All the energy stored in the food we eat is ultimately transformed into energy, and stored into a special type of molecule called Adenosine Triphosphate or ATP. The energy released from the ATP molecules can be used to increase the body temperature or as a source of power to perform chemical or mechanical activities. Every time we contract a muscle we burn the energy stored into the ATP. ATP is available in very small quantities in the muscle cells which can use it to fuel just a few seconds of activity (only between 0 and 4 seconds). Any activity lasting longer than 4 seconds needs to be fueled by ATP created in one of the three energy pathways our organism is capable of; two Anaerobic (without oxygen) and one Aerobic (with oxygen).
Let’s see how they work, as this will clarify what kind of training it is needed to improve endurance.
The first anaerobic metabolism, ATP-CP, provides high power for a short period of time. Once the ATP stored in the muscles has been burned (0 to 4 seconds of exercise), the ATP is re-synthesized from Creatine Phosphate or CP (that is the ATP-CP pathway). The quantity of ATP produced in this way can fuel the muscles for up to another 6 seconds before the CP runs out. This energy pathway together with the ATP stored in the muscles can allow an athlete to run a fast 100 m but not more than that.
The second anaerobic metabolism, Glycolysis, provides a moderate amount of energy for a couple of minutes. After both the ATP and CP stored in the muscles are depleted the body turns to the stored glucose for generating ATP (that is the Glycolysis). The breakdown of glucose in absence of oxygen produces lactate and ions of hydrogen. The production of these ions, and not that of lactate, is the responsible for the sense of fatigue and soreness in the muscles after an high intensity effort. The glycolysis can produce energy lasting no more than few minutes before glucose runs out and lactic acid accumulates. In this conditions it becomes increasingly more difficult to keep the same level of exercise intensity. If the level of oxygen in the blood is enough the aerobic metabolism kicks in to keep producing ATP to fuel the muscles.
Activities lasting more than few minutes are fueled by aerobic metabolism. The ATP in this case is re-synthesized by proteins, fat, and carbohydrate with the help of oxygen. To improve the efficiency of such pathway the cardiovascular system needs to be improved. The more oxygen reaches the muscles the more ATP can be synthesized. The aerobic energy pathway is the primary source of energy used in endurance exercises.
All these three metabolic pathway are used at different level during exercising following the different level of intensity. The two anaerobic metabolic pathways are triggered first. Later, as the breathing and heart rate increase and more oxygen is delivered to the muscles the aerobic metabolism kicks in. This supplies the necessary energy until the intensity of the effort is too high for the cardiovascular system to provide the necessary oxygen concentration. This limit is called, lactate threshold. If this threshold is surpassed the anaerobic metabolism starts increasing again resulting in accumulation of lactic acid and ions. The athlete will then feel the need to reduce the intensity level.
What is the LT
So we have seen that there are three energy systems in the body and that exercising requires all of the three systems to work in cooperation. The level of involvement of the different systems depends on the intensity of the exercises. During rest, as during exercises there is production of blood lactate but removal mechanisms are able to transform it back to something the body can use. When the activity intensity overcomes the lactate threshold, the lactate removal mechanisms can not remove enough lactate, and thus the blood lactate level increases rapidly. Having a higher lactate threshold allows an athlete to keep working at an higher intensity for a longer period of time before reaching his limit. Thus, training aimed at increasing the LT appears crucial for the endurance athlete. Swimming, jogging or cycling are all examples of sports that stress your system aerobically, therefor having an higher lactate threshold makes the athlete perform better. What determines this physiological threshold and how to train it are going to be the topic for the next post.
Keep tuned Until next time
Keep healthy, keep swimming