Substantial physiologic adjustments occur during exercise in the nervous, cardiovascular, and endocrine systems in order to provide maximal oxygen and fuel to the exercising muscles and to remove metabolic toxins from them. These adjustments consist of two phases: the preparatory phase and the active phase.
In competitive sports, thoughts about the future are often a source of psychologic stress. Such thinking can cause dramatic changes in physiologic functions before the start of any exercise that involves strenuous muscular activity and significant mental activity. The commands from the cortex activate hypothalamic and brainstem functions that regulate autonomic and endocrine outflow, as discussed previously.11 Physiologists noticed that when trained runners were preparing to start a race, their resting heart rates doubled as the starter called out the starting commands.12 The rise in heart rate was highest in sprinters starting a 60-yard dash (from 67 to 148 beats per minute).13 The rise was less dramatic in those starting a 220-yard race (from 67 to 130) and even less in middle-distance runners starting an 880-yard run (from 62 to 122).
This preparatory phase obviously involves a top-down process. The anticipation of a planned, intentional activity causes the prefrontal cortex to activate the hypothalamus and the brainstem nuclei that are associated with sympathetic activation and suppression of parasympathetic activity; this results in increases in heart rate, cardiac output, sympathetic outflow to the blood vessels, and epineph-rine secretion. The prefrontal cortical activity also regulates the motor cortex, the premotor cortex, and the supplementary motor cortex. All these centrally generated top-down outflows cause dilation of some muscle vascular beds, increase in muscle tone, and a rise in systolic pressure.
When the actual exercise starts, peripheral physiologic processes begin to exert more influence on the stress response. Feedback from the muscular activity to the brain centers, as well as local metabolic increase, further enhance the sympathetic outflow to the heart and blood vessels, which results in increased cardiac output and a redistribution of blood flow. This in turn provides the exercising muscles with significant increases of oxygen and nutrients. Meanwhile blood flow to uninvolved muscles, such as internal organs, decreases. A trained runner, with all these adjustments taking place, may maintain a level of effort equivalent to 85% of maximum for periods of 3 to 5 hours. Highly trained athletes may maintain sustained heart rates of 180 beats per minute, in comparison with their resting heart rates of 35 to 40.
The increase in sympathetic outflow causes more epinephrine secretion by the adrenal medulla. Epinephrine increases cardiac contractility and dilation of blood vessels in exercising muscles. In addition, epinephrine increases the liberation of free fatty acids from stored fat; the fatty acids circulate to the exercising muscles as fuel for increasing energy demand. The consumption of glucose by the exercising muscles results in a drop in blood glucose levels, which activates the hypothalamus and pituitary gland to increase the release of ACTH; this in turn leads to more secretion of cortisol by the cortex of the adrenal gland, which liberates stored glucose in the liver and fat from adipose tissue. P-Endorphin is secreted by the pituitary gland along with ACTH. P-Endorphin is known as an opiate analogue and associated with analgesic physiologic processes. This analogue is thought to modulate discomfort during physical exercise.
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