Why does the body stiffen up when one sits for a while?
In response to the above enquiry: “Why does your body stiffen up when you set [sic] for a while?”
It has a lot to do with your chair. Throw the chair away immediately.
In order for the body to maintain a bad posture, a crooked posture, a slouchy posture, which the chair forces the body into, the muscles have to tighten up to sustain that posture.