“Heart rate monitoring alone does not tell you how much you are improving on your bicycle; it just tells you how fast your heart is pumping. Your heart rate may be affected by factors that have little to do with actual performance, however, and using only a heart monitor could easily trick you into believing a false conclusion about your fitness, mislead you about your performance, or even undermine your confidence.
Your heart rate is influenced by your level of hydration, by the air temperature, by your core temperature, by how well you slept the night before, by the level of stress in your life, and other factors. The rate at which your heart can pump depends on so many factors that sometimes you really are better off not knowing your heart rate when training or competing, and going on your “perceived exertion” instead. Although heart rate monitors can be valid and useful tools—athletes have been training with them now for more than twenty years, and certainly this has improved the level of fitness of many athletes—heart rate is just one small piece of the puzzle. How fast your heart is pumping is a response to a[…]”
Excerpt From: Hunter Allen & Andy Coggan PhD. “Training and Racing with a Power Meter, 2nd Ed.” iBooks.