Endurance training affects lactate metabolism in two fundamental ways.
When an athlete stresses the aerobic system, the body adapts in several ways. Oxygen can move more quickly to each muscle. Oxygen is necessary to produce the maximum amount of energy from pyruvate. Besides the availability of oxygen, there are other important factors which encourage the muscle to use pyruvate for fuel and speed up the use of lactate in adjacent muscles and the blood stream. Four which are enhanced by endurance training are:
Mitochondria are very dense in slow twitch muscle fibers (Type I) and in some fast twitch fibers (type IIa). There are far fewer mitochondria in type IIb fast twitch fibers. Type IIa fast twitch fibers have the ability to produce aerobic energy while type IIb fast twitch fibers, with very few mitochondria, produce hardly any aerobic energy. Over time, with sustained endurance training, many of the type IIb fibers will convert to type IIa fibers. Since lactate from fast twitch fibers is usually the main source of lactate during exercise, this conversion will cause less lactate to be produced.
Endurance training also increases enzymes that facilitate the conversion of pyruvate into energy in the mitochondria. Thus, more pyruvate can be used for fuel. This means that lactate production will be reduced in most muscle cells and that some cells will be able to use more of the lactate produced in other parts of the body. Both these enzymes help speed up the disappearance of lactate.
Endurance training is one of the keys to these adaptations that help shuttle the lactate about the body. The faster this shuttling process happens, the better the athlete will perform. So if you hear the expression, "building an endurance base", the changes described above are some of the implications of that expression. Even if the race or game involves an all out effort that requires the use of fast twitch fibers, endurance training has an important place. These fast twitch fibers produce lots of lactate and unless this lactate is cleared out of the fibers and shuttled to other areas of the body the fibers will eventually stop contracting. The longer these fibers can contract the faster the athlete will complete his or her event. So even success in the so called "anaerobic"9 events is affected by the body's ability to clear lactate, which is built through endurance training.
Frecuencia cardíaca y el Lactato
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9. The term anaerobic is commonly used in training documents and we have used it many times. We use the term anaerobic in this document primarily to describe the energy produced by the glycolytic system. But we also use the term to describe the high intensity exercise that involves the recruitment of fast twitch fibers. These fast twitch fibers will produce lactate more readily because they cannot handle the pyruvate produced from the glycolytic stage of energy production. The anaerobic process that produces the pyruvate in the fast twitch fibers is not different from the process in the slow twitch fibers which can use the pyruvate more readily. There may be plenty of oxygen available in the body or blood system but many of the fast twitch muscle fibers just cannot use it. Coaches and athletes often assume that anaerobic energy is turned on because oxygen is limited but it is very frequently an issue of fiber recruitment and the ability to utilize oxygen rather than availability.