Question

why do our muscles cramp occur whenever we perform some vigorous work

Answer 1

Because of release of lactic acid in our muscles due to lack of oxygen gives rise to cramps.Delighted to answer you 

Answer 2

When we perform a vigorous work our muscles use up the energy present in them.
To produce ATP(ENERGY) oxygen is required which is all used up in the activity. Thus the muscles use anaerobic respiration. This produces alcoholic substances which cause cramps.
Hope this helps you...