There are around 650 skeletal muscles in the human body. Although the primary purpose of these muscles is to provide movement, we also often hold tension and emotions in the muscles of our body. Recent research backs up what we may have already noticed: that we tend to feel anger in the face and jaw, anxiety in the chest and belly and stress around the neck and shoulders. When we hold tension in our muscles the fibres tend to shorten, which causes tightness. Of course, we can also build up tightness and tension by doing things like sitting at a computer all day without proper breaks.
Interestingly, we have only sat on chairs for a recent part of human history. Apparently, our hunter-gatherer ancestors tended to squat. As early agriculture developed, chairs were mainly for people of high status, like a King’s throne. With the industrial revolution, chairs became more common. Our great-grandparents worked physically harder than we do today, so were less likely to sit during the day. Today we lead much more sedentary lifestyles, often sitting for many hours, exercising little more than our eyeballs and fingers.