Question

why do our leg muscles cramp when we run for a long period

Answer 1

While running our breathing rate become faster and require more energy to work as a result lactic acid is produced in leg muscle which causes sudden cramp in ur leg it is produced in the absence of oxygen

Answer 2

we will use anaerobic respiration while running, anaerobic respiration will occur in absent of oxygen ,it's byproducts are lactic acid &alcohol we feel pain because of the lactic acid