One of the most important aspects of periodization is the intensity at which each workout is executed. For improvement to occur more effectively, training intensity needs to be measured and controlled.
The easiest measure to use is Rate of Perceived Exertion, or RPE. which is how difficult the workout feels. Also easy to use is percent, although what the various percents mean is open to interpretation. I also use my own “Talk Index” defining how easy or difficult it would be to talk at a given output. Then there is heart rate, with ranges for individual training zones. Average pace is a good indicator once an athlete knows their pace range for various intensities. Finally, power can be used on the bike (maybe someday on the run and swim too) with a power meter.
Each of these methods yield results of varying accuracy since our perception of effort is not always perfect and our breathing and heart rate can be affected by other things besides exercise. Power is a very good standard since it is not affected by these factors. As mentioned, currently this is only available for cycling.
It is my recommendation that athletes use a multi-pronged approach in their training, using some or all of the above-mentioned measures. Some athletes relate better to RPE, while others love to watch their heart rate, etc. By following more indicators, an athlete can dial their training into the correct zone and also find the measures that work best for them as an individual.
The longer I work with an athlete, the more we fine tune these measures, one of the keys to that athlete reaching their potential.