Why do our muscle gets sore?

When we begin to gym and perform a new intense physical workout, our muscle begin to sore in the next morning. This soreness is called Delayed Onset Muscle Soreness (DOMS). DOMS usually occurs after eccentric contractions. When the muscle is contracting or lengthening at the same time called eccentric contractions. For example, if we lower our dumbbell, our bicep muscle is slowly relaxing and lengthening. But, at the same time, it is still contracting by holding the heavy weight of the dumbbell. Similarly, in a squat, as we lower ourselves, our quadriceps muscle is lengthening. But at the same time, it is still contracting to hold our upper body weight. Such eccentric contractions generate tension in the muscles, creating minute tears in them. Thus causing the sourness or pain.