart/respiratory rate include heat, humidity, elevation, hydration level, stress, and sleep quality.
A higher demand from your anaerobic energy system will shorten the duration at which you will be able to maintain that given effort/pace. Ie. Running at a 5K effort requires more immediate energy than a 10k, and due to the higher demand of the anaerobic system to maintain a 5K race pace you will physiologically be unable to maintain this 5K race pace for the duration of a 10k distance.
By programming your training around specific RPE ranges you can ensure that you are utilizing the appropriate energy system for the duration and distance that you are attempting to run. Additionally, training within specific RPE parameters will ensure that you are training for intended cardiovascular adaptations that will allow you to maximize your training without over-training, and you will be better able to manage your pacing on race day.
How to use the RPE scale
The RPE chart allows you to subjectively grade the effort level in which you are exercising by correlating it with a couple of self-checks—breathing rate and ability to talk.
RPE levels of 0-2 correlate to everyday activities. These activities rely solely on the aerobic energy system to fuel the processes needed to function.
RPE levels of 3-5 correlate to higher effort activities that rely on your aerobic energy system to fuel your body. This effort allows you to operate at higher heart rates for LONG periods of time.
RPE levels of 6-10 correlate to a greater reliance on the anaerobic energy system to fuel your body, and less reliance on the aerobic energy system. This effort level allows you to operate at even higher heart rates but for SHORTER periods of time. The higher the heart rate gets, the shorter the period of time you are able to operate in that pace.
For someone who is new to running, we recommend that you stay in the RPE of 3 to 6. The higher the efforts, the harder your body has to work (cardiovascular and muscularly), so if you are going to run at a higher RPE you need to make sure that your heart, skeleton, tendons, ligaments, and muscles are able to absorb the stress and strain.
For someone who is experienced (and musculoskeletally prepared) using the full range of RPE from 3 to 10 is appropriate as long as the higher level efforts are prescribed responsibly for the type of race for which you are aiming.
HOW DOES RPE RELATE TO PACE?
Running while being mindful of your RPE (ie. training by ‘feel’) allows you to train your physiology for specific demands as well as manage your race distances appropriately.
Training by feel also provides you a foundational strategy that allows you to change your pace (faster or slower) depending on the conditions in which you are running—heat, cold, humidity, uphill, downhill, flat, elevation, hydration, sickness, blood sugar level, sleep, carbon shoe (yes, even this!), and any other extrinsic/intrinsic factor that will effect your running pace. For example, running an RPE of 4 on a really hot, humid day will yield a pace that is different (slower) than running the same RPE on a cooler, cloudy day.
If your goal is to run a marathon, you will want to operate in the RPE of 4-5 for the majority of the race in order to not hit the wall (run out of energy). You cannot run the same pace on a hot, humid day as you would on a cool day, but you will want to run at the same RPE to mostly rely on the appropriate energy system (aerobic) to maximize your race potential without hitting the wall. Theoretically, if you haven’t hit the wall by mile 20, a more seasoned runner can increase their RPE to 6 for the remainder of the race with minimal risk of completely hitting the wall. Likewise, at mile 23 a runner can up their RPE to 7 and still have energy in the tank to finish. This does not necessarily mean that you are running faster, though. You are just working harder—either because you are trying to run faster than your training dictates, or you are making a final hard push to beat the clock (or fellow competitor). Your ability to maintain or increase a pace at certain RPE levels is dependent on your training.
In order to improve your aerobic capacity (ability to run at a faster pace for longer period of time) you need to train your physiology to force your body to adapt to using the aerobic energy system at faster paces. The only way to do this is to run in the RPE of 6-8 range. The caveat to this is that our bodies typically cannot handle running several days per week at these higher RPE levels because of the musculoskeletal demand that is required to run at this pace. Therefore, training sessions in the RPE 6-8 range should be reserved for 1-2 times per week in order to prevent excessive tissue stress which could lead to injury. The speed adaptation that a runner wants from this type of work happens over training seasons year over year. You cannot force this adaptation change by simply running harder more frequently. You need months and years of adaptive stress to develop your aerobic capacity, especially if you are a seasoned runner already. Employ a run-hard-all-the-time strategy and your body WILL break down.
But if you work your energy system over time and through measured stress to the anaerobic system, an RPE of 4 at a 9:00/mile pace will improve to a sub 9:00/mile pace at the same RPE of 4. This is the beauty of our body—place incremental stress on it, and it WILL adapt and become stronger and more efficient at whatever you’re practicing.
What if you don’t train energy systems appropriately?
If you aren’t holding yourself accountable to specific training paces you run the risk of:
over or under training
hitting the wall during your race or long trading runs
plateauing of your performance from season-to-season
injuring your musculoskeletal system by working too hard too much
FINAL THOUGHTS
Running a marathon, half-marathon, or ultramarathon is primarily performed utilizing the aerobic energy system, if done correctly. In fact, 75- 85% of the ATP produced during these races should be done by the aerobic energy system. If you start tapping into the anaerobic system too much or too soon in your race, you risk hindering your performance by burning through your glucose too quickly during the course of the race.
When you burn through your glucose too quickly it becomes difficult for your body to keep up with your muscle’s demand for ATP, because as you recall you need the glucose to create ATP. When you no longer have enough ATP to power your body, you “hit the wall”. Unfortunately, once you get to the last of your available glucose in the bloodstream it becomes difficult to replenish it. Your body cannot convert glycogen quick enough to fuel the processes that create ATP before you get to the end of your race, even with ingesting carbs (gel packets, gummies, etc). Also, you cannot just simply switch to using oxygen and your aerobic energy system when you start to get in trouble. This is because glucose is still necessary for the aerobic system to use oxygen to create ATP. Therefore, if you are on the struggle bus in the back half of a distance race (ultra, marathon, half marathon) due to hitting the wall, the fuel that you are ingesting will not be converted quickly enough to maintain your pace, and your body will force you to slow down so you don’t reach unsustainable RPE levels 8-10.
Long story short—if you don’t properly train your body to efficiently create and utilize ATP for extended periods of exercise and/or you don’t stick to running within the limits of your energy systems you will sabotage your performance either in training or in your race. Think of your body as a race car, all of the engineering, tuning, and aerodynamics won’t do the car any good during the race if you don’t have enough fuel to power it. Just as race car drivers plan out the use of their fuel when racing in order to optimize their performance, you too must do the same. Minding your RPE is an effective strategy to accomplishing this.