One thing may strike you as strange about this scale–why does the scale go from 6 to 20? This may lead you to consider another "coincidence," namely how a prototypical heart range capacity ranges from resting of 60 to a maximum of 200. This isn't a coincidence at all, as research has demonstrated that there is a very close correlation between heart rate and perceived effort.
Even still, for the lay public, a scale from 6 to 20 isn't overly intuitive to grasp, such that another version of the Borg scale runs from 1 to 10, and this is what many coaches prescribe to their athletes for ease of understanding.
RPE Clamp Protocol
The Borg scale or its variants is commonly employed as a research tool for gauging perceived effort, and also for secondary support for a tool such as a HRM or power monitor. However, one interesting flip on the question is this, "How constant is my workload if I try to maintain a set perceived effort?" In other words, if I went out and did a time trial at a RPE of 16 (between "Hard" and "Very Hard"), does my power output fluctuate over time?
Such a question is of interest both to physiologists and to coaches and athletes. For physiologists, it gets at the central question of "What is the drive for exercise and fatigue?" For coaches and athletes, it helps to calibrate our objective tools like power monitors with individual sensations, helping in turn to adjust training.
This question was again nicely designed and tested by my colleagues at the University of Cape Town from Dr. Tim Noakes's laboratory. His Ph.D. student, Ross Tucker, published a 2006 study using a RPE clamp protocol (2), wherein he had subjects ride at their own self-selected power output at a constant RPE = 16. No power, time, cadence, heart rate, or distance cues were given at any stage of the test, and subjects were told simply to maintain RPE of 16 and adjust wattage (they couldn't see the wattage) as needed. They had subjects do this in 15, 25, and 35 degrees Celsius, but I won't go into detail with those findings.
What they did find was that, in all three environmental conditions, the self-selected power progressively decreased over time in a fairly linear fashion.
Rubber on the Road
So what does a simple but interesting protocol tell us about our drive to exercise and fatigue?? We aren't stable in our perception of effort! This certainly highlights that we might not want to solely rely on our perceived effort to gauge the intensity of intervals or time trials. As if there aren't enough excuses to spring for a power monitor!
? The voluntary drive for exercise remains an important determinant of how hard and fast you can go. One theory is that your brain is deliberately lowering or putting a ceiling on your exercise to prevent you from damaging yourself. However, it can fool you into underestimating your capacity. As the final sprint and spike in power output suggests, you've still got a fair bit of neuromuscular capacity within you even at the point of voluntary exhaustion.
? The above can especially be used during the late stages of a race when the hammer is about to drop. At that point, as REM sings, "Everybody Hurts," but realize that you still have a capacity for burning one more match, and this is exactly the best time to do it when everyone else is mentally suffering.