Muscle fatigue

Muscle fatigue is defined as the inability of a muscle to maintain a particular degree of contraction over time. The onset of fatigue is quite variable and influenced by several factors, including  

Although the exact mechanisms leading to muscle fatigue remain somewhat unclear, several factors have been implicated  

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Green, H. J., 1997. Mechanisms of muscle fatigue in intense exercise. Journal of Sports Sciences 15 247—256. 

F.3 What is the mass percentage composition of I-carnitine, C7HhN03, a compound that is taken as a dietary supplement to reduce muscle fatigue ... 

A decreased glycolytic rate has been proposed as a cause of muscle fatigue and related to pH inhibition of glycolytic enzymes. Decreasing pH inhibits both phosphorylase kinase and phosphofructokinase (PFK) activities. PFK is rate determining for glycolytic flux and therefore must be precisely matched to the rate of ATP expenditure. The essential characteristic of PFK control is allosteric inhibition by ATP. This inhibition is increased by H and PCr (Storey and Hochachka, 1974 ... 

Figure 6. Glycogen content in the vastus lateralis muscle as a function of cycling time at 75-80% of maximal oxygen uptake (VO2 max). Data points are mean values from 10 subjects. For each subject, exercise was performed repeatedly in periods of 15 min separated by 15 min rest periods. At the point of exhaustion and muscle fatigue, muscle glycogen stores were depleted. From Bergstrom and Hultman (1967) with permission from the publisher.

Figure 10. The relationship between the initial glycogen content in vastus lateralis muscle and work time in six subjects who cycled to exhaustion at 75% VO2 max. Each subject cycled to exhaustion on three occasions. The first experiment was preceded by a mixed diet (a), the second by a carbohydrate-poor diet (o), and the third by a carbohydrate-rich diet ( ). The energy contents of the diets were identical. In all experiments depletion of the muscle glycogen store coincided with exhaustion and muscle fatigue. From Bergstrom et al. (1967) with permission from the publisher.

Muscle fatigue during this intensity range may be due to other factors besides lack of CHO substrate. The potential mechanisms include dehydration, electrolyte imbalances, and the onset of muscle injury and soreness. As these potential fatigue factors are common to the range of intensities in this section and exercise below 60% VO2 max, they will be discussed in the following section. 

Bigland-Ritchie, B. (1981). EMG and fatigue of human voluntary and stimulated contractions. In Human Muscle Fatigue Physiological Mechanisms (Porter, R. Whelan, J., eds.), pp. 130-156. Pitman, London. 

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