To understand why training and racing with a heart rate monitor are important one must understand some basic physiology that occurs in the body. Energy, which every athlete seeks, is the capacity to do work. The main sources of energy that the body uses in endurance sports are carbohydrates, fat, protein, and lactate. Usually, but not ideally, in that order. Fat provides more calories per gram than any other source. In fact, fat provides over twice as much energy as carbohydrates. The human body has almost an unlimited supply of stored fat for energy. If one trains the body to use this unlimited supply of energy then they will have the ability to train and race for extended periods of time without fail.
Metabolism is the term that refers to all chemical reactions in the body that enable it to maintain its proper functions of life. Catabolism, a part of metabolism, is any metabolic process whereby cells breakdown larger complex substances into smaller molecules, such as bread being broken down into glucose, a simple sugar that passes through the cells for energy. Anabolism is the process whereby cells convert simple substances into more complex ones, such as glucose being stored in the body as glycogen.
The simplest product of metabolism, and the molecule that is full of potential energy, is Adenosine Triphosphate, or ATP. ATP plays a very important role in endurance sports. An abundance of ATP mean lots of speed, power, and endurance. In contrast, a diminished supply of ATP forces the body to rest and restore its energy supply. In other words, when the body runs out of available energy it "hits the wall" and can go no further until more ATP is supplied.
Oxygen is a necessary component of aerobic metabolism. Aerobic means "with oxygen." For aerobic metabolism to take full effect then many oxygen molecules are required to produce many ATP molecules. Anaerobic metabolism means "without oxygen." Energy (ATP) can be produced when oxygen is not available to the cells but the amount of ATP is limited and the byproduct lactate is produced in excess amounts. Lactate can be reused for energy but without oxygen that energy is limited. The effects of lactate are readily felt when a person exercises at high intensity without enough oxygen. There is a burning sensation in the body and an increase in respiration to remove the excess acid from the body through the lungs.
Because of oxygen demands, the body is limited to the amount of time it can sustain anaerobic metabolism to around 45-70 minutes, depending on the fitness level of the athlete. After 60 minutes of complete anaerobic efforts the body will be forced to slow down so that more aerobic metabolism can provide the energy for the person to sustain a reasonable effort. During anaerobic efforts the body shunts the use of fat as a fuel source and uses the readily available source of glycogen. The human body has approximately 3000 calories worth of glycogen stored within the liver and muscles. If anaerobic efforts are used for approximately 60 minutes then all 3000 calories of glycogen will be used and the athlete will be forced to slow down to provide enough oxygen to use fat as the fuel source.
As stated above, the body requires an abundance of oxygen to sustain endurance. This is because fat molecules require and abundance of oxygen to be metabolized. Without oxygen, fat cannot be used as a fuel source and the athlete will be limited to the amount of glycogen stored in the body. If a person wants to develop greater endurance and to be more metabolically efficient then they must train their bodies to use fat as the primary fuel source and limit the amount of glycogen used in sustained efforts. The higher the intensity then the faster glycogen is depleted to keep up with the demands of the working muscles.
The VO2 Max test is a laboratory test to determine at which intensity a person switches from aerobic to complete anaerobic metabolism. This detection point is referred to as the Anaerobic Threshold (AT). This is the point where oxygen demands exceed oxygen uptake based on ventilatory gases. A corresponding heart rate is measured for convenience to help the athlete during training and racing. When a person surpasses their tested AT then aerobic metabolism is completely supplemented by anaerobic mechanisms. There is no fat metabolism beyond this threshold and the ability to sustain this effort is limited by glycogen supply under anaerobic glycolysis. Once glycogen is depleted then the body must slow down and it will require time to re synthesize this important anaerobic molecule. There is not enough time in a competitive event for this resynthesis to occur and once an athlete passes this "point of no return" their ability to sustain a high effort is over.
There is a term in exercise physiology that is contradictory to common reason. "To gain speed, slow down." If an athlete wants to increase their ability to sustain endurance they must train their body to be more efficient at metabolizing fat. Because fat requires an abundance of oxygen then the athlete must slow down in their training to accomplish this physiological phenomena. Using heart rate as a measure of intensity helps the athlete know where they should keep their effort while training, and even racing. When an athlete is more metabolically efficient then they have the ability to process more oxygen and fat which in turn generates more speed and endurance.
It requires much patience and self control to train at a low intensity, especially when one trains with others. When a person first starts training by heart rate they are often times forced to an extremely slow pace, even walking, to maintain the most aerobically efficient heart rate training zone. This can be frustrating and discouraging but a necessary component to training to develop endurance and even speed. If the event an athlete is training for lasts longer than 60 minutes then the majority of their training must be aerobic. Otherwise they will not develop the necessary aerobic muscle fibers, aerobic muscle enzymes, and aerobic efficiency to hold their speed to the finish line. Building a good aerobic base requires about three months of low intensity work. For athletes who have lost their competitive desires, cannot lose excess body fat, or have experienced chronic illnesses or injuries, a longer base period, up to six months, will help.
In time, weeks to months, the athlete will develop greater aerobic efficiency and their ability to move faster at the same low heart rate will manifest. This is physical proof that their body is adapting to the low intensity training. They will have faster training and race times, not because they were doing any speed training but because their body is more metabolically efficient.
Too often an athlete who feels well trained for an event falls short of their performance goals while racing because they lack endurance. Race endurance and training endurance can often be very different. Too many times athletes train out of their aerobic heart rate zones, either too high or too low, and show up to the race not physically prepared for the demands of the endurance event. It does not matter how well they are prepared mentally if their body does not have the ability to sustain the efforts and speeds they desire. Their race performance disappoints them because they paced themselves incorrectly by starting the race too fast only to fade in the final miles because they lacked endurance for those sustained efforts.
If an athlete wants greater endurance then they must train their bodies to develop greater endurance. Greater endurance comes from sustained aerobic efforts at the right intensity for their given fitness level.
posted by: Nick Gooman of Durapulse Performance Company -firstname.lastname@example.org