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The combination of an inappropriate rapid heart rate response to exercise with heavy or rapid breathing is known as an exaggerated cardiorespiratory response to exercise. [3] Due to the confusion between muscle fatigue and muscle weakness, there have been instances of abnormal muscle fatigue being described as exercise-induced muscle weakness.
Glutamate, one of the most common neurochemicals in the brain, is an excitatory neurotransmitter involved in many aspects of brain function, including learning and memory. [86] Based upon animal models, exercise appears to normalize the excessive levels of glutamate neurotransmission into the nucleus accumbens that occurs in drug addiction. [21]
During intense exercise, lactate has been estimated to provide a third of the brain's energy needs. [39] [42] There is evidence that the brain might, however, in spite of these alternative sources of energy, still suffer an energy crisis since IL-6 (a sign of metabolic stress) is released during exercise from the brain. [26] [34]
In the brain, serotonin is a neurotransmitter and regulates arousal, behavior, sleep, and mood, among other things. [9] During prolonged exercise where central nervous system fatigue is present, serotonin levels in the brain are higher than normal physiological conditions; these higher levels can increase perceptions of effort and peripheral muscle fatigue. [9]
During intense exercise that approaches one's VO 2 max, most of the energy comes from glycogen. A typical untrained individual on an average diet is able to store about 380 grams of glycogen, or 1500 kcal , in the body, though much of that amount is spread throughout the muscular system and may not be available for any specific type of exercise ...
Central fatigue is a reduction in the neural drive or nerve-based motor command to working muscles that results in a decline in the force output. [3] [4] [5] It has been suggested that the reduced neural drive during exercise may be a protective mechanism to prevent organ failure if the work was continued at the same intensity.
Overtraining occurs when a person exceeds their body's ability to recover from strenuous exercise. [1] Overtraining can be described as a point where a person may have a decrease in performance and plateauing as a result of failure to consistently perform at a certain level or training load; a load which exceeds their recovery capacity. [2]
While fatigue is often prominent, it is "more than fatigue following a stressor". [6] Other symptoms that may occur during PEM include cognitive impairment, flu-like symptoms, pain, weakness, and trouble sleeping. [6] [4] Though typically cast as a worsening of existing symptoms, patients may experience some symptoms exclusively during PEM. [6]