Keeping the Big Wheel Turning:
Exercise Metabolism
In This Chapter
▶ Understanding the systems by which your body produces energy (ATP)
▶ Measuring and using oxygen consumption: VO2
▶ Training your metabolism
Your body runs on one fuel source: ATP. But because you can store only a small amount of ATP (like a battery with little charge), as soon as you begin to use it up, you need to create more. Fortunately, you can do so by using your body’s ATP generators!
Some of these systems kick in fast and furious, providing an almost instant supply of ATP; others provide energy at a slower rate. Some get depleted quickly; others can go forever. Some sport activities and movements use one system more than the others. In this chapter, we explain how your body’s ATP systems work and note which activities use one system more than another.
Introducing The ATP-PC Energy System:
Give Me Energy Now!
Your life depends on the energy you get when you break chemical bonds, spe- cifically the bonds holding together a molecule called adenosine triphosphate, or ATP. Adenosine is connected to three phosphates by high-energy bonds.
These bonds hold the energy that drives all the biological actions in your body. To produce energy, you just need to get the bonds to break!
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Breaking (chemical) bonds
An essential component that drives chemical reactions is a catalyst. In the human body, these catalysts are enzymes. It takes a reaction with water and the help of a special enzyme — ATPase — to liberate energy from ATP. Once the energy is liberated (along with a phosphate being lost), you end up with adenosine diphosphate, or ADP (the di part of diphosphate means “two”).
When ADP is present, systems begin to turn on to replenish ATP. Here’s what’s happening:
ATP + H2O → ADP + Pi = ENERGY
Where is ATP stored? Well, if it’s used for movement, where do you think you’d put it? In the muscle! But strangely, muscle stores only enough ATP for about two seconds of work. That’s like having $5 in the bank — certainly not enough to pay the bills. Fortunately, your body has a way to produce the ongoing energy you need.
Replenishing energy as you use energy:
The air compressor analogy
To understand how the human body’s energy system works, think of an air compressor. An air compressor is a big tank that holds air that you can use to fill tires or run machinery. Attached to this tank is a small motor that com- presses air and a gauge that shows the amount of air pressure in the tank.
Now imagine that the tank is full of compressed air and the motor is turned off. How can you make the compressor turn on? Simple: You use some air!
When you use air, the pressure in the tank drops, and that drop in pressure signals the motor to turn on to compress more air.
Your body’s energy system follows essentially the same principle. You have only two seconds of ATP stored in your body, but when you use it, you turn on your body’s energy systems to start making ATP!
Three primary systems provide ATP:
✓ Phosphocreatine (ATP-PC): This system provides an immediate boost of energy that lasts only a few seconds.
✓ Anaerobic glycolysis: This systems provides energy for activities lasting longer than a few seconds (closer to five minutes) but that still need a lot of ATP quickly.
✓ Oxidative (aerobic) system: This system provides energy for activities that don’t need ATP at very high rates but need the ATP to last for a very long time without fatigue (like your entire life!).
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Chapter 4: Keeping the Big Wheel Turning: Exercise Metabolism
As you can see, the systems that make ATP provide energy at different rates.
The moment you start to move, you use your small stores of ATP and turn on all the systems that make ATP. Because your metabolism always needs ATP, you are always making it and always using it! As a result, you can get energy for long walks or high jumps without having to shift gears. The energy will be there for you! The following sections explain these three energy systems in detail.
Phosphocreatine: An Immediate Source of ATP
If the high-energy bond of ATP is broken for activity, that energy needs to be replaced if you want to continue moving. One way your body replaces the used energy fast is to “steal” it from another substance. Phosphocreatine is the most immediate source of energy to remake ATP (see Figure 4-1).
Phosphocreatine is stored within your muscle, and you have enough for about ten seconds of all out, high-intensity exercise.
Figure 4-1:
How phos- phocreatine
remakes ATP.
Illustration by Wiley, Composition Services Graphics
As noted previously, phosphocreatine produces enough energy for about ten seconds of your hardest effort. Say you’re running a 100-meter race. The race begins, and you take off. You’re breaking down ATP fast. But as fast as you use it, phosphocreatine provides the energy to remake ATP. Essentially, it’s your friend handing you $100 bills as quickly as you spend them. Yet as you run along, you continue to deplete your phosphocreatine stores. When it runs out, ATP supply drops quickly, as Figure 4-2 shows, and you must slow down.
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Figure 4-2:
Phospho creatine stores last only about ten seconds.
Illustration by Wiley, Composition Services Graphics
To better understand how the phosphocreatine system works, imagine that you have $100 in your pocket and a friend who replenishes your money as you spend it. You spend $10 on lunch, for example, and immediately, your friend hands you $10. You spend another $50 on some clothes. Immediately, your friend gives you $50! Wow! As long as your friend doesn’t go broke, you can keep spending! But when the friend goes broke, you’re both out of luck.
Creatine loading: Effective aid or expensive urine?
Because phosphocreatine is so important for high-intensity exercise, athletes want to store as much of it as possible. The human body can create some of the components, like creatine, from amino acids. However, some research has shown that supplementing the diet with creatine may also provide positive effects. In laboratory testing, creatine supplementation showed positive improvements in high-intensity exercise performance (producing a better effort at weight training, for example, or leading to increased strength and longer efforts at sprint cycling, resulting in improved sprint time).
The benefits seem to vary: Some subjects responded to the supplementation; others didn’t. Side effects appear to be minimal (nausea and possibly cramping). In addi- tion, the addition of creatine to the cells pulls water with it, causing some water weight gain — an effect that may be beneficial for those needing to cool themselves with sweat but deleterious to those who need speed (the added weight can slow them down). So the jury is still out on the effectiveness of creatine supplementation.
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High-intensity activities that last under ten seconds — activities like the sprinting for 100 meters, high jumping, running to first base in baseball, swing- ing a bat or golf club, or lifting a very heavy weight (one that you can lift maybe only five times) — all get their ATP mostly from phosphocreatine.
Anaerobic Glycolysis: Fast Energy with a Price
Wouldn’t it be nice to be able to generate enough ATP to run hard for longer than the ten-second burst of phosphocreatine? Well, you do have a system of ATP production that works very quickly to give you ATP. Using the sugar in your body, you can produce a lot of ATP pretty fast (not as fast as the phos- phocreatine system can produce ATP, but still plenty fast). However, this ATP production method has a side effect of sorts: It leads to fatigue.
ATP supplies are used up within two seconds of exercise. This loss of ATP immediately turns on the system within the cell that can use the simple sugar, glucose, to provide energy to form more ATP. The chemical reactions that accomplish this take place within the cell.
Your starter fuel: Glucose and glycogen
Because humans run on ATP, you’d think that we should just eat ATP. Not possible! Instead, the energy for ATP is tied up in other molecules. One is the carbohydrate glucose. Your body has the necessary enzymes to rearrange a molecule of glucose so that the energy within the molecular bonds can be used to remake ATP. How sweet is that?
Many versions of sugars exist, but the simplest is glucose, the primary com- ponent of starches, or carbohydrates, like pasta, grains, rice, and sugars, for example. Glucose is a 6-carbon molecule. Plants create glucose through pho- tosynthesis. We humans use glucose for energy.
Although you carry a small amount of glucose in your blood, you actually store glucose in the muscle and liver in the form of glycogen. Think of glycogen as a glucose snowball — a multitude of glucose units connected together. Your body has about 2,000 calories of glycogen — enough to run about 20 miles.
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Getting glucose into the cell
Because glycogen is kept in two locations (the muscle and liver), the process of getting glucose ready for ATP production varies, depending on the location of the glycogen:
✓ From the liver: Glycogen is broken apart into individual glucose units (a term call glycogenolysis). This glucose is dumped into the blood so that it can be transported to the muscle or to other cells that need it. (This is why we have blood glucose.) From there, the glucose gets into the cell by way of special proteins that act like gateways (think of the revolving doors at a fancy hotel).
ATP is required for glucose to get into cells, so you actually use energy to start the process of creating ATP. The end result is a glucose molecule with a phosphate attached (called glucose 6-phosphate) — just what you need to start making some ATP quickly inside the cell.
✓ From the muscle: Location is everything! Because some glycogen already exists in the muscle, you simply need an enzyme to break the glycogen snowball apart and grab a floating phosphate. The result? A glucose 6-phosphate that didn’t cost you any ATP.
Glycogen supercompensation: Carb loading for performance
Having as much glycogen as possible stored in the liver and muscle before any big activity or event (a marathon, the Tour de France, and so on) is clearly advantageous. Normally, you store glycogen after a workout, using a glyco- gen-making enzyme called glycogen synthase, along with carbohydrates in the diet. Briefly storing more glycogen than normal is possible, however. The recipe for loading, or supercom- pensating, is very simple:
1. Reduce your training volume and intensity.
Known as a taper, this action reduces your use of glycogen. Filling the gas tank is easier if you aren’t using as much gas, right?
2. Increase your dietary intake of carbohy- drates.
Maybe bump it up to 60 percent to 70 percent of your diet. The enzymes for glycogen syn- thesis store the extra carbs as glycogen.
3. A day or two before the big event, reduce your training even further and increase your carbs to at least 70 percent.
Doing so drives maximal glycogen super- compensation.
The result? You have extra stores of glycogen, you feel rested, and you probably can run faster and longer than you ever did during training.
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Cooking up ATP, oxygen free:
Anaerobic glycolysis
The cell is a bit of a soup, full of nutrients and enzymes. Chemical reactions can take place quickly in this environment and help produce ATP at a very fast rate. Because oxygen isn’t used, the process of breaking down glucose is anaerobic (which means “without oxygen”).
Two primary steps are involved with anaerobic glycolysis. The first step requires an investment of energy, but the second step doubles your energy. If you want to make money, you have to spend a little, and that’s what happens here (see Figure 4-3):
✓ Step 1: Invest energy and prime the pump: In this first phase, glucose is taken into the cell and “trapped,” by attaching a phosphate and converting glucose into glucose 6-phosphate. Capturing and trapping the glucose takes energy (ATP) and an enzyme. Later, another phosphate is added and another ATP used. This process gets the molecules in a position to produce ATP by converting it to fructose 6-phosphate.
✓ Step 2: Produce double the energy: In this phase, the molecule splits in two, and each one goes through a series of reactions, starting from glucose 3-phosphate (G3P), to 1–3 biphosphoglycerate (1–3 BPG), to 3 phosphoglycerate (3-PG), during which you generate two ATP). After water removal, phosphoenolpyruvate (PEP) is converted to a strong acid, pyruvic acid, and two more ATP are generated. You just doubled your money — you went from 2 ATP to 4 ATP — and have a net gain of energy!
The reason you can get one more ATP is because muscle glycogen doesn’t take energy to trap the glucose, so one less ATP is invested!
This outcome almost seems too good to be true: fast energy, double your energy . . . so what’s the catch? The end result of glycolysis, pyruvic acid, or pyruvate, is a strong acid that very quickly causes fatigue.
One way to slow the acid build up is to convert the pyruvate to a less-acidic acid — lactic acid. An enzyme (lactate dehydrogenase, or LDH) converts pyruvic acid into lactic acid:
Pyruvic acid ——– Enzyme (LDH) ——– Lactic acid
Unfortunately, lactic acid can be just as bad, as we explain in the next section.
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Figure 4-3:
Producing ATP, using anaerobic glycolysis.
Illustration by Wiley, Composition Services Graphics
The metabolic bad boy: Lactic acid and fatigue
The unfortunate side effect of anaerobic glycolysis is the accumulation of lactic acid. Your body doesn’t respond well to acid. Proteins begin to break down, and cells can die. To prevent too much acid from building up, a number of fatigue-related changes take place when lactic acid accumu- lates from exercise. These changes are designed to slow you down and prevent damage.
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When we talk about acid in this context, we’re really talking about a particular ion. Think of the unit of acidity, pH. pH stands for the “power of hydrogen.”
The hydrogen ion (H+) is what makes acid, well, acid. The concept can be con- fusing sometimes, because a higher concentration of H+, or more acidity, is reflected by a lower pH value. So the accumulation of lactic acid really means the accumulation of hydrogen ions and a drop in pH.
Understanding how H+ causes fatigue
Following is a step-by-step description of how H+ can cause fatigue; Figure 4-4 shows this process:
1. Hydrogen ions block muscle contraction.
H+ ions compete with another ion, calcium (Ca++) for the affection of a protein (troponin). Calcium normally binds to troponin in the muscle to cause muscle contraction. As H+ builds up, it competes with Ca++ for troponin binding and starts to block contraction. The result? Less force produced and fatigue.
2. Hydrogen ions slow nerve signals to the muscle.
Normally, nerve signals can skip along a nerve like a stone across water.
H+ in the cell slows the skipping down. As a result, you can’t get a coor- dinated signal to the muscles. Uncoordinated signals interfere with motor skills. Your running strides are altered, and you experience more fatigue.
3. Hydrogen ions block an enzyme necessary for anaerobic glycolysis.
Early in the anaerobic glycolysis process of making ATP, a particular enzyme, phosphofructokinase, is very sensitive to H+. When H+ levels rise, phosphofructokinase slows its ability to help run glycolysis! When this happens, you can’t make more lactic acid. Of course, you can’t make more ATP either. Result? Even more fatigue.
4. Hydrogen ions cause pain.
You may have felt the pain of lactic acid in your muscles when you work hard. And when the pain is high, you reduce your effort (which means a reduced motor stimulus leaving your brain); that is
also fatigue.
Lactic acid causes fatigue at the brain (called central fatigue), as well as at various locations around the muscle and cell (peripheral fatigue). Head to Chapter 3 for a more in-depth discussion of the brain’s role in movement.
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Figure 4-4:
How lactic acid causes fatigue.
Illustration by Kathryn Born, MA
Recovery from exercise: Getting rid of lactic acid
Because lactic acid is such a major player in fatigue (and fatigue means poor performance), getting rid of lactic acid fast is desirable if you want to get back in action quickly. Here are some suggestions:
✓ Breathe it off. Thank goodness for chemistry! Blood contains a sub- stance called bicarbonate (HCO3–). When you breathe, the HCO3– in your blood can combine with the nasty H+ to form a weaker, and less nasty, acid called carbonic acid.
H+ + HCO3– ——– H2CO3
Can you see two substances in there? Well, when blood passes through the lung, it’s converted into carbon dioxide and water:
H2CO3 ——– H2O + CO2
The extra CO2 makes you breathe harder. You’ve probably found yourself breathing hard when you’ve pushed the intensity a bit too much. Maybe it’s because lactic acid is building up!
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Chapter 4: Keeping the Big Wheel Turning: Exercise Metabolism
✓ Use your muscles. As you’ll see when you study muscles, muscle fibers favor either fast or slow ATP production. Aerobic fibers, known as slow twitch muscle fibers, are unique. They actually use lactic acid just like glucose and produce ATP! Slow twitch fibers are used mostly during light exercise. So when the big sprint is over and all that lactic acid has built up, what should you do? Walk! Light jog! If you use your slow twitch fibers, you’ll get rid of the lactic acid more quickly.
The Oxidative (Aerobic) System:
It Just Keeps Going and Going
Mitochondria are nature’s batteries. These organelles contain enzymes that can rearrange molecules through a number of steps to ultimately create ATP.
The reason mitochondria are so good at producing ATP is because, as long as they have a source of fuel and oxygen available, the only waste they produce is carbon dioxide and water, which you can breathe out.
Mitochondria (see Figure 4-5 for an image of a mitochondrion) are located in the cell. From a standpoint of exercise, you’ll find that they may be more numerous in some areas than in others. As Chapter 10 explains, different muscle fibers have different characteristics; some are aerobic muscle fibers and have a lot of mitochondria; others are anaerobic fibers and have few mitochondria.
Figure 4-5:
A mitochon- drion.
Illustration by Wiley, Composition Services Graphics