Energy Resources

The actual source of energy used by the muscles in running or any other kind of activity is ATP (Adenosine Tri Phosphate). It can be generated either aerobically or anaerobically (with or without oxygen). All running events use a combination of the two systems. Distance running, which we will define here as any event over 1 mile, primarily uses aerobic metabolism; but it is important to understand the contributions from anaerobic metabolism.

Anaerobic Energy Sources
Anaerobic energy sources are used at the start of exercise and when the intensity of exercise is greater than that which can be supported by the available oxygen supply using aerobic sources. The point where this occurs is called the “anaerobic threshold”. It occurs at a pace near your 2 mile race pace or about 10% faster than your 10 K race pace (0.9 x 10K pace/mile). The anaerobic threshold is not an immediate transition from aerobic to anaerobic metabolism but is more like the point where the anaerobic contribution increases rapidly with intensity.
There are two sources of energy in the anaerobic system, the phosphate system and the lactate system. The phosphate system consists of small stores of high energy ATP and Creatine Phosphate (CP) in the muscle. The phosphate system is the primary system used in events taking 10 to 20 seconds( short sprints). When the store is exhausted, additional energy must be generated to replenish the phosphate pool or keep the muscles working. The stores can be regenerated and used over and over again.
The lactate system utilizes glycogen (sugar) stored locally in the muscles and remotely in the liver and the glucose present in the blood. The higher the energy requirement the more the local stores in the muscle are favored. Anaerobic glycolysis or the breakdown of the glucose quickly for intense exercise results in the formation of lactic acid. Lactic acid is often thought of as a “waste product”. In the presence of oxygen, however, it is easily converted to ATP and becomes a fuel. There is also evidence to suggest that lactic acid can be shuttled back and forth to other muscle groups via the bloodstream. Lactic acid may not accumulate if the intensity of the exercise is low enough for it to be oxidized to make more ATP. When the rate of production exceeds the rate of removal, the lactic acid begins to accumulate, bloodstream pH level, (acidity), rises and the muscles do not function as well and begin to “burn”. An example of the result of lactic acid accumulation is the “tying up” of the 800 meter runner the last 100 yards after starting too fast. Examples most of us have felt are the burning from too many sit ups (lactic acid in the abdominal muscles) or the quadriceps burning felt when bicycling or running up a steep hill.

Aerobic Energy Sources
At exercise intensities below the anaerobic threshold, energy conversion is primarily aerobic. The energy for aerobic metabolism comes from two sources glycogen (muscle and liver glycogen, blood glucose) and fat. Aerobic glycogen conversion is the most readily available source of energy and the primary energy source up to about 30 minutes of exercise. After 30 minutes, fat has been mobilized from fat stores and becomes a major contributor. There is always a combination of glycogen and fat usage with the relative contributions at any time determined by the intensity of exercise. More intense exercise will tend to burn more of the most readily available fuels, first muscle glycogen, then liver and blood glycogen and finally fat. As the intensity of the exercise decreases, a higher ratio of fat is used. At paces more than 30% slower than your 10K race pace (1.3 x 10K pace/mile), you should be utilizing the highest ratio of fat for fuel.
There is a finite storage of glycogen which can be increased somewhat through training. These stores will run out usually after 1 hour 45 minutes to 2 hours of hard running. On the other hand, there are nearly unlimited supplies of fat making it the fuel of choice for longer events. One pound of fat contains enough energy to run over 50 kilometers (31.2 mi). Fat, however, cannot be easily metabolized without the presence of muscle glycogen. Therefore if muscle glycogen levels are badly depleted, fat is available but cannot be utilized leading to a drastic drop in performance (“hitting the wall”). This is why it is important not to start too fast in long training runs or races and “waste” your muscle glycogen stores.
At lower intensities more glycogen from the liver and bloodstream can be utilized. This spares some of the important muscle glycogen stores so that efficient aerobic metabolism using fat can be maintained for a longer duration. In ultra-marathon events liver glycogen depletion has been observed indicating that ingestion of sugar or sugared drinks may be necessary. There is evidence it will enhance performance in marathons.
In exercise lasting over 4 hours, protein may be broken down, first from muscle enzymes and then from muscle tissue itself, to make the necessary glycogen for fuel. This method of energy conversion is extremely inefficient and the body’s last resort for survival.

The ability to perform work with our muscles is dependent on our muscular composition and our muscular fitness level. There are two major types of skeletal muscles: slow red and fast contracting. Slow red muscle, because it has lots of blood vessels to carry the nutrients, lots of myoglobin to transport oxygen and lots of energy factories known as mitochondria, consumes oxygen well and generates ATP or energy with aerobic metabolism. Because of its aerobic ability and its resistance to fatigue, this is the primary muscle type of the long distance runner. Slow red muscles tend to be long and thin, note the slender legs of most marathoners.
Fast contracting muscle may be either white which uses the anaerobic phosphate or lactate energy systems or red which has the same characteristics as the slow red, but can use either anaerobic or aerobic metabolism to work. Note that fast white muscles usually are those that can hypertrophy or get big and are the primary muscles of body builders, sprinters and jumpers. People with a high percentage of fast white muscle fibers do not perform well in endurance events.
With a couple of exceptions which are mentioned below, we are born with a muscle make up consisting of a fixed ratio of both kinds of fibers and cannot change it. The most notable exception is that fast red muscle may be converted to use either aerobic or anaerobic metabolism through appropriate training. Some studies have shown that ultra-distance runners who have broken down huge amounts of muscle tissue during strenuous events rebuild with slow red fibers. This would be considered a rather extreme way to alter muscle makeup, however.

Training Requirements for Events
Aerobic endurance training makes major changes in muscles. It increases size and the number of mitochondria, (ATP factories), as well as the amount of enzymes they produce. It also increases the amount of myoglobin and the number of blood vessels, which enhance the ability to get remote fuels to the muscles. These changes occur in red muscles not in white; they, therefore, generally increase aerobic metabolism and may actually decrease the ability for anaerobic metabolism. Remember that we always use a combination of both, but you must look at your major energy needs to see what kind of training is best for you.
Training programs for any event should utilize a base building period, during which stamina is increased before any rigorous specific training is done. The specific training is only half of the training regimen; the other half being easy recovery days during which actual adaptation takes place. Here are some specific training requirements for the energy systems for various events.