Figure 10.1 Sprinters use the process of glycolysis to generate the energy required for top performance.
For endurance athletes, the most important aspect of glycolysis is actually what happens after the glycolytic reactions take place. The pyruvic acid created during glycolysis can be funneled into a complex series of energy-creating reactions called the Krebs cycle. In addition to breaking down the pyruvic acid produced from glucose, the Krebs cycle also metabolizes fats; overall, it furnishes more than 90 percent of the energy required to run in a sustained manner. Since glycolysis provides muscles with quick energy and also jump-starts the Krebs cycle, it is a paramount player in muscular energy production. In fact, without glycolysis the muscles would grind to a halt after only 10 to 15 seconds of intense activity.
Fortunately, glycolysis usually proceeds normally inside muscle cells, and it also keeps pace with a runner’s level of activity: the faster the athlete runs, the hotter the glycolysis fires burn. This has a very interesting consequence: When an athlete is running at a quick pace, pyruvic acid is produced via glycolysis at high rates, but not all of the pyruvic acid that is produced can be instantaneously shuttled into the Krebs cycle. As pyruvic acid waits to be admitted to the Krebs cycle process, an enzyme called lactate dehydrogenase converts some of the pyruvic acid to lactic acid. When an athlete is at rest or running at slow to moderate intensity, modest amounts of pyruvic acid will be formed; almost all of it will go into the Krebs cycle, and there will consequently be relatively little lactic acid lingering in the muscles. When an athlete runs strenuously, however, everything changes: To supply the energy required for the fast running, glycolysis proceeds at a high rate, and thus lots of pyruvic acid ends up waiting in the Krebs cycle queue. Unusually large amounts of lactic acid can then be created in the muscles, and some of this lactic acid surfeit can be dumped into the blood.
Lactic Acid’s Real Role
Two popular myths in running are that the burn felt in the leg muscles during fast running is caused by this buildup of lactic acid and that the soreness experienced the day after an especially tough workout is produced by the same troublesome compound. Two other widespread misconceptions are that lactic acid is a waste product formed in muscles during vigorous exercise and that lactic acid shows up in the muscles when athletes run out of oxygen or enter a mysterious process called oxygen debt. A final untruth is that lactic acid causes fatigue during intense running; unfortunately, this unfounded principle is still accepted as gospel by many coaches and runners.
Science tells us that all five of these assertions about lactic acid are untrue: Lactic acid doesn’t produce burning sensations, it doesn’t induce soreness, and it’s not a form of metabolic garbage that must be eliminated from muscle cells as quickly as possible. The burn experienced during high-speed running is probably a protective mechanism created by the nervous system in order to stop runners from damaging their muscles with too much high-speed effort. The soreness experienced 24 to 48 hours after a tough workout is most likely the result of an inflammatory process occurring in muscle cells that have been partially damaged by very strenuous running; lactic acid is not involved.2-5
In addition, oxygen shortfalls are not required in order to make lactic acid appear in the muscles and blood, and lactic acid does not induce fatigue. The truth is that lactic acid is produced in the body all the time, even when athletes are at rest, because it’s a natural byproduct of the key energy-producing process of glycolysis. Furthermore, running velocity at lactate threshold occurs at 60 to 88 percent of O2max, that is, at an exercise intensity at which oxygen is not yet limiting since O2max has not been reached.
The concentration of lactic acid in the muscles and blood can rise significantly whenever a carbohydrate-containing meal is consumed; many of the ingested carbs are broken down glycolytically to pyruvic acid, which is then converted to lactic acid. If lactic acid really caused muscle soreness and fatigue, runners would experience muscle pain and tiredness every time they wolfed down their favorite carbohydrate-rich meals!
Instead of being a dangerous compound that wreaks havoc inside muscle cells, lactic acid (or, more accurately, lactate, which is just lactic acid without a hydrogen ion) plays a paramount role in carbohydrate processing throughout a runner’s body. Lactate can move out of the muscles and travel through the bloodstream to the liver; the liver can then use lactate to produce glucose, a runner’s most important source of carbohydrate fuel. This is an incredibly significant role for lactate because the liver relies on glucose to maintain normal blood sugar levels.
In addition, up to 50 percent of the lactate produced during a very tough workout or race may be used eventually to synthesize glycogen in the muscles. Glycogen is the key storage form of carbohydrate in the body. This is important because the muscles use carbohydrates as the major energy source during high-quality workouts and competitive endurance performances. Far from damaging tissues or inducing soreness, the glycogen that comes from lactate provides the energy needed to carry out subsequent, high-quality workouts; the glycogen can be broken down into countless molecules of glucose, which then undergo glycolysis.
During exercise, lactate is also an irreplaceable source of immediate energy for muscles and other tissues because lactate can be converted back to pyruvate, which can then quickly enter the energy-producing Krebs cycle. Enhancing the ability to use lactate can improve a runner’s race times rather dramatically. Thus, lactate can go two ways in muscles: (1) into glycogen formation, or energy storage, or (2) into energy creation via pyruvate’s entry into the Krebs cycle. Developing the ability to process lactate effectively helps athletes run faster and longer. You’ll learn how to do this in chapter 27.
Lactate’s Movement Through the Body
Lactate moves through a runner’s body in important ways after meals. Most of the carbohydrate from ingested food enters the bloodstream as glucose and moves directly to the liver. The liver picks up a large quantity of this glucose from the blood and converts a significant fraction of it via glycolysis to lactate. This lactate is then released from the liver into the bloodstream, destined for all points around the body.
Why does the liver like to ship out carbohydrate as lactate? Why doesn’t it simply keep the carbohydrate packaged as pure glucose? Glucose tends to enter body tissues, including muscles, rather sluggishly; it must be guided by an important hormone called insulin, and the overall process can be rather lethargic. That’s why your blood sugar levels can remain elevated for a couple of hours after a carbohydrate-rich meal.
Lactate, on the other hand, does not depend on insulin and can enter muscle and other cells very quickly. In other words, lactate represents quick energy for your muscles and other organs. This is why the heart is a huge sink for lactate: It picks lactate right out of the bloodstream to support its beating around the clock and uses it to supply its vast energy demands during strenuous exertion. It’s a good thing that lactate doesn’t really cause fatigue. Otherwise, your heart would have to take a break now and then, which would not be good. Lactate can be viewed as a kind of shortcut mechanism for getting energy into the muscles, heart, and other tissues.
This overall process means that blood levels of glucose and lactate rise after a high-carbohydrate meal. However, lactate levels don’t appear to rise as fast as glucose concentrations, primarily because lactate is rapidly removed from the blood once it appears, while glucose is taken away more slowly. By changing some of the absorbed glucose to lactate, the liver quickens the disposal of blood carbohydrate. A key benefit of this glucose-lactate conversion is that the amount of insulin that pours into the blood from the pancreas after meals decreases. This limiting of insulin production may help to enhance body composition, since one feature of insulin is that it coaxes glucose into adipose cells, where it can be converted readily to fat.
Lactate Shuttle
Overall, lactate is the primary player in an extremely important process called the “lactate shuttle.” Described in detail
by the noted George Brooks and his colleagues at the University of California at Berkeley,6 the lactate shuttle involves the following chain of events:
Lactate is formed in ample amounts in tissues in which glycogen and glucose are being broken down at high rates via glycolysis. This happens in the leg muscles during vigorous running; as mentioned previously, pyruvate is actually formed first, but pyruvate can be readily converted to lactate.
The lactate formed from pyruvate can slip quickly out of muscle cells and into surrounding tissues and the blood. This lactate escape from the cells enables glycolysis, the conversion of glucose to pyruvate and lactate, to keep going at high rates. If pyruvate could not be transformed into highly dispersible lactate, pyruvate might build up to overly generous levels within muscle cells; this would shut down glycolysis via a feedback mechanism and thwart energy production. The muscles would have to reduce their rate of force production because of the lack of energy, and a runner would have to slow down. As lactate is released from hard-working muscle cells, it can be picked up by nearby muscle cells that are not so overflowing with lactate, or it can enter the bloodstream and be transported to other muscles and tissues throughout the body, including cardiac muscle fibers in the heart.
The muscle cells and tissues receiving the lactate have a couple of options: They can use lactate as an energy-rich fuel by converting it back to pyruvate and sending it into the Krebs cycle, or they can use it as a building block for glycogen storage to satisfy future energy needs.
The lactate shuttle demonstrates that lactate is very far from being a soreness-inducing toxin, a metabolic waste product, or a key inducer of fatigue, as some have described it. Lactate’s easy diffusibility prevents glycolysis from shutting down, and its high-octane fuel status helps the muscles, heart, and other cells meet their immediate energy requirements or else store significant amounts of energy for later use.
Physiology of the Lactate Threshold
These descriptions of lactate’s activities and roles make it easier to understand the often-misunderstood phenomenon called running velocity at lactate threshold. At the beginning of a moderate to difficult workout, lactate levels in the blood initially rise because glycolysis is working to provide the energy required to initiate running. If there were plenty of oxygen around, the pyruvate formed from glycolysis would enter the Krebs cycle and would be broken down all the way to carbon dioxide and water, releasing a lot of important energy in the process.
However, the workout has just begun, so heart rate is just beginning to increase, and the capillaries leading into the muscles are not yet in their fully open position. Therefore, blood and oxygen flow to the muscles is still somewhat limited. As a result, a fair amount of pyruvate will be converted to lactate, and lactate will begin piling up inside leg-muscle cells and spilling out into the blood. If blood lactate level is measured at this early stage of a workout, it can be surprisingly high, even when an athlete is moving along at a moderate pace.
If running is continued at a moderate intensity, blood lactate concentration will quickly drop. As heart rate increases and capillaries dilate, oxygen will pour into muscle cells, pyruvate will be oxidized for energy, and the lactate spillover process will abate. Blood lactate concentration will decrease and then hold steady, which means the entry and exit rates of lactate into and out of the blood are equal. Some lactate may continue to move into the blood from muscle cells, but other muscles, the heart, and various tissues around the body will remove it approximately as fast as it appears.
Lactate levels might continue to hold steady even as the intensity of the workout is gradually increased. As long as an athlete is not going too fast, that is, as long as oxygen is moving into muscle cells at an adequate rate, and the muscle cells are doing a good job of taking care of the pyruvate produced by glycolysis and thus limiting lactate spillover, blood lactate concentration will remain steady.
However, as running velocity increases, a speed is eventually reached at which glycolysis tears along so fast that the leg muscles begin to have difficulty breaking down most of the pyruvate into carbon dioxide and water via the Krebs cycle. Once this speed is attained or surpassed, lactate begins building up inside the muscle cells, and the lactate-spilling process may accelerate so much that lactate levels in the blood may increase significantly. This happens when the rate of lactate spilling into the blood is greater than the rate of lactate uptake from the blood.
This point may be reached because not enough oxygen is getting into muscle cells to handle all of the pyruvate being produced. Causes vary: The heart may be unable to pump oxygen-carrying blood at the needed rate; the capillary density around muscle fibers may be too limited; there may not be enough enzymes available to guide pyruvate through the Krebs cycle at very high rates; or muscle cells may be somewhat lacking in mitochondria, the tiny structures inside muscle cells in which the key reactions of the Krebs cycle take place.
This threshold velocity at which blood lactate levels begin to increase dramatically may also be reached if the muscles and tissues are not very good at clearing large amounts of lactate from the blood once they appear, a fact that has important implications for training. Whatever the underlying mechanism, the rate of lactate appearance in the blood suddenly outstrips the rate of lactate disappearance, and so blood lactate levels begin to climb somewhat precipitously. The running speed above which this lactate increase begins to occur is the running velocity at lactate threshold (see figure 10.2). Any higher speed produces a significant buildup in blood lactate. Any lower speed is associated with relatively low, stable blood lactate levels.
Figure 10.2 Above a specific running velocity, a runner’s blood-lactate level begins to increase dramatically. This specific speed is termed the lactate threshold velocity (LTV).
Every endurance runner has a running velocity at lactate threshold; even the fittest elite runner eventually reaches a velocity at which lactate begins to build up in the blood. The actual value of running velocity at lactate threshold, usually expressed in meters per second, reveals a lot about the overall fitness and performance capability of a runner. If running velocity at lactate threshold is reached at a relatively slow speed, for example, it often means that the oxidative energy systems in the muscles are not working very well based on one of the causes described previously. If the oxidative energy systems were operating at a high level, they would easily break down the modest amounts of pyruvate and lactate produced at the relatively slow speed, and lactate would not pour out into the blood.
If running velocity at lactate threshold is attained at a modest speed, it might also mean that the heart is not capable of sending oxygenated blood to the muscles at an adequate rate; this could thwart the breakdown of pyruvate and increase lactate production. Since blood lactate depends not only on lactate formation and spillage but also on how well the muscles and other tissues can remove lactate from the blood once it appears, a low running velocity at lactate threshold can also mean that the muscles, heart, and other tissues are not very good at extracting lactate from the blood.
Impact of Training on Running Velocity at Lactate Threshold
In practical terms, a key goal of training should be to move the running velocity at lactate threshold to progressively faster speeds; doing so will mean that cardiac output and the oxidative energy systems are improving and that the muscles are getting better at pulling lactate out of the blood and using it for energy. Having a high running velocity at lactate threshold means that an athlete can process pyruvate at greater rates and thus has the energy needed to run fast and long during endurance competitions.
A strong link exists between running velocity at lactate threshold and how difficult running feels, or the perceived exertion. In general terms, any running speed above running velocity at lactate threshold tends to feel difficult, while exertions completed below that velocity are comparatively comfortable. As an athlete moves up the velocity scale, perceived exertion increases dramatically. Thus, as running velocit
y at lactate threshold increases over time in response to appropriate training, previously uncomfortable paces suddenly begin to feel more comfortable and sustainable because they are now below the velocity at lactate threshold, and athletes complete their races at much faster paces than before. For many endurance athletes, improving running speed at lactate threshold can be the key to unlocking better performances. A variety of different scientific studies have suggested that running velocity at lactate threshold can sometimes be the single best predictor of endurance performance.7, 8
Runners and coaches sometimes wonder why running velocity at lactate threshold is such a great fitness indicator and race predictor. The reason for this predictive power is that this measurement includes information about lactate dynamics, and thus indirectly about oxygen use and running economy. Runners cannot have poor running economy and great velocity at lactate threshold. Poor economy means that lots of energy must be used to maintain a particular pace, and high rates of energy consumption generally mean heavy-duty carbohydrate (glycogen and glucose) breakdown rates. Ramped-up glucose metabolism means ample glycolysis, resulting in high rates of lactate production. It’s difficult to have a great running velocity at lactate threshold if lactate is flooding the blood at moderate speeds because of poor economy.
Running Science Page 15