The soreness in the back is usually caused by the following reasons.1. It is possible that the muscles in the back are strained, or some chronic sterile inflammation. For example, long-term use of the back, shoulders to lift things, do heavy physical labor, or back to do some fitness exercises and sports. In addition to soreness, there is usually pain. If it is myofasciitis, there is usually a very obvious pressure pain in the inflamed area.2. It is possible that after doing strenuous exercise the day before, it leads to lactic acid in the local muscles, which accumulates on top of the muscles in the back. This stimulates the muscles, resulting in localized soreness, pain, weakness and other sensations. But this situation generally rest for two or three days, the symptoms will be able to slowly improve.