Lately, a common question heard in the locker room and on the roads is "what kind of heart rate monitor are you using?" You see them in magazines. Your training partner just got one. What is going on? It seems as if everyone has become "wired" and has started babbling in physiology techno-speak: anaerobic threshold, Vee-Oh-two-max, and Krebs cycle mitochondrial enzymes.
What is a heart rate monitor, and how can it help improve your performance? Is it just a fad, or is there some scientific basis for training with these devices? The goal of this article is to provide the answers to these questions and to familiarize you with concepts used in exercise physiology.

Physiology 101

Like any combustion engine, your body uses oxygen and fuel to generate energy. The cardiovascular system delivers oxygen to the skeletal muscles, which then use this oxygen to "burn" various fuels (carbohydrate and fat) to yield mechanical energy. A unique feature of your body is its ability to change in response to the demands placed on it. By working out hard, you overload your aerobic systems. During rest, your body adapts to make you stronger. This is accomplished by improvements in cardiovascular and muscular function. The heart becomes stronger and more efficient, and the skeletal muscles become better at extracting oxygen from the bloodstream. Within muscle cells, the mitochondria boost their enzyme systems to oxidize fuels.
All of these changes occur slowly over time. For continued improvement, you must continue to overload these systems. As you adapt, however, you require harder workouts to do this. How do you know if you are training at the right level? Physiologists have discovered that the rate of oxygen "burned" in the muscles is the best measure of aerobic work. To determine this requires expensive equipment and specialized testing facilities. Basically, an individual runs on a treadmill while heart rate, and volume of inhaled and exhaled air are measured. Samples of exhaled air are periodically taken and the oxygen concentration determined. The difference between the amount of oxygen breathed in and out during the test is what the muscles have consumed to burn fuel. The rate of oxygen consumption, in liters per minute, is called V O2. The test is done at progressively harder levels until the individual maxes out. The maximum rate of oxygen consumption is called the V O2 max.

Using the numbers to improve

Research on V O2 has shown that there is a threshold below which no additional gains are achieved in aerobic exercise. For most people, this is a pace that allows for casual conversation during the workout, and is approximately 55% of V O2max. Above this level you are sufficiently overloading your cardiovascular and muscular systems to bring about improvement. The lab equipment is much too bulky to take with you on the road. So, how do you know if you are above this level in your workouts?

Maximum heart rate is related to maximum VO2

The heart rate is much easier to measure and is a very good approximation of VO2. The relationship between percentage of maximum heart rate and percentage of V O2 max is very predictable and is independent of age, gender, or level of fitness. As you can see from the graph, 55% VO2 max corresponds to about 70% max heart rate. Thus once you have determined your maximum heart rate, you have a very convenient method of monitoring your workouts. Because heart rate monitors are reasonably inexpensive and lightweight they are the training tool of choice for many elite endurance athletes.

There are many types of heart rate monitors. Most consist of a chest electrode/transmitter which identifies the electrical signals from the heart and beams them to a receiver. The receiver is a watch-like device that displays the heart rate in beats per minute. You can usually set high and low limits for your workout. Additionally, some wrist units have functions that calculate average beats per minute, time spent in the target range, and can record data from your workout for analysis later.

Correctly using a heart rate monitor

Now that you understand the science, how do you use it to get faster? First, you must determine what your maximum heart rate is. Most people have seen the equation:

Max Heart rate (bpm) = 220 - age

This is only a rough approximation, and there is considerable variation between individuals of the same age. You can directly determine your maximum heart rate by an exercise stress test. It is vitally important that you undergo a physical examination prior to any stress test -- especially if you are over 35.

Once you have determined your maximum heart rate, you can construct a target zone for your workouts. The aerobic training zone is usually between 70% and 90% max heart rate. Most training schedules incorporate different types of workouts (eg. long slow distance, high intensity intervals). You can construct different target zones depending upon the type of workout you are performing. The heart rate monitor helps you stay in that zone so that you can achieve your goal for that workout.
The biggest advantage of using this approach in your training schedule is the ability to account for improvement. Suppose a 30 year old male averages 9 minutes per mile for a 10k workout with an average heart rate of 145 beats per minute. As he improves, his average heart rate will decrease for the same 9 minutes per mile. If he focuses only on keeping a constant time of 9 minutes per mile, he will reach a point where his workout no longer challenges his aerobic system. At this point, his workout can only maintain aerobic fitness and no further improvement can occur. If, however, he focuses on keeping an average of 145 beats per minute for each workout, he will apply a constant overload to his aerobic system. Over months, his average time per mile should steadily decrease. Another advantage is the ability to account for variable terrain and wind. Take our 30 year old and make him run into a 20 mph headwind. Clearly, he must perform extra work per mile. The heart rate monitor helps him keep the applied load constant even though his time will necessarily be slower.

Final words

If used wisely, a heart rate monitor can be a great asset to any endurance athlete's training schedule. A poorly designed schedule reduces the heart rate monitor to an expensive toy that you will quickly tire of. To start, you must determine your goals. Whether it is dropping 2 minutes off your 10k time, dropping 10 pounds, or completing your first Ironman, set realistic goals. From here you can construct a workout plan that incorporates different heart rate target zones for different types of workouts. The best book I've read on this subject is Serious Training for Serious Athletes by R. Sleamaker.
For those just beginning to use a heart rate monitor I recommend an observation period before a new schedule is introduced. The data from the monitor is used to give you an idea of what your own heart rate is for your current schedule. Finally, it is very important to avoid becoming a slave to the heart rate monitor. You should always pay attention to what your body is telling you. A healthy exercise schedule blends both science and "feel". The heart rate monitor is not a speedometer. If you ignore warning signals from your body, you may find the monitor flashing "call 911" (emergency).