As a sports scientist, I've had the opportunity to participate in a great deal of research concerning the causes of "hitting the wall." Any athlete who has experienced this extreme muscle fatigue knows that it can make crossing the finish line a difficult - if not impossible - goal to achieve. Numerous studies have pointed to dehydration and carbohydrate depletion as the causes of exercise-induced fatigue. To some extent, this is true.
Today, endurance athletes know they can prolong activity by practicing carbohydrate loading prior to extended training sessions or long competitive events. They are also aware of the importance of drinking fluids before and during exercise to prevent dehydration and heat-related illness. However, many athletes - professional or otherwise - do not know the full extent of what causes muscle fatigue. We will explore several other factors that contribute to fatigue, including depletion of muscle fuels, low blood glucose, increased lactic acid levels and central fatigue.
Dehydration
Water is an essential macronutrient in every function of the body. During exercise, it's important to make sure you drink water because of its vital role in cardiovascular function and temperature regulation.
When you exercise, your body loses water through sweating and evaporation. Sweat is your body's coolant. During an intense workout, your muscles generate heat, which is carried by your blood through capillaries near the surface of your skin. Your sweat glands release perspiration that evaporates, cooling the skin and the blood underneath. Cooled blood then flows back to cool your body's core.
Sweating is therefore an essential mechanism for regulating body temperature. However, when your body loses water, it limits the capacity of your blood to carry vital nutrients (such as glucose, fatty acids and oxygen) to working muscles. The capacity of the blood to remove the by-products of metabolism, including carbon dioxide and lactic acid, is compromised as well. The result is an increased demand on the circulatory system, which is approximately 70 percent water.
I have seen some cyclists set out for a 50- to 75-mile ride with only two small water bottles. On a hot day, the weight lost through sweat may be as much as five to six pounds. Even if a rider drinks both bicycle-style bottles, equaling about 40 ounces of water, he or she will replace only 2.5 pounds of lost fluid. In such cases, dehydration is inevitable. At the very least, this limits the cyclist's athletic performance. More seriously, it puts the athlete in jeopardy of experiencing heat-related illness and even circulatory collapse.
Even during mild dehydration -as little as 2 percent of your body weight- can impair athletic performance. Athletes must drink fluids to combat the sweat loss that naturally accompanies vigorous exercise. Although it may be impossible to offset, all of the water lost through sweating - even partial replacement - can minimize the risk of overheating.
Pure water is not the best way to rehydrate during and after exercise. To restore the body fluids you sweat out during exercise, you should consume a beverage that contains agents such as glucose and sodium, two ingredients found in most sports or energy drinks. Glucose and sodium help maintain blood volume and aid the absorption of water into your body. These two ingredients also increase thirst, which will prompt you to continue drinking. The more you drink, the more completely you'll restore lost body fluids.
Overheating
An athlete's temperature, normally about 98.6 degrees Fahrenheit, may increase to 104 degrees or more during intense exercise. The circulatory system transports the heat generated by muscles to the skin to be dissipated. While a certain percentage of blood is used to regulate body temperature, large quantities of blood are still required to meet the energy and metabolic needs of working muscles. These demands may overtax the circulatory system, resulting in an inadequate removal of body heat and a corresponding rise in the athlete's body temperature.
Even in mild weather, you can run the risk of overheating. The threat becomes more severe when weather conditions are hot and humid. Sweat doesn't evaporate well in this sort of climate because the surrounding air is already saturated with water. Without the cooling effects of sweat evaporation, your body is unable to maintain a constant body temperature that's within the normal limits. If you continue to exercise in this state, you will increase your chances of suffering from heat exhaustion.
The hazards of exercising in hot, humid conditions were made abundantly clear at the 1996 Olympic Games in Atlanta, Georgia. A cycling road race was held on a day when temperatures exceeded 85 degrees, with unfavorable conditions of high humidity and no cloud cover. Several athletes from New Zealand and Denmark dropped out, and a Swedish rider who collapsed after the finish line had to be hospitalized. The symptoms these athletes developed undeniably indicated heat exhaustion. (See Table 3.1.).
Research has proven that athletes involved in endurance sports other than distance cycling experience similar risks of overheating. In the 1970s, studies conducted by Dr. David Costill at the Human Performance Laboratory at Ball State University found that athletes who drank fluids during a two-hour run lowered their body temperatures by two degrees compared with athletes who did not rehydrate. Without fluid intake, one athlete's temperature reached 105.5 degrees during exercise, but it only reached 103.6 degrees when he drank fluids. Body temperature above 104.5 degrees is extremely dangerous, causing great physical and mental stress. Therefore, fluid replacement is absolutely critical during training or competition - especially on a hot day.
If you're preparing for competition, it's wise to drink extra fluids in the few days before you compete because drinking ensures maximum tissue hydration at the start of an event. You should also drink fluids before and frequently during a long event to keep your body temperature at safe levels.
Depletion of Muscle Fuels
During intense short-term exercise, fatigue can result from depletion of muscle glycogen. This is because glucose is the only fuel source your muscles use to generate energy through the anaerobic pathways, which last from 10 seconds to several minutes. During long-term exercise, the aerobic pathway kicks in for energy production. In addition to glucose, fatty acids and amino acids are burned as fuel for aerobic metabolism, providing a wider range of energy resources.
Studies have shown, however, that glycogen depletion contributes to muscle fatigue even during long-term exercise. When subjects exercised to exhaustion at 80 percent of their maximum capacity, the glycogen content of their muscles dropped to near zero in about 90 minutes. Endurance was increased when glycogen storage capacity in the muscles was enhanced through carbohydrate loading. This suggests that your muscles' initial glycogen content plays an important role in exercise performance. Because glycogen is a crucial fuel for energy production, your muscle cells attempt to conserve glycogen during extended exercise. As you continue to exercise, fat stores are mobilized and fatty acids are used in approximately equal amounts as glycogen to provide energy. Finally, protein begins to provide a greater percentage of energy.
If you exercise primarily to burn fat, you'll be happy to know that it's possible to train your muscles to become more efficient in using fat as a fuel source by completing several extended training sessions, each lasting more than two hours. This method stimulates the enzymes responsible for the conversion of stored fat into energy, which will enable you to burn a higher percentage of fat and conserve glycogen for more strenuous efforts. The increased capacity of trained athletes to use fat, and the tendency of their muscles to release more fatty acids from fat tissue, suggests that training can shift the proportion of energy produced through the metabolism of fat.
Low Blood Glucose
In addition to providing necessary energy for muscle contraction, glucose is a vital source of energy for the brain and nervous system. Although fatty acids and amino acids can be used for voluntary muscle movement, glucose is the only fuel that can be used in sufficient amounts for nervous system function. In fact, 50 to 60 percent of the glucose supplied by the liver is used strictly for brain and nervous system function.
During the early phase of exercise, most of the energy supplied by carbohydrates comes from muscle glycogen. As exercise continues and muscle glycogen stores run low, glycogen contributes less and less as a source of energy. Figure 3.1 shows the proportions of nutrient fuels that are used during exercise. After about two hours of endurance exercise, muscle glycogen stores decrease rapidly. This reduced reliance on muscle glycogen is balanced by an increased reliance on blood glucose for fuel.
After two to three hours of exercise, the majority of carbohydrate energy appears to be derived from glucose, which is transported from circulating blood into the exercising muscles. This causes blood glucose to decline to relatively low levels. The liver, which has been supplying some amount of glucose from its glycogen stores, reduces its output due to depletion of liver glycogen. Fatigue occurs because there is not enough blood glucose available to compensate for the depleted muscle glycogen.
Studies by Christensen and Hansen in the 1960s demonstrate that subjects who exercised to exhaustion and then consumed 200 grams of glucose extended performance by one hour. In this experiment, the nervous system's fuel had been depleted and restored. These results suggest that exhaustion may sometimes be a phenomenon of the central nervous system and not only the result of depleted muscle fuel stores.
As a long race continues, many athletes consume sports drinks, carbohydrate gels and sports bars in an attempt to avoid fatigue. The use of these products helps athletes keep blood glucose levels elevated to maintain central nervous system function, in addition to providing carbohydrates to working muscles. Research by Edward Coyle, Ph.D., from the University of Texas has shown that athletes are capable of absorbing up to 80 grams of carbohydrate per hour during exercise. This can delay fatigue by as much as 30 to 60 minutes, because the working muscles can rely primarily on blood glucose for energy.
Increased Lactic Acid Levels
Lactic acid is a by-product of anaerobic metabolism that cannot be used effectively by your working muscles. Instead, lactic acid diffuses into your bloodstream to be transported to your heart, liver and non-working muscles, where it is converted back into glucose. As you begin to exercise harder, more lactic acid builds up in your muscles and must be removed by your blood. The lactic acid level of your blood, therefore, continues to increase as exercise intensity increases. If this level of intensity is maintained, you will soon reach your lactate threshold, defined as the point at which the level of lactic acid in your blood is greater than your body can metabolize. Figure 3.2 illustrates how blood lactate concentration increases with an increase in exercise intensity.
Most coaches and scientists consider the lactate threshold to be an excellent indicator of an athlete's potential for endurance performance. The ability to exercise at a high intensity without accumulating lactic acid is very beneficial. Generally, in two athletes with similar oxygen up takes, the athlete with the higher lactate threshold will perform better in endurance activities. Laboratory and field experiments suggest that training can alter the amount of lactic acid produced and tolerated by athletes. This adaptation probably results from an increase in the number of capillaries that deliver oxygen to the muscles.
Lactic acid build-up causes burning pain and muscle fatigue if it is not removed quickly from the muscles. Although lactic acid can be tolerated for short periods of time, your muscles should be allowed to relax at every opportunity. This allows your bloodstream to carry the lactic acid away and supply your tissues with oxygen for aerobic metabolism.
Central Fatigue
In addition to focusing on the causes of muscle fatigue, recent research has also centered on mental fatigue during exercise. This is commonly called central fatigue because it results from impaired function of the central nervous system. Although central fatigue does not affect your muscles directly, it can reduce your capacity to perform.
Dr. Eric Newsholme of Oxford University has uncovered a correlation between levels of the amino acid tryptophan in the brain and the degree of mental fatigue. When tryptophan enters the brain, it can depress the central nervous system, causing sleepiness and fatigue. Normally, there are sufficient amounts of the branched-chain amino acids (BCAAs) leucine, isolucine and valine in the blood to regulate the entry of tryptophan into the brain. Supplementation before and during exercise has been proven to increase performance during a soccer game and after a 30-kilometer race. Likewise, in a study of 193 marathoners, branched-chain amino acid supplementation improved performance in the slower runners. Additional research is being conducted in this area of study.
Conclusion
Today, sports scientists and nutritionists know that dehydration and carbohydrate depletion are not the only two factors that cause fatigue. Factors such as elevated lactic acid levels and central fatigue also contribute to exhaustion. By making sure that you're properly hydrated before and during exercise, and by consuming enough of the nutrients your body needs to fuel activity, you can greatly reduce your chances of "hitting the wall."
Bio:
Edmund R. Burke, Ph.D., is director of the Exercise Science Program at the University of Colorado at Colorado Springs. He is also a Fellow of the American College of Sports Medicine (ACSM) and Certified Strength and Conditioning Specialist (CSCS) with the National Strength and Conditioning Association (NSCA). His most recent publication is Optimal Muscle Recovery (Avery Publishing Group, Inc., $14.95).
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