Joe Dunbar provides an overview of the body's energy systems and the foe of all endurance athletes - Lactic Acid.
The term lactic acid, or lactate, is used by some in the field of research in sports science, while others may use the term in describing that pain felt in the body during exhaustive exercise, but what is it? It is usually feared by most athletes, as the term lactate is commonly associated with that intense pain felt in the legs and arms at the end of a race or hard session, especially in short events like the 400m or 800m. However, lactate is not all bad, because it can act as a good marker that may help in the planning of effective training programs.
In describing lactate, it is important to have a brief understanding of energy systems, the mechanisms the body uses to supply energy to the working muscles. This does not mean that we have to plunge deeply into the realms of biochemistry and physiology, but a few basic principles have to be established.
For the muscles to contract, they need energy, and put simply, this can be supplied by molecules of ATP. This ATP, however, must be supplied to the muscle when exercise continues as the amount stored in a muscle at any one time is enough for only a very short period. There are indeed three methods the body can use to supply energy, in the form of ATP, to the working muscles. Two of these methods, or sources of energy supply, can be considered anaerobic (they do not require oxygen immediately for their chemical processes) and the other is considered aerobic, where a steady supply of oxygen is available to meet the demand of exercise. The anaerobic sources can be split again, into either lactate or alactate. When the muscle's immediate stores of ATP are exhausted (within a fraction of a second), chemical changes to creatine phosphate, also stored in the muscle, can give the muscle extra ATP. This is considered an alactate source of ATP, because no lactate is produced. Again, this source of ATP is quite short lived, so the body has to utilise another method to continue to supply a working muscle with these units of energy.
Glycolysis is the term used to describe the energy pathway that is used in producing molecules of ATP from a molecule of glucose. A series of biochemical reactions takes place within the muscle cells whereupon, if oxygen is present, another series of reactions is embarked upon and the process is considered aerobic. Provided there is plenty of oxygen and other fuels needed in the chemical reactions, and the exercise maintains at a steady state, it is possible to maintain aerobic exercise for a long time. However, if there is not enough oxygen present then lactic acid is produced, an oxygen debt is incurred and the process is considered anaerobic. This is all very well, after all if the body can still produce ATP to fuel the muscle without oxygen, there is no problem, is there? Well, it is not quite as simple as that, because if the acid conditions continue, the functioning of the body will become impaired and the muscles will fatigue very quickly. In this respect lactate can be considered as bad news for the athlete, as it is one of the factors that will lead to fatigue, at least in the shorter, middle distance events.
To put the idea of energy systems into practical terms it is useful to give an everyday example. If a bag of sugar is sitting on a table and a person picks it up, the muscles would not need oxygen, as you could still do this activity while holding your breath. The exercise would be anaerobic and alactate. If the person then decided to hold their breath and pick it up and put it down quickly, 40 times, the work would still be anaerobic, but they would probably begin to feel discomfort, as lactate would probably build up, causing local fatigue. If the bag was then lifted and put down 100 times, at a steady rate, the exercise would probably be aerobic because the body would be able to take in sufficient oxygen to meet the demands of the exercise.
It should be remembered that the energy systems are all working together at the same time. It is just that the amount of energy or ATP derived from each system may vary according to the conditions and intensity of the exercise. For example during a 10 second sprint there would be contribution from all of the energy sources. The ATP in the muscle would be used immediately, but at the same time some ATP would be supplied by anaerobic glycolysis (producing lactic acid as one of the waste products) and some from aerobic sources. It is just that most of the ATP would be supplied anaerobically. If the exercise was of longer duration the contribution from anaerobic sources would be less, but made up by a greater contribution by the aerobic source.
If an athlete starts a steady five mile run, it might take a few minutes for the heart rate to increase and the blood supply to be at its optimal level. This might mean that there is not quite enough oxygen to fuel this exercise fully aerobically and in the short term the ATP might be supplied more by anaerobic sources. As the athlete warms up, however, his/her body will become better adapted for the exercise and will shift to become exercising in a more aerobic state. In terms of lactate, then, it is possible that at the start of a steady run, there may be an increase in lactic acid in the muscles that will gradually be reduced as aerobic sources slowly meet the demands of the exercise.
If the athlete is performing a run at slightly greater intensity, for example in a five mile race, after adequate warm up, it is likely that there will be an element of lactate build up. This is because there is a contribution of anaerobic energy production involved, as well as aerobic energy production. If the athlete produces a finishing kick at the end of this five mile race, he/she will still be using aerobic source, but will also use his/her anaerobic sources to a greater extent, not only in terms of lactate build up, but also in the alactate department The key to judging the kick correctly is knowing what is the furthest point from the finishing line that maximum effort can be thrown in, before lactic acid takes over and limits the ability of the muscles to perform.
In exercise, the greater the intensity of exercise, the higher the lactate levels are likely to be. So, you would expect to see greater lactate levels at the end of an 800m race than a marathon, because the intensity of exercise would be much greater in the former. Lactate is produced in working muscles, as a waste product. It can be oxidised, either when the exercise is halted, or slowed down so that the aerobic capacity of the individual is sufficient to meet the demand of the exercise being performed. As the lactate is produced in the muscles it leaks out into the blood and is carried around the body. This is why measuring blood lactate is a good reflection on how hard the individual is working. There is very good correlation between the muscle lactate (which is much more difficult to try to measure) and blood lactate.
When lactate is produced in some muscles during very hard exercise, it can be taken to other muscles which are not working, or other organs of the body, to be oxidised. It can also shift across to different fibres to be oxidised. It has been found that
fast twitch fibres generally produce more lactate than slow twitch.
This is most likely to be the reason why at exhaustion, a sprinter will have greater lactate levels than, say, a marathon runner. The muscles of a sprinter are likely to have a much greater ratio of fast twitch fibres, while the marathon runner will have fibres of predominantly slow twitch.
Further, speed work is likely to increase the maximum amount of lactate present in the muscles during intense physical exercise, probably because the body learns to recruit more fast twitch fibres, that may also become better developed.
Given that high levels of lactate will be detrimental to endurance performance, one of the key reasons for endurance training is to enable the body to run at a greater pace with a small amount of lactate. This can be done by long, steady state endurance running, which will develop the aerobic capacity, by means of capitalisation (formation of more small blood vessels, thus enhancing oxygen transport to the working muscles) and by creating greater efficiency in the heart and lungs.
If the aerobic capacity is greater, it means that there should be more oxygen available to the working muscles and this should delay the onset of lactic acid at a given work intensity.
However, another way of developing the aerobic capacity is threshold running and interval work. Here the body works at an intensity that produces a greater amount of lactate, thus the anaerobic system is challenged as well as the maximum aerobic capacity.
VO2max is often quoted as a good indicator of an individual's fitness. The VO2max is a good general indicator of endurance fitness and was, up to the 1970's, the gold standard measurement of aerobic ability. In recent times, however, sports science has progressed, giving a more sensitive measure of an athlete's condition by measuring blood lactate during exercise.
Joe Dunbar
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