Answer:

Most weight trainers do not know the reason their muscles get sore after a workout. Muscle soreness is not caused by lactic acid, as many people believe.

• It is caused by microtrauma to the muscle fibers.
  
• This causes calcium molecule leakage from the muscle fibers (as well as an accumulation of histamines, potassium, prostaglandins and local edema).

• The sensation of pain and soreness comes due to the pressure of localized edema (fluid retention) on nerve endings, not by the muscle damage itself.
• Negative-accentuated training often results in severe muscle soreness as it causes the most muscle damage.

• Muscle soreness can be an indication of growth.
  
• Light training of the same exercise can help decrease it.

Should you train a muscle when it's still sore?