(DOMS) = Delayed Onset Muscle Soreness.
Doms are a result of an unfamiliar exercise workout, or an increase in type, intensity, and/or duration in training. In simpler terms, soreness occurs when exercises are different to what you are used to and your muscles have to work harder. It is a myth that DOMS are caused by lactic acid build up. They appear to be a result of inflammation caused by microscopic tears in the connective tissue, especially significant with a greater emphasis on the eccentric phase (the lowering down on a bicep curl). They often occur a day or two after training, soreness can last 2-5 days and anybody can develop DOMS – even elite athletes!
Muscles respond to stimulus and the mechanisms to adaptions are clear. There are 3 primary mechanisms for hypertrophy: mechanical stress, metabolic stress and muscle damage. All of which happen in the absence of DOMS.
Aiming for soreness is not the purpose of training; muscle growth is. This is relevant for whether you are looking to build muscle, tone up or lose fat. Muscle gain will speed up the process of fat loss. Soreness indicates how well you recover from exercise. It shows how your body functions and mechanisms work as we are all different.