Sore muscles after physical activity are common. This is known as Delayed Onset Muscle Soreness (DOMS). It can be due to a change in your exercise routine. You start a new exercise programme, or do it for a longer time or make it more intense that puts extra strain on your muscles, more than the usual. When we make our muscles work extra hard, it may cause microscopic damage to the muscle fibres. It can result in muscle soreness or stiffness. However, the soreness is just a part of adjustment that leads to higher and better stamina and muscular strength.