Muscle soreness after exercise is mainly due to lactic acid accumulation caused by sudden heavy exercise overload, which is a normal physiological reaction and usually relieves itself within a few days.
Causes of muscle soreness after exercise:
Muscle aches that occur from a few hours to 24 hours after exercise generally last about 1 ~ 3 days. The reason is nothing more than muscle injury, muscle spasm or abnormal connective tissue. It is generally believed that abnormal connective tissue is the biggest cause of muscle soreness.
Refer to Baidu Encyclopedia entry? Muscle pain