Question: (Continued from last week.) What causes fatigue?
Answer: Last week, I mentioned several factors associated with fatigue in activities of different duration. These factors were as-so-ci-ated with metabolic depletion (ATP, glycogen) and metabolic accumulation (lactic acid, inorganic phosphate). There are many other possible causes of fatigue, including fatigue at the nerve junctions (or somewhere else in the nervous system), and psychological fatigue, a feeling of being tired when there is really no physiological reason to feel tired.
Today, I will discuss exercise performances of different durations in terms of the factors involved in fatigue and the training methods that are used to decrease the effects of fatigue.
- Ultra short-term performances (around 10 seconds): This category involves such activities as the shot put, high jump, long jump and the 50- to 100-meter dashes. All of these events require a high amount of energy in a very short period of time. People who do well in these events usually have a high quantity of Type II fast-twitch muscle fiber (the same type used by Michael Jordan). Fatigue usually occurs in less than 10 seconds and is related to a depletion of creatine phosphate and an increase in the by-product of ATP use, inorganic phosphate.
People who do short-term, high-energy things also train using a series of short-term, high-energy intervals with the hope of increasing the amount of creatine phosphate stores as well as strength-ening the muscles used in the activity and improving any skills needed to perform the activity well.
- Short-term performance (10 seconds to 2 minutes): Doing all-out exercise for this period of time requires energy from anaerobic glycolysis, a metabolic pathway that produces lactic acid as a byproduct. The elevated hydrogen ions that accumulate with these activities can affect the contractile process in the muscles as well as the body's ability to produce ATP. In other words, we become fatigued because the muscles don't have enough energy to keep up the work at that level, and their contractile ability is also affected.
People train for this type of activity by running a series of longer intervals to help the body become better at producing energy at high rates and by changing its ability to buffer the acids that accumulate. Many team sports, such as football, basketball and volleyball, use this type of energy for the performance portion of the sport, then need longer-lasting energy to support the one- to two-hour total time of the activity.
- Moderate-term performance (2 minutes to 15 minutes): As the time of the performance increases, the amount of energy from aerobic sources becomes more important. This type performance includes 800-meter runners and those who run 5-kilometer races. On the high side, the ability of the heart to pump blood and the muscles to extract oxygen becomes important, so training involves intervals of longer duration, to increase the lactate threshold and to train the aerobic system.
- Intermediate-length performance (15 minutes to an hour): Most of the energy for this type activity comes from the aerobic system and requires longer workouts to develop. These athletes tend to have more aerobic-type muscle (with more mitochondria and aerobic enzymes), larger hearts (to pump blood more efficiently) and a lanky build. Training involves longer intervals or overdistance training with a more moderate intensity to make these changes. Fatigue occurs because the athlete runs faster than ATP can be produced, or because of dehydration as a result of the heat produced while exercising.