The first article in this series, Should You Train According to Heart Rate?, explained that heart rate training can be an ideal tool for runners to use because it's predictable. It also covered how to find your target heart rate, as well as the lower and upper limits of your heart rate. Review the first article before attempting to structure a heart rate training program based on the advice in this article.
Once you know your actual max HR, knowing exactly what target HR to prescribe is where the task becomes complicated because there is great variability among runners concerning how long a given percentage of max HR can be sustained. This will depend, in part, on your general physical fitness level and your specific lactate threshold—the point above which lactate begins to quickly accumulate in the muscles and blood. For example, a new runner may feel discomfort after only a few minutes of running, even at 60 percent max HR, while a competitive runner with years of experience may run at 90 percent max HR without much discomfort.
It is paramount, therefore, to take into account your present fitness level when deciding what pace to run. The other major factor that determines what HR you should use is the goal of individual workouts.
Heart Rate for Aerobic Endurance Workouts
Continuous, aerobic running lasting 30 to 60 minutes or longer should be performed at about 70 to 75 percent max HR (60 to 65 percent HRR). These runs target cellular changes within the running muscles, such as increases in mitochondrial and capillary volumes. For this type of workout, 70 to 75 percent max HR is all you need to cause those changes.
Complete most of your running at 70 to 75 percent max HR during base building, when you're increasing weekly mileage. If the length of the run is well within your aerobic capacity and is a regular part of your training, it is possible for your heart rate to remain nearly constant throughout the run, as long as the terrain remains flat and it is not excessively hot. During very long runs, however, when glycogen levels are getting low and body temperature rises, heart rate will begin to drift upward as the body fatigues.
Heart Rate for Lactate Threshold Workouts
Perform workouts that target improvements in the lactate threshold at about 80 to 90 percent max HR. This intensity feels "comfortably hard." The more fit you are, the higher your lactate threshold is in relation to your max HR, so you'll need to increase the intensity to train the lactate threshold. By raising your lactate threshold, you will be able to run harder for longer periods of time. Training in this HR zone may take place in the latter portion of the base phase and the early competitive phase of the training year.