This video will examine the courses for muscle fatigue during a single part of exercise. As you'll see there is no one single course for muscle fatigue. Further the cause of fatigue will be event specific. In other words what contributes to fatigue when you're doing bench presses will be very different from the courses of fatigue near the end of a marathon. First, let's begin by defining by what is meant by muscle fatigue. Muscle fatigue is defined as the inability to maintain a given exercise intensity or power output resulting in a decrease in work or performance capacity. Shown here is the textbook example of the onset of muscle fatigue. They are in the early stages of exercise. Power output or force production is well maintained. However over time, the ability to maintain this level of force production declines, thereby fulfilling our definition of fatigue. For years physiologists have been studying the mechanisms for muscle fatigue. The issue is extremely complex and, as already mentioned, the causes will vary based upon the type of exercise involved. Any step between when the decision is made in the brain for muscle movement and the eventual contraction of the skeletal muscle are potential sites for a fatigue. However generally the causes of fatigue are located in the peripheral sites beginning with the neural muscular junction and beyond. As such I will only focus on those sites. As a reminder, the type of muscle fiber recruited will play a significant role in the cause of fatigue. Type 2 X fibers, which are recruited during short-term, high intensity exercise, are quick to fatigue. While Type 1 fibers, used during more long-term, lower intensity exercise, are slow to fatigue as long as there's ample fuel for ATP production. First, let's examine the most common causes for fatigue when engaging in short term, high intensity exercise. For example, when bench pressing a heavy weight, you may be able to manage completing five repetitions, but just cannot squeeze out the sixth. Or while sprinting 200 meters, you cannot maintain your speed during the final 20 meters. Frequently the cause of fatigue is the depletion or accumulation of sub substance or metabolite. While many factors can potentially contribute to the onset of fatigue, I will discuss the three most common causes for muscle fatigue during a short term high intensity exercise. These include the depletion of both ATP and Creatine Phosphate stores in muscle. As well as the accumulation of hydrogen ions or an increase in muscle acidity. First, the depletion of ATP resulting from a mismatch between ATP utilization and production will result in fatigue. As a reminder, ATP is required for cross bridge cycling and tension development. If ATP is being consumed at a faster rate than it can be made, then levels will drop below resting values. At this point the individual must reduce their exercise intensity, thereby lowering the rate of ATP utilization, allowing ATP production to keep pace, or the individual must stop exercising all together. Second, during short-term high intensity exercise, creatine phosphate stores are rapidly depleted. As creatine phosphate is the main immediate energy source in muscle for ATP production, its depletion will compromise one's ability to maintain ATP levels. Shown here are the ATP and creatine phosphate levels in muscle in response to a bout of maximal exercise. Notice that the creatine phosphate levels, which are abbreviated PC for phosphocreatine deplete very rapidly at this intensity. When levels are extremely low, this negatively impacts our ability to maintain ATP production and thus ATP levels in muscles decline dramatically leading to exhaustion or fatigue. The third common cause for muscle fatigue during short term, high intensity exercise is an increase in muscle acidity or an accumulation of hydrogen ions. This metabolic acidosis can contribute to fatigue in a number of ways. The high concentration of hydrogen ions can interfere with calcium's role in cross bridge formation and thus tension development in muscle. This will result in a reduction in force output by the muscle. Also, hydrogen iron concentration will inhibit anaerobic glycolysis. As you remember, anaerobic glycolysis is a major source for ATP production during short-term high intensity exercise. Thus a decline in creatine phosphate stores, in conjunction with a reduction in ATP production from anaerobic glycolysis will lead to a rapid onset of muscle fatigue. As will be discussed in the video on performance enhancing drugs, this is the concept whereby buffer loading can potentially lead to an improvement in performance during high intensity explosive exercise. Having a greater buffering capacity prior to engaging in these intense activities will allow for a neutralization or buffering of the negative effects associated with metabolic acidosis. Shown here is an example indicating that when trained women were given sodium bi-carbonate prior to one bout of maximal exercise, they were capable of performing at a higher peak power and intensity for 60 seconds. When compared to when they performed the exact same test with a placebo or no supplement at all. This is just one of many studies suggesting that metabolic acidosis is a factor contributing to fatigue during short term high intensity exercise. Now let's examine the three most common causes for fatigue during long term lower intensity exercise. As I have discussed previously, the depletion of carbohydrate stores is a common cause for fatigue during this type of exercise. Additionally, over time, a decline in intramuscular calcium levels can also contribute to fatigue. Finally, an accumulation of heat or increasing body and muscle temperature is a potential factor leading to the development of fatigue. Participation in distance events such as a marathon are performed at submaximal or moderate exercise intensities that can be tolerated or maintained over a prolonged period of time. The depletion of muscle glycogen frequently coincides with the unset of fatigue as a major source for ATP production no longer exists. This is why the technique of carbohydrate loading known to increase muscle glycogen stores prior to competition can improve ones performance. When muscle glycogen is depleted, the individual is left with two choices. Reduce the exercise intensity and thus the rate of ATP utilization or stop exercising completely. Both scenarios adhere to our definition of muscle fatigue. Additionally, the depletion of liver glycogen will eventually result in significant reduction in blood glucose levels or a hypoglycemia. As the muscles and brain rely on blood glucose for fuel, the ability to continue to exercise is not sustainable once a state of hypoglycemia has been reached. Again this is why the practice of consuming dilute carbohydrate drinks during the course of distance events can lead to an improvement in performance. During hours of prolonged exercise, the cytoplasmic reticulum in muscle cells are repeatedly stimulated to release calcium for cross-bridge formation and tension development. Over time, some of the calcium release can leak out into the extracellular fluid and/or be taken up by mitochondria. This can result in less calcium being available for muscle contraction, thereby impairing force or power output. Finally, an increase in body and muscle temperature can lead to fatigue. During prolonged exercise, these temperatures can approach 42 degrees Celsius, or 108 degrees Fahrenheit. This will result in a greater percentage of your blood being diverted to the skin for thermoregulation. Meaning less blood is going to the active muscles. Further, failures to keep properly hydrated will cause a reduction in plasma volume, reducing your cardiac output and blood delivery to the muscles. In summary, muscle fatigue is defined as the inability to maintain a given exercise intensity or power output, resulting in a decrease in work or performance capacity. The most common sight of fatigue during excercise is located within the muscle itself. The causes of fatigue are task specific, and may result from the depletion or accumulation of key variables. Finally, causes of fatigue during intermediate events such as a 10k run, likely involve factors mentioned for both short term high intensity exercise as well as long term lower intensity exercise.
