Your Power Plant

You need a lot of power to propel your body through 26.2 miles of running. If you were driving your car in the race you would be using gasoline to produce that power. Of course your body can’t use gasoline. Instead it uses a substance called ATP. ATP is an abbreviation for adenosine triphosphate. ATP is your body’s power source. Just as your cars internal combustion engine won’t run without gasoline, your body won’t run without ATP. Unfortunately you can’t just run up to the pump and fill your body with ATP - your body must manufacture it.

Your body uses four primary systems to produce ATP - the aerobic lipolysis system, the aerobic glycolysis system, the anaerobic glycolysis system and the phosphagen system. Aerobic means “with oxygen”. The aerobic system depends upon a sufficient delivery of oxygen to your muscles cells. Anaerobic means “without oxygen. Both the anaerobic and the phosphagen system will work without the presence of oxygen.

Phosphagen System

This is the most straight forward system and also the one of least interest to a marathon runner. When you need energy very quickly, this is the system your body chooses. The creatine phosphate system works very quickly and without oxygen by breaking down a compound called phosphocreatine. The problem with this system is the limited supply of the phosphocreatine.

There is only enough of this substance to energize your muscles for 5 to 20 seconds of work. Not quite enough to carry your through your marathon! This system is used for the first few seconds of a very intense activity such as sprinting and heavy weight lifting. It is not a major contributor to energy production for long distance runners. Competitive distance runners will sometimes draw on this energy producing system during the final all out sprint to the finish line.

Anaerobic Glycolysis System

Anaerobic means “without oxygen”. Just as the phosphagen system, the anaerobic glycolysis system is able to produce ATP without the assistance of oxygen. This system uses glucose to produce ATP. Glucose is the storage form of sugars in your body and bloodstream. The glucose comes from digestion of carbohydrates you have eaten and from stores of glycogen (converted glucose) in your muscles and liver.

When you are running at a very fast pace, such as 400 meter to 800 meter pace, your body is not able to extract enough oxygen from your blood stream to assist with ATP production. This system dominates in that situation. The main limiting problem with anaerobic energy production is the moderate amount of energy that is produced and also the fact that glycogen is only partly broken down. A by-product of this partial breakdown is a compound called pyruvic acid. The pyruvic acid that is produced in eventually converted into another compound called lactic acid. You may have heard or read that lactic acid is a “garbage” by product that causes fatigue and muscle soreness. That is not true. It is actually a valuable compound that serves a number of purposes including the production of even more energy. There is more on that subject later in this chapter.

Aerobic Glycolysis System

This system and the related aerobic lipolysis system are the most complex and are also the most important systems to a marathon runner. Aerobic means “with oxygen” This system for producing ATP dominates when a sufficient supply of oxygen is available to your muscle cells. When you are running at marathon pace most of your ATP is produced using the aerobic system. The aerobic glycolysis system uses glucose (carbohydrates) in the presence of oxygen to produce ATP. Thanks to the ready supply of oxygen, glucose is broken down completely which produces a large amount of energy. The aerobic system becomes the dominant system when you are running at more moderate levels. Race speeds from 1 mile up to ultra marathons use the aerobic systems extensively.

Aerobic Lipolysis System

The aerobic lipolysis system is the marathoners best friend. Instead of breaking down glucose for energy, this system completely breaks down fat. You body’s supply of glucose is limited, but even the most lean marathon runner has enough body fat to fuel many marathons. In addition, fat generates vast amounts of ATP. In fact, fat produces more than twice the amount of ATP than glucose. Fat yields 9 calories of energy per gram compared to only 4 calories per gram of glucose.

Aerobic and Anaerobic Interaction

There is no single point at which your body switches from strictly aerobic energy production to only anaerobic energy production. You are always using both systems. Even when you are resting, there is some anaerobic energy production going on. As you increase the intensity of your running you begin to rely more and more on the anaerobic system. When you are walking you are using mostly the aerobic lipolysis system, although the other two systems are also operating. When you reach marathon pace there is a greater contribution from the aerobic glycolysis system with a decrease in lipolysis. As you increase your pace even further to 10K race pace, aerobic glycolysis really heats up along with anaerobic glycolysis while lipolysis continues to decrease. There is a continuum of sorts; as your pace increases you depend more upon both aerobic and anaerobic glycolysis. When your pace decreases to more moderate levels your lipolysis system comes more into play. It may help to think about glucose or carbohydrate as the fuse and fats as the firecracker. The fats or fatty acids provide the big bang, but require a jump start from the burning of glucose or carbohydrates. The carbohydrates burn more easily but also with less intensity.

Continue to The Fatigue Factor

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