One reason for using several methods is to give some idea about how much uncertainty there is about the precise heart rate figures. The trouble is that people are all different, what works well for me might not work for you. Some of the older methods, such as Dr. Kanopka s work entirely on age. Opinions vary regarding how many training zones should be defined. But generally speaking lower intensities are used in combination with longer distances, to develop basic endurance and stamina, and to exercise the body s fat burning metabolism.
Higher intensities develop power, and exercise the body s carbohydrate burning metabolism.
Very high intensities, that can only be maintained for very short duration s are used to develop high powered sprints, mainly fueled by carbohydrates, but also by stored muscular glycogen, using stored phosphates to liberate the required oxygen supply. So if you want to improve your 50 mile time, practice 50 mile rides, if you want to improve your sprints, practice sprinting or hill climbing. The methods that work best for me are either Peter Keens, or the Karnovan method. Both of these give very similar results. What I mean by this is that the descriptions of subjective feelings match the quoted THR figures best.