INTRO. TO HEART RATE MONITORS
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.
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.
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.