It was in the morning the day before yesterday, when I got up on my bed I felt a stimulating pain in my back. The strained back has came back again I recognized in the moment. I visited my acupressurist after a few hours later. This is a story I heard from him. The reason for the strained back is too long and strong tension of your mascle. It's just like you are in a shirt which is two size smaller than your real size. It gets torn with any kind of body action such as coughing, putting on pants, lifting up something heavy and so on. In this case, the body action itself does not matter at all. The problem exists in the fact that my back muscle had been over-tensioned for a long time. My torn muscle will recover within a few weeks automatically, but it will tear again if I wouldn't change my shirt into my size. He also told me that stretching is the best way to change my shirt size. All I can do now is cooling down the broken muscle so that it recovers earlier and after the recovery I have to remember to stretch my back muscle. This story might not be scientific, but it is persuasive at least for me.