The diagram here (click to enlarge) illustrates what happens to heart rate, rating of perceived exertion (RPE), speed, and power while riding a bike steadily uphill, coasting downhill, and then on flat terrain. (Please pardon my crude drawing—an artist I’m not.)
Note that heart rate was slow to respond as the rider started up the hill and continued to increase in the early part of the descent. This lag is common with heart rate.
RPE increased on the climb also as fatigue gradually set in. It rather quickly decreased on the downhill side before rising again as pedaling was resumed on flat terrain.
Speed remained constantly low on the uphill, increased on the descent, and eventually settled in at a steady rate with the return to flat terrain.
Following a steady level on the climb, power quickly responds to the transition from climbing to coasting and from coasting to pedaling on the flat section. It reflects some of the changes taking place in the other three, but does so rather quickly.
The higher the power up the hill and on the flat terrain, the greater the rider’s performance. The same can be said of speed. Only power and speed are directly related to performance. Heart rate and RPE tell us nothing about performance—they simply reflect what the rider is experiencing. But when compared with power, heart rate and RPE also tell us something about the rider’s fitness. When power is high and heart rate and RPE relatively low compared with previous rides on that same hill, we know that the athlete is fitter and faster.
Both the performance and the experience of creating the performance are important. These may be referred to as “output” (power and speed) and “input” (heart rate and RPE). The most reliable for training and racing purposes are power and heart rate (the exception being in bicycle road racing when RPE is critical to dealing with all of the sudden changes in intensity). Comparing these two makes measuring progress in training much more meaningful. If you know only your heart rate you are lacking the most critical piece of information—what you are accomplishing with your input. Training with a power meter is like watching a movie in 3D instead of only in 2 dimensions—much greater depth and meaning.