so what if there's some tension in the soft tissue during effort? what matters is the balance between tension and relaxation. too much tension and you become stiff and unable to change. too loose and you have no power. that's tai chi. the balance between yin and yang, not only yang.
If it were that simple, I don't think the whole yin-yang thing would be such a big deal. It would also be wildly uninformative, because it comes down to 'Never too much, never too little.' It's like saying 'Always practice correctly.'
Secondly, doesn't the balance between yin and yang imply the change from yin to yang and vice-versa? Your interpretation doesn't seem to allow for that.