Monitoring training using your heart rate is not a new concept, and there are many good reasons to use it as long as you are aware of some limitations. Other metrics that measure output, such as pace and watts, have sought to replace heart rate, but I believe it is beneficial to get information from multiple sources and getting to know a little about your own body is a good place to start. Heart rate training first became popular was due to the direct relationship between heart rate and increasing workload. Heart rate quickly became an easy way to quantify the intensity of an exercise session or a portion of an exercise session. As technology has advanced, so have the ways to interpret this information. Now even basic heart rate monitors have all of the basic stopwatch functions as well as current heart rate, average heart rate, and, oftentimes, high/low settings. The more advanced gadgets have the ability to download the information to analyze an entire exercise session.
Heart rate is a relative measure of intensity and highly individual. For example, at the same running speed one athlete might have a heart rate of 120 beats per minute (BPM) while another athlete at the identical speed could have a heart rate of 140 BPM. Without any further information we cannot say which athlete is more fit, which athlete is working harder, or which athlete is older. If each athlete happened to have the same heart rate profile then we could say that the athlete at 140 BPM is at a higher relative intensity level. However, if the athlete operating at 140 BPM has a max heart rate and threshold heart rates that are much higher, then we cannot say with certainty which athlete is at a higher relative intensity. Also from this example you can see how heart rate-based calorie equations could be inaccurate.
Age-based heart rate equations do not work for most people. Across the population, there is indeed a relationship with age and max heart rate, but individually there is much variation. For decades age-based heart rate zones have been used with all sorts of complexity. For most people these equations will NEVER be correct, no matter what correction factor is used. Max heart rates can vary by as much as 40 beats at any age so as long as age is part of the equation, those outliers will never be able to use those zones. Because of this, training with heart rate has led to much confusion and frustration. Alternatives to an age-based heart rate equation include metabolic testing, lactate threshold testing, simple field testing, or using a race file to determine heart rate zones.
There are a few limitations of training with heart rate. Since heart rate lags behind the workload, it is more of a measure of work that you just completed and not the instantaneous intensity level. Therefore, heart rate is less useful for short, high intensity intervals. If heart rate is your only metric to use during interval training, be aware of this limitation by gradually ramping heart rate depending on the length of the interval. A common mistake many people make when interval training with heart rate is trying to get heart rate up to a certain number as quickly as possible. This results in a hard anaerobic effort followed by a reduced power output or pace for the remainder of the interval.
Heart rate can also be affected by temperature, dehydration, and fatigue. This could also be considered a benefit of heart rate since it is telling you something about what is going on inside your body in response to external stress. For example, I often see artificially high heart rates during the later stages of an indoor cycling session due to heat and dehydration (this is termed cardiac drift). Fatigue can affect heart rate in different ways, but most often I notice higher perceived exertion at the same heart rate when I am fatigued.
Heart rate is most useful for prolonged, steady-state workloads at moderate intensity levels. For long, easy workouts and sustained bouts at sub-threshold intensities, it can be a very useful tool to let you know if you are going too hard or not hard enough. For mountainous terrain, speed can be a very poor indicator of intensity so I find myself relying more on heart rate than any other metric during my outdoor workouts.
Finally, heart rate can also be a great indicator of improved fitness. A lowered resting heart rate indicates that the actual size and strength of the heart has increased. One of the most obvious indicators is the ability to do more work at the same submaximal heart rate. To measure improved fitness you need to combine heart rate with another metric such as time plus distance, pace, or power output.
Vail Valley resident Josiah Middaugh is an Xterra national champion as well as a coach and trainer at Dogma Athletica and a client at Vail Integrative Medical Group. Email comments about this column to firstname.lastname@example.org.