I was amazed to learn that a Swedish psychologist, Gunnar Borg, in the 60s, was studying the perception of physical exertion using lumberjacks for physical stress measurement and relating the amount of energy expended to ‘how they felt’ (from a great article in ACSM Health and Fitness Journal). From this study came the measurement “Rating of Perceived Exertion”, or the RPE scale, that many triathlon trainers (among others) use when training athletes.
Borg developed a RPE scale of 0 – 10 (originally 0 – 20, but that was a bit too difficult to use) and it has become a widely used scale….in fact, I often use it myself when training, to see if I’m working in the right ‘zone’ for optimum effect. RPE does tend to track fairly well with Heart Rate (HR)…the chart is a handy reference for the RPE scale; ZONE numbers are HR zones, used in the same manner as RPE for measuring ‘how hard you’re working’, in much the same way. For example, during an easy run I would want to have a bit of difficulty carrying on a conversation (using breathing as the criterion) or RPE 3-4. Checking my HR monitor perhaps see I’m in HR Zone about right where I’d want to be. So I could likely rely on my breathing and ‘level of difficulty’ for training.
On the other hand if I wanted to do [what we call a Tempo run, RPE 6 or 6+] and didn’t have a HR monitor, I’d go by my breathing and pick up my pace until I could only do a few words at a time or around HR Zone 3…it would be fairly hard or uncomfortable but bearable for a pretty long time.
How you FEEL, the basis of RPE, while exercising is determined by body functions such as heart rate, oxygen intake, etc; psychological state like mood, experiences, fatigue; performance factors like duration, distance, people watching; and other body functions like temperature, pain, and so on. A real mess of things that create How Hard You Feel You’re Working, but through all those variables, tends to be reasonably accurate for training.
Sometimes older or new exercisers may report a higher stress number because they aren’t used to working hard, and stronger and more fit people might report a lower number than they’re actually working at, but generally the RPE number the person reports has been shown to correlate pretty well with heart rate (one of the easiest functions to monitor) for close to 90% of people,
Truth being told here, I’ve reported a lower stress number (lower RPE) to ‘look good’ (during a treadmill test), while a few times I’ve imagined a higher stress number than my heart rate was showing because I was feeling crappy and didn’t want to work so hard! But I use RPE often, while training, to figure out if I’m in ‘the zone’ I want to be.
At the end of the day, to quote an old saw (speaking of lumberjacks), I can train by RPE and, together with other data such as time, distance traveled, etc , and the result can be accurate enough to judge improvement, performance, and how good my workout actually was.