Science of Sport: What Buses Teach Sprinters, Middle-Distance Runners, And Long-Distance Competitors About Training
By Owen Anderson, Ph. D. - Copyright © 2002-2004
On a day when you happen to step inadvertently in front of a bus, your continued existence suddenly hinges on whether your muscles contain enough high-energy phosphates. True, your visual system plays a role in your survival, too: After all, you do need to see the bus bearing down on you with all of its thunderous mightiness. But it is the job of your muscles to get you out of harm's way - to jump clear of the bus, and to jump free in just a small whisker of time. Such sudden leapfrogging is the province of the phosphates which are floating around in your muscular protoplasm.
You see, your muscles need energy to clear the bus bumper, and they can't wait for your heart to get revved up, for a cascade of oxygen-rich blood to hurry through your arteries, and for the incoming oxygen to assist with the breakdown carbohydrate or fat in order to release the energy required for your fancy jitney jumping. Depending on this rather laborious process would leave you dead; that looming double-decker has no time for the niceties of aerobic metabolism.
Your muscles need energy in a very small fraction of a second, and that energy comes from a unique high-energy phosphate compound called adenosine triphosphate, aka ATP. ATP is always present in your muscle cells - and indeed in all the living cells in your body; without ATP, your cells would quickly stop working and die. ATP is often called the universal energy donor, because it provides energy to all cells, not just a select few, but perhaps a better name would be your body's primary energy currency. As it happens, the energy contained in the food that you eat, be it carbohydrate, protein, or fat, must be first stored as energy within ATP molecules before it can be used by any cell in your body. To put it another way, although we often talk about muscles "burning" carbohydrate for energy, the carbohydrate is as directly useful to your muscle cells as Iraqi dinars or Italian lire would be to you as you shop at your neighborhood grocery store. Change those dinars or lire into pounds sterling, and you are good to go; change the carbohydrate energy into ATP energy, and your muscles can actually do some work. ATP provides all of the energy needed for muscular contractions (thankfully, for those key moments when you are positioned in front of a speeding bus), for the secretion of gastric juice after a meal, for the cognitive processes which take place in your brain as you read this article, and for every single activity you undertake. Carbohydrate, fat, and protein are not directly usable by cells, but ATP is, and in fact it is the only energy molecule that is directly serviceable.
Since ATP is so important, you might think that your cells would stock up on the stuff in a big-time way, but that is not the way it works. Stubbornly, your cells, including your muscle cells, refuse to stockpile the precious high-energy phosphate; rather, they are much more interested in storing carbohydrate and fat. Since ATP is stockpiled to a limited degree, and since muscular exercise depends on a steady, often expansive supply of ATP to provide the energy needed for muscular contractions, there is a strong need for muscles to have dependable "metabolic pathways" which can provide ATP at a rapid, reliable rate. If there is not going to be much ATP "at the ready" (there's only enough, in fact, to spring away from three or four buses), then there has to be some way to manufacture it in a quick and predictable manner.
As it turns out, athletes - and sedentary sofa spuds - have three such ATP-creating pathways, e.g., three unique and distinct series of chemical reactions which have the sole purpose of creating ATP. The simplest and most rapid pathway (and thus the one most useful for a confrontation with a careening bus) involves a rather generous little fellow found inside muscle cells called phosphocreatine. Phosphocreatine can not provide energy for muscle contractions directly (only ATP can do that), but it is very willing to donate a high-energy phosphate group to a chemical called ADP to form that energy kingpin, ATP. This critically important reaction (in which phosphate and ADP combine to make ATP) is catalyzed by an enzyme called creatine kinase, which is consequently found in significant concentrations inside muscle cells (the ubiquity of creatine kinase in muscle tissue explains why high levels of creatine kinase in the blood are associated with muscle breakdowns, including the kinds of catastrophes which can occur in cardiac muscle tissue after a heart attack).
One nice feature of phosphocreatine is that its intramuscular levels are not as tightly capped as ATP concentrations. Basically, your muscles worry a lot about keeping ATP levels fairly modest, but they allow phosphocreatine concentrations to soar quite dramatically. This, of course, helps explain why creatine supplementation has been so successful in high-power athletics; creatine added to the diet is absorbed readily and makes it way to the muscles, where it combines with the phosphates which are always lying around to make phosphocreatine. If you have a substantial amount of phosphocreatine in your muscle cells, you should be able to generate a lot of ATP in a very short period of time. You will be a great bus jumper - and if you are an athlete you will have the potential to improve your high-jumping and short-sprint abilities, too.
All physiological systems have their limits, however, and the system we have just described, which also goes by the moniker "ATP-PC system" or even "phosphagen system," has definite limitations. A key factor to bear in mind is that the phosphagen system, even in an individual who has "creatine-loaded" his/her muscles, can probably provide energy for no more than about eight to 10 seconds of intense muscular exertion. This is of course an adequate quantity of energy to help you survive your bus dilemma (unless the bus perversely decides to follow you down the street), and the system works wonderfully well for high-jumpers, power weightlifters, 50-meter sprinters, pole vaulters, cricket bowlers, soccer players racing across the pitch during the opening moments of play, and other athletes whose sports call for short bursts of high-intensity exertion. Unfortunately, however, the phosphagen system is of little help in activities which last longer than 10 seconds. If you wanted to run as fast as possible for 200 meters for example, your phosphagen system would ordinarily get you less than half-way toward your goal. Without another ATP-generating pathway to bank on, you would fall, fatigued, in a miserable heap, well short of your 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 - but why creatine has not been strongly tied to better times in endurance athletes (1).
The fact that the phosphagen system "works" for only eight to 10 seconds or so might seem a bit puzzling to you. After all, if creatine is still hanging around inside muscle fibers after it donates its phosphate to ADP, why can't creatine simply pick up some of the phosphate which is a natural constituent of cells and thus form phosphocreatine again, rejuvenating the ATP-creation process? That's pretty good thinking, but the hitch is that phosphocreatine reformation actually requires ATP and thus generally occurs only during recovery from exercise, when the ATP which is present is not being used to help muscles contract.
Fortunately, you and the rest of the animal kingdom possess an important second ATP-producing pathway which allows fervent activity 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 which is already lying about nor on a simple reaction between phosphocreatine and ADP, but it can get going fairly quickly (in fact, it can really get rolling after - you guessed it - eight 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 (glucose or glycogen) within muscle cells to form two molecules of pyruvic acid or lactic acid. As was the case with the phosphagen system, not even a droplet of oxygen is required for this to happen. However, glycolysis does not hinge on phosphocreatine; rather, the energy locked up in a glucose molecule is utilized in a way which allows a phosphate group to link up with ADP, forming our old friend ATP once again. For every molecule of glucose which is split during glycolysis, two robust molecules of usable ATP are formed. Glycolysis is the dominant ATP-production system for strenuous activities which require longer than 10 seconds to complete - but less than about 120 seconds (two minutes) for completion. It's important to note that your ability to generate energy via glycolysis is sometimes referred to as your "anaerobic capacity," since no oxygen is required to make glycolysis hum along.
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, hydrogens are stripped away from segments of carbohydrates, proteins, and fats and passed on to special hydrogen- (energy-) accepting molecules. These hydrogens actually contain the potential energy found in the original food molecules, and this energy can be used to combine phosphate with ADP to make - you guessed it - ATP! The pathway is termed aerobic because oxygen is the final hydrogen acceptor in the overall process, and in fact without oxygen the entire series of energy releasing reactions would grind to a halt; along similar lines, if the rate of oxygen provisioning to muscle cells can not be increased, then the rate at which ATP is generated aerobically can not be increased. If you understand the aerobic ATP-generating pathway, then you also can understand why increases in VO2max ("maximal aerobic capacity") often lead to improvements in endurance performance. To put it simply, if your muscles can use oxygen at a higher rate (to accept hydrogens), you can generate ATP at a higher rate, too, and you thus have the potential to exercise more intensely during your endurance competitions.
An important point to make is that the three ATP pathways are generally associated with three speeds of movement. Athletic events lasting 10 seconds or less are usually associated with incredibly intense exertion, and thus the phosphagen system is linked with ultra-high-speed movement. 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 our minds to the performance of maximal speeds, the glycolytic ("anaerobic") system to the use of fast speeds, and the aerobic process to the utilization of modest velocities.
Modes of training have also fallen into three general "baskets." Athletes whose events last no more than 10 seconds tend to train by engaging in short intervals of work lasting less than 10 seconds. If they possess some physiological awareness, they say that they are working on their phosphagen systems.
Meanwhile, athletes who compete for 10 to 120 seconds run a bit slower during training, and their work intervals generally last from 10 to 120 seconds, as you might expect. Such athletes may talk about building anaerobic capacity or - less commonly - maximizing their glycolytic potential. Finally, athletes who compete for longer than 120 seconds tend to have an abhorrence for training intervals lasting less than two minutes and work at slower speeds over intervals lasting from two to 10 minutes and during continuous efforts which may last for considerably longer. These endurance athletes are all agog about 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 thinking correct? Should the 100-meter, "phosphagen-based" sprinter, for example, completely eschew longer, glycolytic or aerobic running? Should the 90-second, glycolysis-loving athlete avoid phosphagen-enhancing efforts or exertions lasting more than two minutes, since such activities would tax the "wrong" energy producing systems? And should the aerobic, endurance athlete, be quarantined from phosphagenic and glycolytic efforts?
We can start with the easiest answer: The "phosphagen athlete" does not need to worry about conducting training efforts which use the glycolytic or aerobic systems. Even with an impressive dose of rhetoric, 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 their slow-twitch brethren. Of course, stroking 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 their muscles (so that force can be produced more quickly and efficiently).
How about the endurance athlete? Should he/she engage in the type of training which is the ordinary providence of the phosphagen or glycolysis athlete? To answer that question completely, let's picture a real life situation. We could use any endurance sport as an example, but let's say that you are a well-trained competitive cyclist and you are participating in a 100-K road race (surely, a true endurance event). You are doing very well, but there is that athlete about 25 meters ahead of you whom you would like to "pick off." You know that it's going to be tough, but you shift into a higher gear and pick up your rpms. In 10 seconds or so, you're past the rascal, but you're a little fatigued from your sprint and so you're scared he/she will pass you back. So, you keep up your sudden surging for a full 60 seconds before falling back into your normal velocity, and when you look back over your shoulder you are satisfied that you have left your competitor in the dust, or at least a bit behind you on the tarmac.
What ATP systems did you rely on for your sudden sprint - your incredible burst of speed? Did you utilize your old-reliable phosphagen system to catch up with the fellow in 10 seconds - and then your glycolytic system to power past him/her over the next 50 seconds?
If you answered these questions affirmatively, you certainly deserve an "A" for following and understanding what we have said so far, while resisting the temptation to fold up your newsletter (or turn off your computer) and turn on the telly. Unfortunately, however, your very reasonable answers would be wrong: 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 be produced via the aerobic pathway.
To understand the error of your ways, you simply need to remember that the rules we have established so far (in which 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) apply when the exercise begins from a relatively quiescent physiological state. When you start from "physiological ground zero," the phosphagen system is ready to go, but it takes about 10 seconds for glycolysis to get kicking and two minutes for oxygen to really penetrate your cells in significant amounts (thus permitting aerobic pathways to take hold).
Everything changes when you have been exercising for awhile, however; in fact, everything changes when you have been exerting yourself for as little as two minutes. Let's turn to our biking example, again: While you were cruising along during your 100-K race, you were probably working at about 85% of your maximal aerobic capacity (VO2max). During your sudden one-minute sprint, you probably soared to 95% of VO2max or so. In other words, your aerobic ATP-generating system had enough "room" to handle the upswing in cycling intensity; you simply stepped up the rate at which you were using oxygen to "catch" hydrogen inside your muscle cells. You were going fast, but your aerobic system was good enough to handle your fastness. True, glycolysis probably perked up as you pedaled furiously along. However, for each molecule of glucose broken down, the aerobic pathway generates about 19 times as much usable energy, compared to glycolysis. Thus, it's hard to argue that glycolysis (or your anaerobic capacity) got you through your sprint; the glycolytic-system's contribution was in fact pretty puny. Forget about the phosphagen system, too, which gave up the ghost after just 10 seconds of your ride.
That makes it seem as though the endurance athlete does not need to worry about glycolysis, the phosphagen system, or even about the fast training speeds associated with improving those systems, but hold on! If we change the event slightly, our picture comes into a different focus. For example, think about the 1500-meter runner competing to the best of his ability; by definition, this individual 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 world record - to complete). Two minutes into the event however, our athlete has already reached VO2max, and thus the "kick" which occurs during the last lap can not be propelled by advanced use of the aerobic pathway; glycolysis must fill the bill. Thus, it's clear that endurance athletes who reach VO2max during their competitions must train like glycolytic competitors, too, in addition carrying out their aerobic training. In general, athletes who compete in events lasting 12-13 minutes or less will "hit" VO2max as they compete, and thus their fate in competition might depend strongly on glycolytic capacity.
What about the plus-13-minute crowd? Perhaps surprisingly, they also need to train like the glycolytic gladiators. Even a marathon runner or 100-K biker, each of whom might get less than 1% of total ATP during competition from glycolysis, should spend significant amounts of time training fast, using work intervals as short as 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 our ATP-creating paradigm: There are other factors besides ATP-pathway development which are important for athletic success. Maximal speed is one: As an athlete's maximum rate of movement increases, usual race paces will feel easier and more sustainable. One way to enhance max velocity is to carry out the short-interval, high-intensity efforts from the realm of glycolytic training.
In addition, economy of movement is critically important to the endurance athlete. Economy is simply the oxygen cost of moving at a specific speed (that is, the rate of oxygen consumption associated with that speed), and as economy improves (e. g., as the cost/rate drops), specific speeds are sustained at a lower percentage of VO2max 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 - you guessed it - work intervals which often last from 10 to 120 seconds - is one of the most potent ways to upgrade economy.
Don't forget, too, about our continuity rule. 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, and an even greater amount will be produced via glycolysis, with the aerobic pathway chipping in almost nothing. However, as the workout continues (assuming, for example, that the athlete utilizes typical recovery intervals of 30 seconds), the rate of oxygen consumption will rise dramatically. In fact, after the seventh or eighth interval, the athlete may find himself/herself exercising right at VO2max, and he/she will probably stay right at VO2max for the remainder of the exertion. Exercise scientists believe that training at VO2max is one of the very best ways to enhance the aerobic ATP pathway, so we have a workout seemingly designed to enhance glycolysis which actually is incredibly good for aerobic ATP production.
What about the athlete who competes in events lasting from 10 to 120 seconds? How should he/she train? Fast starts are essential in such competitions, so he/she will have to do some training which bears a resemblance to the sub-10-second, phosphagenic athlete's work. 10-second max efforts, with very long recovery intervals to allow the phosphagen system to restore itself, will do the trick. The 10-120 athlete will also have to do some traditional aerobic work, too, using intervals or efforts lasting longer than two minutes. The reason for this is that even if the competition lasts only 30 seconds, the aerobic pathway chips in 20% of the needed energy; if the event lasts 60 seconds, aerobics add 30% of the juice. Thus the 10-120 athlete, even though he/she may never hit VO2max 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 little piece of the energy pie during races. ©
(1) Journal of the American College of Nutrition, Vol. 17, pp. 216-234, 1998
By Owen Anderson, Ph. D.
Copyright © 1998-2004 by Running Research News