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

Muscle cramps are often the result of dehydration.

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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.