DOMS - what is it?
DOMS or delayed onset muscle soreness is something that everyone has experienced regardless of training history. You don't even have to have trained to experience it. You have just decorated the bedroom and you have sore shoulders, that soreness is DOMS. Often thought of as a sign of a good training session, nothing could be further from the truth.
In tech speak the pain is caused by an increase in the acute loading that is sufficiently above your chronic loading level that you aren't ready for. You react to this in an adverse manner that is painful. The unwanted pain of DOMS is a secondary reason why you should build up the volume and intensity over a period of weeks. The primary reason for the gradual build-up is to reduce the risk of injury. DOMS is painful but it isn't an injury. A good rule of thumb is to use increases of no greater than 10% per week.