Explain why some people’s leg muscles tired quickly and developed cramp when they were running

Muscle cramps are often the result of dehydration.

Explanation:

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The 'Dehydration/Electrolyte Theory'

It speculates that a significant disturbance in fluid or electrolyte balance, usually due to a reduction in total body exchangeable sodium stores, causes a contraction of the interstitial fluid compartment around muscles and a misfiring of nerve impulses, leading to cramps. And

The burning sensation we sometimes feel during exercise is caused by a build-up of lactic acid. The burning sensation we sometimes feel during exercise is caused by a build-up of lactic acid.

what statement best describes the difference between the sympathetic and parasympathetic nervous systems?

the sympathetic nervous system prepares the body for intense physical activity and is often referred to as the fight-or-flight response. the parasympathetic nervous system has almost the exact opposite effect and relaxes the body and inhibits or slows many high energy functions.