High overload training is a very effective stimulus for muscle growth (hypertrophy) as it causes small injuries to muscle fibers. Scientists call this trauma “micro tears.” These tears are thought to trigger the whole integrated hypertrophy response within muscle cells.
Muscle cell repair begins immediately after the damage is created. Injured tissue releases chemicals called prostaglandins that co-ordinate the inflammation process. In turn, this creates swelling and therefore, the sensation of pain in your muscles. The inflammation process involves a series of immune cells and other protein complexes that perform specific functions at various stages (such as removing damaged tissue or helping the regeneration of new tissue to occur).
Each phase in this procedure is completed over a period of time, and the whole process is thought to peak 36-48 hours after the damage has occurred. Therefore, like most bodybuilders you probably feel that your muscle soreness is at its greatest precisely at this time, which is the second day after a workout.
If I were you, I wouldn’t be concerned; the soreness is just a secondary response to the regeneration process that is happening to your muscles. After a while, most bodybuilders grow to enjoy the pain! In fact, I know a lot of bodybuilders that are not happy unless at least one part of their body is sore!
Source: Medicine & Science in Sports & Exercise
