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 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?
Dr. Albert Einstein, in his infinite wisdom, discovered that the total energy of an object is equal to the mass of the object multiplied by the square of the speed of light. His formula for atomic energy, E = mc2, has become the most recognized mathematical formula in the world. According to his equation, any change in the energy of an object causes a change in the mass of that object. The change in energy can come in many forms, including mechanical, thermal, electromagnetic, chemical, electrical or nuclear. Energy is, after all, all around us.
The lights in your home, a microwave, a telephone, the sun, all contain energy. Even though the solar energy from the sun that heats the earth is quite different from the energy used to run up a hill, energy, as the first law of thermodynamics tells us, can neither be created nor destroyed. It is simply changed from one form to another.
ATP Resynthesis: Where All Energy for Running Comes From
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 of ATP, which requires water, a proton and energy, heat is produced. Since our muscles don't store much ATP, we 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.
Like many other animals, humans produce ATP through three metabolic pathways that consist of many enzyme-catalyzed chemical reactions. Which pathway you use for the primary production of ATP depends on how quickly you need it and how much of it you need. Sprinting 100 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.
Phosphagen System: The Gateway to Quick Energy
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. 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 exercise lasting up to about 10 seconds. However, since there is a limited amount of stored CP and ATP in skeletal muscles, fatigue occurs rapidly.
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