We usually talk of energy in vague terms. “I don’t have a lot of energy today,” or “You can feel the energy in the room.” But what really is energy? Where do we get the energy to move? How do we use it? How do we get more of it? Ultimately, what controls our movements?

As you may have learned in high school biology class, the energy for all physical activity comes from the conversion of high energy phosphates (adenosine triphosphate, ATP) to lower energy phosphates (adenosine diphosphate, ADP; adenosine monophosphate, AMP; and inorganic phosphate, Pi). During this breakdown, or hydrolysis, of ATP, which requires water, a proton, energy, and heat are produced: ATP + H2O → ADP + Pi + H+ + energy + heat. Since your muscles don’t store much ATP, you must constantly resynthesize it. The hydrolysis and resynthesis of ATP is thus a circular process—ATP is hydrolyzed into ADP and Pi, and then ADP and Pi combine to resynthesize ATP. Alternatively, two ADP molecules can combine to produce ATP and AMP: ADP + ADP → ATP + AMP.   

Like many other animals, humans produce ATP through three metabolic pathways that consist of many enzyme-catalyzed chemical reactions. Two of these pathways, the phosphagen system and anaerobic glycolysis, do not use oxygen to create ATP and are therefore referred to as anaerobic. The third pathway uses oxygen to create ATP and is therefore referred to as aerobic.

Which pathway your muscles use for the primary production of ATP depends on how quickly they need it and how much of it they need. Racing 800 meters, for instance, requires energy much more quickly than running a marathon, necessitating the reliance on different energy systems. However, the production of ATP is never achieved by the exclusive use of only one energy system, but rather by the coordinated response of all energy systems contributing to different degrees. Think of three dials that are always being adjusted to optimize the production of energy. When you race 100 meters, the phosphagen dial is turned up very high, while the other two dials are turned down low. When you run a marathon, the aerobic system dial is turned up very high, while the other two dials are turned down low. When you race a 5K, the aerobic system dial is turned up high, the anaerobic glycolysis dial is turned to medium, and the phosphagen system dial is turned down low.

Simplistically speaking, running faster comes down to increasing the rate at which ATP is resynthesized so it can be broken down to liberate energy for muscle contraction.

Phosphagen System

During short-term, intense activities, a large amount of power needs to be produced by the muscles, creating a high demand for ATP. The phosphagen system (also called the ATP-CP system) is the quickest way to resynthesize ATP. Creatine phosphate (CP), which is stored in skeletal muscles, donates a phosphate to ADP to produce ATP:

ADP + CP → ATP + C

No carbohydrate or fat is used in this process; the regeneration of ATP comes solely from stored CP. Since this process does not need oxygen to resynthesize ATP, it is anaerobic, or oxygen-independent. As the fastest way to resynthesize ATP, the phosphagen system is the predominant energy system used for all-out sprinting lasting up to about 10 to 15 seconds. However, since you have a limited amount of stored CP and ATP in your muscles, fatigue occurs rapidly when you sprint.

Anaerobic Glycolysis

Anaerobic glycolysis is the predominant energy system used for all-out running lasting from 30 seconds to about two minutes and is the second fastest way to resynthesize ATP. During anaerobic glycolysis, carbohydrate, either in the form of glucose in the blood or its stored form of glycogen in the muscles and liver, is broken down through a series of chemical reactions. Every molecule of glucose broken down through glycolysis produces two molecules of usable ATP. Thus, very little energy is produced through this pathway, but the trade-off is that you get the energy quickly, so you can run fast.

You rely on anaerobic glycolysis when oxygen is not supplied fast enough to meet your muscles’ needs for ATP. When this happens, your muscles lose their ability to contract effectively because of an increase in hydrogen ions, which causes the muscle pH to decrease, a condition called acidosis. The concentration of other metabolites, including potassium ions and the two constituents of ATP (ADP and Pi) also increase. Acidosis and the accumulation of these other metabolites cause a number of problems inside muscles, including inhibition of specific enzymes involved in metabolism and muscle contraction, inhibition of the release of calcium (the trigger for muscle contraction) from its storage site in muscles, and interference with muscles’ electrical charges, ultimately leading to a decrease in muscle force production and running speed.

Aerobic System

Since humans evolved for aerobic activities, it’s not surprising that the aerobic system, which is dependent on oxygen, is the most complex of the three energy systems. The metabolic reactions that take place in the presence of oxygen are responsible for most of the energy your cells produce. Races longer than two minutes (800 meters to ultramarathons) rely most heavily on the aerobic system. However, aerobic metabolism is the slowest way to resynthesize ATP.

The aerobic system uses blood glucose, muscle and liver glycogen, and fat as fuels to resynthesize ATP. The aerobic use of carbohydrates produces 38 molecules of ATP for every molecule of glucose broken down. Thus, the aerobic system produces 19 times more ATP than does glycolysis from each glucose molecule. If that sounds like a lot, using fat gives you much more ATP—a whopping 130, give or take, depending on the specific fatty acid being used.

Running performance, whether recreational or elite, is most dependent on the aerobic system. The more developed the aerobic system, the faster a person will be able to run before he or she begins to rely on the anaerobic energy pathways and experiences the consequent fatigue.

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