Adenosine Triphosphate (ATP) - The Energy Source for Muscle Contraction

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Before discussing the various systems by which your body can provide energy to your muscles, we first need to define what muscle "energy" actually is. We know that your muscle cells need an energy source to be able to contract during exercise. At the highest level, the energy source for muscle contractions is the food you eat. A complex chemical process within your cells, called cellular respiration, ultimately converts the energy stored in the foods you eat into a form that is optimized for use at the cellular level of your muscles. Once food energy has been converted by cellular respiration it exists at the cellular level in the form of a molecule called adenosine triphosphate (ATP).
The composition of an ATP molecule can be inferred from its name. It is composed of three (or "tri") phosphate groups attached to an adenine (or "adenosine") nucleotide. The energy that is stored within an ATP molecule is released for your muscles to use when the bond between the second and third phosphate groups is broken. Breaking this bond releases the third phosphate group on its own and thus reduces the ATP molecule to adenosine diphosphate (ADP). The ADP molecule can be restored back to its ATP form by replenishing the missing phosphate group (this is called rephosphorylization).
Three Exercise Energy Systems
The cellular respiration process that converts your food energy into ATP is in large part dependent on the availability of oxygen. When you exercise, the supply and demand of oxygen available to your muscle cells is affected by the duration and intensity of your exercise and by your cardiorespiratory fitness level. Luckily, you have three exercise energy systems that can be selectively recruited, depending on how much oxygen is available, as part of the cellular respiration process to generate the ATP energy for your muscles. They are summarized below.
The Alactic Anaerobic Energy System
This energy system is the first one recruited for exercise and it is the dominant source of muscle energy for high intensity explosive exercise that lasts for 10 seconds or less. For example, the alactic anaerobic energy system would be the main energy source for a 100 m sprint, or a short set of a weightlifting exercise. It can provide energy immediately, it does not require any oxygen (that's what "anaerobic" means), and it does not produce any lactic acid (that's what "alactic" means). It is also referred to as the ATP-PCr energy system or the phosphagen energy system.
The alactic anaerobic energy system provides its ATP energy through a combination of ATP already stored in the muscles (about 1 or 2 seconds worth from prior cellular respiration during rest) and its subsequent rephosphorylization (about 8 or 9 seconds worth) after use by another molecule called phosphocreatine (PCr). Essentially, PCr is a molecule that carries back-up phosphate groups ready to be donated to the already used ADP molecules to rephosphorylize them back into utilizable ATP. Once the PCr stored in your muscles runs out the alactic anaerobic energy system will not provide further ATP energy until your muscles have rested and been able to regenerate their PCr levels. Creatine supplementation is a method used to extend the duration of effectiveness of the alactic anaerobic energy system for a few seconds by increasing the amount of PCr stored within your muscles.
The Lactic Anaerobic Energy System
This system is the dominant source of muscle energy for high intensity exercise activities that last up to approximately 90 seconds. For example, it would be the main energy contributor in an 800 m sprint, or a single shift in ice hockey. Essentially, this system is dominant when your alactic anaerobic energy system is depleted but you continue to exercise at an intensity that is too demanding for your aerobic energy system to handle. Like the alactic anaerobic energy system, this system is also anaerobic and so it does not require any oxygen. However, unlike the alactic anaerobic energy system, this system is lactic and so it does produce lactic acid. It is also referred to as the lactic acid system or the anaerobic glycolytic system.
In contrast to the alactic anaerobic energy system, which uses ATP stored from previous cellular respiration in combination with a PCr phosphate buffer, the lactic anaerobic energy system must directly recruit the active cellular respiration process to provide ATP energy. The cellular respiration process consists of a very complex series of chemical reactions, but the short summary of it is that it ultimately converts food energy (from carbohydrates, fats, and proteins) into ATP energy. When oxygen is not available for cellular respiration, as is the case for the lactic anaerobic energy system, lactic acid is produced as a byproduct.
The Aerobic Energy System
During continuous aerobic exercise your intensity level, relative to the high intensity levels that recruit your alactic anaerobic and lactic anaerobic energy systems, must be reduced so that the energy demand placed on your muscles equals the energy supply (compare this to the alactic anaerobic and lactic anaerobic systems, where demand usually exceeds supply and energy stores are quickly depleted). The energy supply at this lower intensity level, in contrast to the alactic anaerobic and lactic anaerobic systems, which do not require oxygen, now becomes dependent on how efficiently oxygen can be delivered to, and processed by, your muscles. A continuous supply of oxygen allows you to maintain a reduced intensity level for a long period of time. If you are able to extend an exercise activity beyond approximately two minutes in length it will be due to the fact that you are working at an exercise intensity level that can be accommodated by your aerobic energy system. By five minutes of exercise duration the aerobic energy system will have become your dominant energy source. As an example, the aerobic energy system would be the main energy contributor to a marathon runner. The aerobic energy system does not produce lactic acid, but unlike the other two energy systems, it does require oxygen.
Just like the lactic anaerobic energy system, the aerobic energy system must directly recruit the active cellular respiration process to provide ATP energy. Food energy is converted into ATP by your muscle cells through a very complex series of reactions. The difference, relative to the lactic anaerobic energy system, however, is that since oxygen is now available to your muscles no lactic acid will be produced as a byproduct. The generation of ATP energy by the aerobic energy system can be continued as long as oxygen is available to your muscles and your food energy supplies don't run out.
Relative Contributions - Aerobic vs. Anaerobic Energy Systems
The table shown below compares experimentally measured (accumulated oxygen deficit method) energy contributions of the aerobic and anaerobic energy systems for various track running events. A quick review of the table illustrates how the aerobic energy system's contribution increases with increasing event distance, and vice versa for the anaerobic energy system.
Energy Contributions of the Aerobic and Anaerobic Energy Systems to Track Running Events
    Males    Females
Event    Aerobic Energy Contribution    Anaerobic Energy Contribution    Aerobic Energy Contribution    Anaerobic Energy Contribution
100 m    21%    79%    25%    75%
200 m    28%    72%    33%    67%
400 m    41%    59%    45%    55%
800 m    60%    40%    70%    30%
1500 m    77%    23%    86%    14%