The phosphagen system becomes less important in running events that last longer than 10 seconds. For the runner who wants to run as fast as possible for 200 meters, the phosphagen system would ordinarily get him or her less than halfway to the finish line. Without another ATP-generating pathway to use, the runner would fall, fatigued, in a miserable heap well short of the target. Incidentally, this limitation of the phosphagen system explains why creatine supplementation has often been linked with better performances in athletes engaged in high-power, short-duration sports—and why creatine has not been strongly tied to better times in endurance athletes.13
The fact that the phosphagen system works for only 8 to 10 seconds or so is puzzling to many serious runners. After all, if creatine is still present inside muscle fibers after it donates its phosphate to ADP, why can’t creatine simply pick up some of the phosphate that is a natural constituent of cells and thus form phosphocreatine again, rejuvenating the ATP-creation process? The problem is that phosphocreatine reformation actually requires ATP and thus generally occurs only during recovery from exercise when the ATP that is present is not being used to help muscles contract.
Glycolysis
Fortunately, there is a second ATP-producing pathway that allows running to be sustained for a longer period of time. This second system takes a little longer to get started since it does not depend on ATP that is already present in muscle cells or on a simple reaction between phosphocreatine and ADP. This second system can get going fairly quickly; it can really get rolling after 8 to 10 seconds, making it a nice complement to the phosphagen system. This second pathway is called glycolysis, and it involves the breakdown of carbohydrate (either an existing glucose molecule or a glucose molecule cleaved from glycogen) within muscle cells to form two molecules of pyruvic acid or lactic acid.
As was the case with the phosphagen system, not a single molecule of oxygen is required for this to happen. A further advantage is that glycolysis does not depend on phosphocreatine; rather, the energy locked up in a glucose molecule is used in a way that allows a phosphate group to link up with ADP, forming ATP in the process. For every molecule of glucose split during glycolysis, two robust molecules of usable ATP are formed. Glycolysis is the dominant ATP-production system for strenuous activities requiring longer than 10 seconds but less than about 120 seconds, or two minutes, for completion. The ability to generate energy via glycolysis without the use of oxygen is sometimes referred to as a runner’s anaerobic capacity since no oxygen is required to make glycolysis proceed at the necessary rate.
Aerobic Pathway
In activities lasting longer than two minutes, the well-known aerobic pathway for ATP production holds sway. During aerobic ATP production, which occurs inside special cellular structures called mitochondria, hydrogen atoms are stripped away from segments of carbohydrates, proteins, and fats and passed on to special hydrogen-accepting molecules. These hydrogen atoms actually contain the potential energy found in the original food molecules, and this energy can be used to combine phosphate with ADP to make ATP!
The pathway is termed aerobic because oxygen is the final hydrogen acceptor in the overall process, and without oxygen the entire series of energy releasing reactions would grind to a halt. If the rate of oxygen provisioning to muscle cells cannot be increased, then the rate at which ATP is generated aerobically cannot be increased either. If runners understand the aerobic ATP-generating pathway, then they can also comprehend why increases in O2max, or maximal aerobic capacity, often lead to improvements in endurance performance. If the muscles can use oxygen at a higher rate to accept hydrogens, then ATP can be generated at a greater rate, too, and runners thus have the potential to exercise more intensely during endurance competitions.
Training Implications
The three ATP pathways are generally associated with three speeds of movement. Athletic events lasting 10 seconds or less are usually linked with incredibly intense (i.e., maximal) exertion, and thus the phosphagen system is depended on most heavily for movement at high speeds. Competitions lasting from 10 to 120 seconds are also carried out at fast speeds, although not as fast as the shorter-duration exertions. Finally, competitions engaged in for more than 120 seconds are conducted at fairly moderate speeds compared with the torrid movements linked with shorter efforts. As a result, the phosphagen system has been wedded in runners’ and coaches’ minds to the performance of maximal speeds, the glycolytic (i.e., anaerobic) system to fast speeds, and the aerobic process to more modest velocities.
Modes of training have consequently fallen into three general types. Athletes whose events last no more than 10 seconds tend to train by emphasizing short intervals of work lasting 10 seconds or less. If such runners have some physiological knowledge, they might proclaim that they are working on their phosphagen systems during training. Athletes who compete for 10 to 120 seconds tend to run a bit slower during training; their work intervals generally last from 10 to 120 seconds, as one might expect. Such athletes may talk about building anaerobic capacity since the glycolytic system provides most of the energy for their efforts or—less commonly—about maximizing their glycolytic potential.
Finally, athletes who compete for longer than 120 seconds tend to abhor training intervals shorter than 2 minutes; these athletes work at slower speeds over intervals lasting from 2 to 10 minutes and at continuous efforts that may last for considerably longer. These endurance athletes are extremely attracted to the process of maximizing their aerobic systems, and they may even speak about improvements in heart function, upswings in breathing capacity, vine-like growths of capillaries around their muscle fibers, and the increased ability of their muscles to use oxygen.
Is this traditional thinking about training correct? Should the 100-meter, phosphagen-based sprinter, for example, completely eschew longer glycolytic or aerobic running? Should the 50-second, glycolysis-using athlete avoid phosphagen-enhancing efforts or exertions lasting more than 2 minutes since such activities would seemingly tax the wrong energy producing systems? And should the aerobic endurance athlete stay away from phosphagenic and glycolytic efforts?
Science suggests that it is best to start with the easiest answer: The phosphagen athlete does not need to worry about conducting training efforts that use the glycolytic or aerobic systems. It is not possible to construct a compelling argument for such training especially since scientific evidence suggests that longer-duration work intervals might convert fast-twitch muscle cells into slow-twitch fibers!
Of course, upgrading the phosphagen system is not the whole story for such athletes. Simple manipulations of phosphocreatine and creatine kinase may well help an athlete sprint faster, but by themselves they will not produce an athlete’s best-possible performances. Performance, after all, is not just a chemical story. Short-distance sprinters will also want to upgrade leg-muscle size in order to produce more propulsive force and also improve nervous system control of muscles so that higher amounts of force can be produced in shorter periods. These changes can occur independently of upswings in phosphagen use in muscle cells. Furthermore, it is possible that advancements in the phosphagen system might not produce any improvements in running performance at all unless nervous system function and leg-muscle size are enhanced.
Should the endurance athlete engage in the type of training that is the ordinary providence of the phosphagen or glycolysis athlete? To answer that question completely, it is necessary to picture a real-life situation. Consider the case of a well-trained runner competing in a half-marathon competition. The runner might notice an athlete about 15 meters ahead who needs to be picked off. The runner knows that it’s going to be tough, but he or she shifts into a higher gear and begins to accelerate. In 10 seconds or so, this runner is ahead of the other competitor, but there is fatigue from the sprint and the troubling awareness that the competitor might end up coming back. So, this runner keeps up his or her sudden surging for a full 60 seconds before falling back to normal velocity. When the runner looks ba
ck, he or she is satisfied that the competitor has been left in the dust.
What ATP systems did this athlete rely on for the sudden sprint? Did he or she use the phosphagen system to catch up with the competitor in 10 seconds and then the glycolytic system to power past the competitor over the next 50 seconds? Such thoughts are certainly reasonable, but the truth is that most of the energy for the sprint, both the high-speed 10-second component and the follow-up 50-second surge, would have to be produced via the aerobic pathway.
To understand this, it is important to remember that the rules established so far apply when the exercise begins from a relatively quiescent physiological state. In other words, the phosphagen system controls exercise lasting 10 seconds or less, glycolysis dominates exertion requiring 10 to 120 seconds, and the aerobic pathway swamps everything else. When an athlete begins from physiological ground zero, the phosphagen system is ready to go, but it takes about 10 seconds for glycolysis to come up to speed and as long as 2 minutes for oxygen to really penetrate muscle cells in truly significant amounts, thus permitting aerobic pathways to take precedence.
However, everything changes when a runner has already been in motion for awhile; in fact, everything is altered when an athlete has been running for just 2 minutes. In the example just mentioned, when the runner was cruising along during the half-marathon race, he or she was probably working at about 85 percent of maximal aerobic capacity, or O2max. During the sudden 1-minute sprint, the runner probably soared to 95 percent of O2max or so. In other words, the aerobic ATP-generating system had enough room to handle the upswing in running intensity; the runner simply stepped up the rate at which he or she was using oxygen to catch hydrogen and increase energy supply inside the muscle cells. The runner was going fast, but the aerobic system was functioning well enough to handle the speed.
Yes, glycolysis would have perked up as running speed increased. However, for each molecule of glucose broken down, the aerobic pathway generates about 19 times as much usable energy compared to glycolysis alone. Thus, it’s hard to argue that glycolysis, or anaerobic capacity, provided the lion’s share of the energy for the sprint; the glycolytic-system’s contribution was in fact pretty puny. The phosphagen system, too, was nonproductive because it was exhausted just 10 seconds into the race.
That makes it seem as though the endurance runner does not need to worry about glycolysis, the phosphagen system, or even about the fast training speeds associated with improving those systems. If we change the event slightly, however, the analysis comes into a different focus. For example, the 1,500-meter runner competing to the best of his or her ability is an endurance athlete whose performance depends primarily on the aerobic pathway for ATP generation since the event requires at least 3:26—the current male world record—to complete.
Two minutes into the event, however, the athlete has already reached O2max, and thus the kick, or increased speed, that occurs during the last lap cannot be propelled by advanced use of the aerobic pathway; glycolysis must fill the bill. It’s clear that endurance athletes who reach O2max during their competitions must train like glycolytic competitors, too, in addition to carrying out their aerobic training. In general, athletes who compete in events lasting 12 minutes or less will hit O2max as they compete, and thus their fate in competition might depend strongly on glycolytic capacity.
What about the plus-12-minute crowd? Perhaps surprisingly, they also need to train like the glycolytic competitors. Even a marathon runner, who might get less than 1 percent of total ATP during competition from glycolysis, should spend significant amounts of time training fast, using work intervals as short as 15 to 30 seconds. From an ATP-generation standpoint, this would not seem to be the case, but it is important not to get too trapped by the ATP-creating paradigm. There are other factors besides ATP-pathway development that are important for athletic success. Maximal speed is one: As an athlete’s maximal rate of movement increases, usual race paces feel easier and more sustainable. One way to enhance maximal running velocity is to carry out the short-interval, high-intensity efforts from the realm of glycolytic training (see chapters 16 and 28).
In addition, economy of movement is critically important to the endurance athlete. Economy is simply the oxygen cost of moving at a specific speed (i.e., the rate of oxygen consumption associated with that speed). As economy improves, along with a parallel drop in cost or rate, specific speeds are sustained at a lower percentage of O2max and feel appreciably easier, allowing the athlete to graduate to higher speeds in competitions. As it turns out, scientific research indicates that high-speed training, using very intense work intervals that often last from 10 to 120 seconds, is one of the most potent ways to upgrade economy (see chapters 25 and 28).
A continuity rule is also important. When an endurance athlete begins a workout by blasting along very quickly for 30 seconds, a significant amount of the energy will come from the phosphagen system; an even greater amount will be produced via glycolysis, with the aerobic pathway chipping in comparatively little energy. However, as the workout continues—assuming, for example, that the athlete uses typical recovery intervals of 30 seconds or so—the rate of oxygen consumption will rise dramatically over the course of the workout. In fact, after the seventh or eighth interval, the athlete may find himself or herself exercising right at O2max and will probably stay at O2max for the remainder of the exertion. Exercise scientists believe that training at O2max is one of the best ways to enhance the aerobic ATP pathway. So a workout seemingly designed to enhance glycolysis can actually be very good for aerobic ATP production.
What about the athlete who competes in events lasting from 10 to 120 seconds? How should he or she train? Fast starts are essential in such competitions, so the runner will have to do some training that bears a resemblance to the work of the phosphagenic athlete who competes in events that are over in about 10 seconds. Ten-second maximal efforts, with long recovery intervals to allow the phosphagen system to restore itself, will do the trick. This athlete will also have to do some traditional aerobic work using intervals or efforts lasting longer than 2 minutes. The reason for this is that even if the competition lasts only 30 seconds, the aerobic pathway chips in 20 percent of the needed energy; if the event lasts 60 seconds, aerobics add 30 percent of the kilocalories. Thus this athlete, even though he or she may never hit O2max during competitions, will still need to develop the aerobic system to a certain extent to make sure it is there, waiting, to chip in its piece of the energy pie during races.
Conclusion
Intramuscular, stored carbohydrate provides most of the fuel for running that lasts for 3 hours or less. In general, competitive runners at all distances ranging from 100 meters to the marathon should avoid eating and training strategies designed to enhance fat metabolism since carbohydrate will actually be the key fuel for such endeavors. At first glance, an understanding of the bioenergetics of running suggests that endurance runners should never carry out short, high-speed intervals. The truth, however, is far different. While it is true that sprinters should never run long, endurance runners should often run fast. This is because endurance athletes require high-velocity work to upgrade their neuromuscular systems, boost maximal running speed, and enhance running economy. Plus, such anaerobic training is actually highly aerobic in nature, with O2max being attained as fast intervals are repeated over the course of a session.
Chapter 44
Eating for Enhanced Endurance and Speed
Runners can eat in ways that either promote or limit their endurance capacities and overall performances. Fortunately, optimal eating is straightforward and relatively easy to accomplish with a diet that revolves around heavy carbohydrate consumption, moderate protein ingestion, and the intake of reasonable levels of health-promoting omega-3 and monounsaturated fat. Elite Kenyan runners follow the best eating plan in the world for endurance running and yet routinely ingest no more than 12 food items, most of which are rich in carbohydrate and low in fat.
Carbohydrat
e Loading
Since the early 1970s, exercise scientists have known that high-carbohydrate diets foster enhanced endurance performances. In a classic study carried out in Sweden, 10 fit physical education students ran a 30K (18.6 mi) race, trained normally for 3 weeks, and then repeated the 30K competition.1 Four of the 10 runners consumed a mixed diet with moderate amounts of carbohydrate during the week leading up to the first 30K race. The other 6 participants ran the first race after a special program of carbohydrate loading (i.e., manipulating dietary intake in order to maximize glycogen storage in the muscles).
Approximately 1 week before the first race, these six runners exercised heavily for at least 2 hours to deplete muscle glycogen stores. After the race, the individuals followed a carbohydrate-free diet for 3 days while carrying out moderate training. Following this carbohydrate fast, the athletes consumed about 2,500 calories (625 grams) of carbohydrate each day for 3 days in association with light training. This pattern represents a depletion-repletion method of carbohydrate loading that was once quite popular among endurance athletes.
Prior to the second 30K competition, the two groups switched dietary plans so that each participant in the study ran a 30K with and without carbohydrate-loaded leg muscles. Muscle biopsies were taken from the leg muscles of each runner before and after each race to assess actual glycogen levels. The depletion-repletion carbohydrate-intake strategy had a major impact on average muscle glycogen concentrations. The average was 35 grams per kilogram of muscle with the special loading regimen and just 17 grams per kilogram with the mixed, moderate-carbohydrate diet.
Running Science Page 60