Hi,
Delayed Onset Muscle Soreness (DOMS) is the pain or discomfort often felt 24 to 76 hours after exercising and subsides generally within 2 to 3 days. Once thought to be caused by lactic acid buildup, a more recent theory is that it is caused by tiny tears in the muscle fibres caused by eccentric movements, or unaccustomed training levels. The dismissal of the lactic acid theory was that rapid dispersal of lactic acid which could not explain pain felt the next day.

More recent research (Ji-Guo Yu) claims that DOMS is not caused by the pain from damaged muscle cells, but from the reinforcement process. The muscle responds to training by reinforcing itself up to and above the previous strength by adding new sarcomeres - the segments in the muscle fibrils. This reinforcement process causes the cells to swell, and put pressure on to nerves and arteries, causing the DOMS.

Training does not make an existing DOMS worse, because the reinforcement process is already underway. As training will induce further damage to the muscles and thus more hypertrophy and reinforcement, it can add more swelling and pain, and lengthen the period of DOMS. Training in constant DOMS would be uncomfortable though one may be able to adapt to it. Perpetual DOMS may not be an ideal condition but could be seen as the ideal method of assuring velocity in muscle growth and fitness.

Suggestions for reduction of DOMS:

Stretch slowly. Studies linking DOMS to stretching have been conducted and have mixed results-some say stretching causes DOMS and others say it doesn't. It is normally a good idea to warm up before exercising to warm the muscles, preparing them for and making them more efficient at doing the coming exercises. Stretching after exercising, especially if someone is new to stretching, should progress slowly. Stretching should only be done to the point of slight discomfort and held for 10-30 seconds (10 or less for beginners).

Recovery from DOMS
To lessen pain after exercise thorough warming up is required. Warmer muscles are more flexible and less prone to damage.

Cheers.