Running is naturally an aerobic sport, and performance is primarily governed by your body’s ability to move oxygen from the air and get it to your muscles. Heart rate monitors are tools designed to allow you to objectively monitor the performance of the systems responsible for this task. This, in turn, allows you to make sure you are getting the most out of your training.
There are three basic variables that control the quantity of oxygen that your cardiovascular system can transport. Firstly, the composition of your blood determines how much oxygen it can carry in a given volume. Secondly, your stroke volume determines the amount of blood that your heart moves in each beat. Finally, your heart rate determines the number of beats per minute.
The first two parameters are basically fixed in nature. With sustained training, they will improve, but that process occurs over the period of weeks and months. As such, your heart rate is the only mechanism that your body has to adapt to short-term demands. By monitoring this value, like watching the tachometer in your car, you can objectively determine your training intensity at any given time.
Unlike a car, however, the human body can adjust itself to deal with changing demands. When your systems are repeatedly put under stress, they will make adaptations to better deal with those stressors in the future. Training programs, such as the one that you are following now, stress your body in a controlled manner in order to trigger a specific set of adaptations suited to the target race.
The catch, however, is that the adaptations that the body makes are very specific to the stresses that are applied. Each of the different types of runs in your schedule has different objectives, so it is critical that they are done at the correct intensity. If they are done too hard or too easily, they will trigger different adaptations than intended and leave you improperly prepared when it comes to race day.
The tricky part of this is that it is often difficult to figure out exactly what the correct intensity is. Trying to tie it down to a specific pace is problematic as the mapping between intensity and pace is dependent on a number of variables. Further complicating this, as your fitness improves you will be able to run at faster paces at a given intensity.
Naturally, this is exactly the reason why heart rate monitors were created. By targeting specific heart rates during your runs, you will be able to precisely target the correct intensity and trigger the intended adaptations. As you get stronger, your heart rate for a given pace will become lower, allowing you to see when you need to step things up a bit to keep the stressor in place.
To simplify this process, physiologists have developed a structure composed of five different heart rate zones. Each zone encompasses a range of heart rates, and carries with it specific training characteristics. Nearly all modern HRMs provide tools to determine which zone you are in, as well as to analyze how much time is spent in each. Naturally, depending on what you are trying to achieve, the importance of each zone to your training program will vary.
The lowest zone on this scale, zone 1, is for extremely low intensity work such as a light walk or slow jog. Naturally, it produces little stress on your body, and therefore produces little to no training benefit in and of itself. With that said, it does serve to get the blood flowing and helps to warm up your tissues prior to heavier work. Similarly, at the end of the workout it is also potentially useful for clearing excess lactate from muscle tissues. As such, this zone is primarily used for the warmup and cooldown segments of your workouts.
The next step up the ladder is zone 2, which generally represents the bottom edge of what is considered aerobic exercise. For most people, running in this zone will require a conscious effort to slow yourself down and is basically akin to the long-slow distance runs that you do on Sundays. Due to this, it is often the most under-appreciated zone despite its critical role in improving a runner’s efficiency.
The energy that your body needs is provided from two primary sources – carbohydrates and fats. The former is naturally the ideal fuel; however your body can only store a maximum of about 2500 Calories in this form at any given time. The later, on the other hand, provides energy at a much slower rate; however each pound of fat can provide about 3500 Calories. If you are carrying 30 lbs of fat, for instance, that translates into a store of 105,000 Calories at your disposal. As such, when it comes to long distance running your ability to make use of that energy becomes critical to your success.
Whenever you run, your body burns a mixture of these two fuels. The higher your heart rate, the larger the percentage of carbohydrates you will consume. In zone 2, the majority of your energy is drawn from your fat stores. Because of this, spending time in this zone stresses the metabolic pathways responsible for burning fat. Over time, this will improve the efficiency of these systems and, in turn, allow a larger percentage of your energy to be drawn from your fat stores even at higher intensities.
For endurance sports, this is a critical adaptation as significant depletion of your carbohydrate stores becomes more and more likely as the length of your runs get longer. While complete depletion is unlikely in the half-marathon distance, it takes some time to regenerate those supplies between training sessions so it can negatively affect your ability to train properly. Every Calorie of energy that comes from fat is a Calorie that remains in your carbohydrate reserves, allowing you to recover for the next session faster.
Moving up another level, zone 3 is generally the range where people will naturally run without any conscious effort to control pace. This relates to the steady runs in your training schedule and will generally provide the bulk of the mileage that you rack up on a weekly basis. Running in this zone begins to stress the cardiovascular system, and triggers the body to make a number of changes to improve the transport of oxygen to the muscles. Primarily, it increases the density of capillaries in the muscles responsible for running, making it easier to get oxygenated blood to your muscle fibers. Secondly, it builds the strength of your heart muscles; thereby increasing the amount of blood it can pump with each beat (the stroke volume, mentioned above). This, in turn, means that your heart rate can run at lower levels for a given output level – allowing you to run faster in each of the zones covered here. As such, these adaptations are critical to pretty much every type of running and form an important part of any training plan.
Next up is zone 4, which is likely one of the most important zones for the half marathon distance. Running in this zone will typically require you to push yourself to maintain the pace; however it is slow enough that you can sustain it for significant distances. As such, this is the level that you will typically run races longer than 10K (including the half marathon). With that said runs in this zone take a lot out of you and will require significant recovery time, so there are limits to how much mileage you can accumulate in it.
This zone represents the level at which you begin to reach the limits of your ability to supply sufficient oxygen to your muscles. When this happens, your body must rely on its anaerobic pathways. This produces a byproduct called lactate that will accumulate as long as the oxygen deficit persists. When it builds up to a sufficient concentration, it causes pain and will begin to negatively affect the ability of your muscles to produce power.
Fortunately, your body is capable of clearing this substance from your blood in parallel with its production allowing you to push a little harder than you would otherwise be capable of. As such, there is a point within this zone called the lactate threshold where the rate at which lactate is produced exceeds the capacity of your body to clear it out of your system. When you run at or below this level, you will be able to sustain it for an extended period. If, however, you exceed this threshold, lactate will build up and eventually force you to slow down. The further you go beyond the threshold, the faster that this will become an issue.
As with the other zones, training at this threshold triggers a number of adaptations that will help to improve your ability to run at race pace. Primarily, it stresses the metabolic pathways responsible for processing lactate in your bloodstream. This, in turn, results in improvements in these systems that allow you to clear it faster and more efficiently – pushing the threshold to higher intensity levels. Secondly, repeated exposure to significant concentrations of lactate will trigger your tissues to develop a tolerance to it. This, in turn, will increase the concentrations necessary to cause problems and allow you to run beyond the threshold for a longer period of time.
Your lactate threshold is the primary physiological characteristic that will determine how fast you will be able to run long distance races like the half marathon. As such, training sessions focused on working in this zone, such as your tempo runs, are critical to getting the most out of your goal race. While they don’t represent a huge portion of your weekly mileage, it is important that you focus on running them at the correct intensity.
Finally, we come to zone 5, which represents maximal effort exercise such as speedwork and hill training. At these intensities, you will accumulate blood lactate very quickly so you cannot sustain it for very long. Exercise in this zone will increase the maximum volume of oxygen (VO2Max) that your body can transport, and it helps to trigger a number of adaptations that increase your maximum speed. Working out in this zone is critical for short distance races like the 5K and 10K (as this is the zone you will race them in), however its utility is somewhat limited for endurance distances like the marathon and half marathon so I won’t go into any further details.
With the basics covered, I’ll move along to the equipment itself. Heart rate monitors operate by using a simplified electrocardiogram device that wraps around your chest. Two small sensor pads capture the electrical activity within your heart, and a small microprocessor then processes that data. The resulting information is then relayed to a wrist unit, which provides a real-time readout of this value, allowing you to use this value as feedback to adjust your training.
With all but the most basic units, this data is also recorded in on-board memory and can often be uploaded into a computer for post-run analysis. Further, higher-end models also possess the ability to monitor other parameters such as pace, distance and elevation providing the heart rate plots with critical context. These capabilities are nearly as important as the real-time displays, as it allows you to look back at your runs and evaluate what you did right and what you did wrong. In addition, it allows you to precisely monitor your progress over the long term and determine the effectiveness of your training regime.