When using the Karvonen Method, you should retest your resting HR once every few months to recalculate a target range since resting HR decreases as cardiovascular fitness improves. However, there is a limit as to how much the resting (or running) heart rate will decrease in response to training. The goal is not a heart rate of zero. The lower resting heart rate in endurance-trained runners results from a combination of an increased stroke volume (the volume of blood pumped by the heart's left ventricle with each beat) and an increased activity from the parasympathetic nervous system. Since max HR decreases with age by about 1 beat per minute per year, you should also readjust your target HR as you get older.
It is important to remember that the formula "220 - age" provides only an estimate of the max HR, and may be off by more than 10 to 15 beats per minute. All people of the same age do not have the same max HR. In fact, 68 percent of the population will have a max HR within one standard deviation of the population's average, with 95 percent falling within two standard deviations of the average. This rather large margin of error can lead to prescribing a running intensity that is either too low or too high to achieve the optimal benefit.
The equation tends to overestimate max HR in highly trained runners and underestimate max HR in untrained people. A more accurate way to determine your max HR would be to measure your HR while you perform an all-out running test, such as a race or a time trial.
Heart Rate Monitors
Although it can be relatively expensive to purchase a heart rate monitor, and wearing it may take some getting used to, both the time and the cost are worth the knowledge gained from using it. One of the best uses for heart rate monitors is to slow the pace of recovery runs enough so that you recover sufficiently from the previous day's interval workout so you're ready to handle another interval workout the next day.
The heart rate monitor allows for precise determination of the degree of effort. However, it is important to remember that, although the heart rate at any given running intensity can reflect your physical working capacity, there are limitations associated with using heart rate as a single independent variable. Heart rate can vary apart from fitness level, and is often related to emotional state, environmental conditions, amount of sleep or elapsed time after a previous meal.
You can use a heart rate monitor during workouts to link your workout paces with your heart rates. Using these numbers, you can generate an entire heart rate profile. Once this profile is established, you could specifically target your workouts based on heart rate at specific times of the training year. Over months and years of training, pace changes for workouts can be matched with heart rate changes, making the measurement of fitness gains more objective and observable.
Finally, by monitoring training using heart rate, over time you will begin to understand what a given heart rate feels like. This is important because becoming more aware of your body and the link between your physiology and your perceived exertion is a vital step toward high athletic performance. At the very least, it gives you an appreciation for the wonderful adaptations of the runner's body.race.