haven't bought a power meter yet but started reading Allen and Coggan's power meter book as well as Friel's Training with an HR Monitor. Both books are very positive on power meter training but I'm a little confused. I think Friel wrote about heart rate as the "input" to the system and power as the "output" and both wrote of the problems training only with HR because "low" HR can indicate fitness or not enough effort, and "high" HR can indicate a lack of fitness or appropriate hard effort. However, it makes more sense to me to train the input to a system rather than the output. That is, if really believe in periodization, ie. that I want to do a particular workout in a given zone, if I train according to the output, eg. power, I could be way out of the proper heart rate zone, eg. if I feel crummy or am tired during a given workout, then my heart rate could be too high if I try to maintain a given level of power for an aerobic zone workout. Seems like HR takes into account all those confounding factors, appropriately. What am I missing?