Muscle Energetics
The best way to define energy is to describe what it does. Energy is the ability to do work or cause motion. Common forms of energy in our world include heat, light, sound, electrical energy, mechanical energy, and chemical energy. Most metabolic processes in the body use chemical energy, which is held in the bonds between the atoms of molecules and is released when these bonds are broken. The muscles are no different. When muscles work, they require energy so that they can contract. The unique feature about muscular contraction is that the chemical energy is transformed into mechanical energy - movement. Although extended muscle activity depends on the provision of important nutrients such as carbohydrates, fats and even protein, the basic source of chemical energy for muscle contraction is adenosine triphosphate or ATP, which has the following basic chemical formula:| Adenosine - PO4 ~ PO3 ~ PO3 |
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| Figure 8. |
The bonds attaching the last two phosphate radicals to the molecule, designated by the symbol ~, are so-called high energy phosphate bonds. When these bonds are broken (and new bonds are formed), a large amount of chemical energy is released. In fact, each of these bonds stores about 11,000 calories of energy (per mole of ATP) in the body. (A mole is equivalent to Avagadro's number - 6.02 x 10 - of molecules.) Therefore, when one phosphate radical is removed from one mole (6.02 x10 molucules), 11,000 calories of energy that can be used to energize the muscle contraction process are released. Then, when the second phosphate radical is removed, still another 11,000 calories become available. Removal of the first phosphate converts the ATP into adenosine diphosphate or ADP and removal of the second converts this ADP into adenosine monophosphate or AMP.
Figure 8 shows the breakdown of ATP first to ADP and then to AMP, with the release of energy to the muscle for contraction. The energy, then, for muscle contractions is actually produced by the breaking apart of ATP into ADP and then again into AMP. But in order to have an adequate supply of ATP, the AMP must be recycled back into ADP and then it must be recycled back into ATP. To do so involves adding a phosphate molecule to each step.
The left-hand side of Figure 9 shows the three different metabolic mechanisms that are responsible for recycling AMP and ADP back into ATP in order to provide a continuous supply of ATP in the muscle fibers. Why do we have three different metabolic systems in our bodies to produce the ATP? Well, each one serves a different metabolic need that we may have depending on the level of movement or activity that we participate in. The longer and more intense the muscular activity, the greater is our need to supply ATP more rapidly to those muscles. Let's examine how our bodies provide this important fuel.
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| Figure 9. The three important metabolic systems that supply energy for muscle contraction: (1) The phosphagen energy system, (2) the glycogen-lactic acid system, and (3) the aerobic system. |
Unfortunately, the amount of ATP that is present in the muscle cells, even in the well-trained athlete, is only sufficient to sustain maximal muscle power for 5 or 6 seconds, maybe enough for a 50-meter dash. Therefore, except for a few seconds at a time, it is essential that new ATP be formed continuously, particularly during the performance of athletic events. However, different kinds of athletic events require different amounts of energy. Some athletic events such as a 100-meter dash, weight lifting or certain football plays require a quick "burst" of energy. In this case, the amount of ATP present in the muscle cells is not enough to sustain the muscle power that is needed. Therefore, the body must rely on the phosphagen system, which utilizes a substance called creatine phosphate (which also contains a high energy phosphate bond) to recycle ADP back into ATP to provide fuel for our muscles (Figure 9). This is accomplished when the high energy phosphate bond in creatine phosphate breaks down and releases phosphate and energy. The phosphate molecule and the energy then combine with ADP to form ATP. In simple terms, the reaction goes like this:
| Creatine ~ PO4 ® Creatine + | PO4 + Energy |
| PO4 + Energy + ADP ® ATP |
Certain athletic events, such as longer track events (200- or 400-meter dash events and longer), basketball, tennis or soccer require extra ATP to fuel the muscles. During these times of heavy physical activity, glucose molecules come to the rescue. The extra ATP that is needed by the muscles is provided by a system that breaks down glucose in the absence of oxygen. This system - the glycogen-lactic acid system - is the anaerobic step of glucose breakdown (Figure 9). In this step, each glucose molecule is split into two pyruvic acid molecules, and energy is released to form several ATP molecules providing about 30 to 40 seconds of maximal muscle activity in addition to the 10 to 15 seconds provided by the phosphagen system. The pyruvic acid will then partly break down further to produce lactic acid. If the lactic acid is allowed to accumulate in the muscle, you will experience muscle fatigue (often characterized by pain such as cramps).
The aerobic system in the body is utilized for those sports that require an extensive expenditure of energy, such as a marathon race or cross-country skiing. Lots of ATP must be provided to your muscles in order to sustain the muscle power that you need to perform such events without excessive production of lactic acid. Therefore, in the presence of oxygen, pyruvic acid breaks down into carbon dioxide (CO2), water, and energy by way of a very complex series of reactions known as the citric acid cycle, providing essentially unlimited time (as long as nutrients in the body last) to continue muscle utilization (Figure 9). As you know by now, oxygen travels around the body in the blood vessels by attaching to the hemoglobin of the red blood cells. However, in the muscles, some oxygen can be stored in a special chemical substance found within the muscle fibers called myoglobin(remember, myo = refers to muscle). The myoglobin is similar to hemoglobin in its oxygen-binding capability but it can only store small amounts of oxygen. Therefore, for very heavy, sustained exercise, new oxygen must be provided from outside the body if you expect to keep working.
In summary, the three different muscle metabolic systems that we have at our disposal to supply the various degrees of energy that are required for various activities are: (1) the phosphagen system (for 10-15 second "bursts" of energy), (2) the glycogen-lactic acid system (for another 30-40 seconds of energy), and (3) the aerobic system (providing a great deal of energy that is only limited by your body's ability to supply oxygen and other important nutrients). Many sports require the utilization of a combination of these metabolic systems. These systems are outlined in Table 1.
| Table 1. The three different metabolic systems that we have at our disposal enable different degrees of muscle activation. | ||
| Muscle Metabolic Energy Systems | Duration of Maximal Muscle Activity | Activity Types |
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| Phosphagen System | 10-15 seconds | Power surges |
| Glycogen - Lactic acid system | 30-40 seconds more | Intermediate athletic activities |
| Aerobic System | Unlimited time (as long as nutrients last) | Prolonged athletic activities |
Not only do we have different metabolic systems to provide energy for our
muscles but you have also learned that we have different "kinds" of muscle
fibers, the Type I (slow twitch) fibers and the
Type II (fast twitch) fibers,. These different muscle
fiber types allow you to produce different kinds of movement. In a special
activity that was designed for Student Investigation 3.3, you will be
examining how your quadriceps muscles are able to support you in a certain
position; this will enable you to determine how well you have been endowed
with slow twitch fibers. In the meantime, however, let's finish this
section by briefly discussing the topic of animal
experimentation.