The theory of muscular contraction starts with two observations:
- We can feel the exertion of muscular contraction when we contract our contraction muscles (contractors), and we can feel relaxed when we contract our extension muscles (extensors) - see picture below.
- When we feel tensed psychologically or mentally, our contractors contract, and when we feel relaxed, our extensors contract.
"Use Jing (or Yi) instead of Li" (用勁/意不用力) is an important tai chi essential. If we can feel we are exerting our muscles, we are overly using our contractors. When we are balanced in our muscular contraction, we do not feel the exertion (or Li). Our internal muscular feeling will be "muscles-as-one" or "body-filled-with-molten-lead".
In our working lives, most of the time we overly use our contractors. The result is that our joints (mainly our shoulder and hip joints) will likely be moved into slight misalignment when we approach middle-age, if not earlier, resulting in various kinds of pain in our body. And in our hectic lives full of psychological stress (which is normal rather than unusual), our emotional stress will be trapped inside our muscles and joints.
The practice of tai chi aims at releasing such misalignment and trapped energy, and to bring about good physical and mental health. In order that such benefits can be achieved (rather than just bragged about), a student of tai chi has to understand the theory of muscular contraction in tai chi and put its implications into practice. Needless to say this is only the beginning of everything.