This is not to say muscles do not benefit by tightening. Flexing your muscles is a form of tightening them. Lifting weights is a form of tightening them. Most any exercise requires a tightening of muscles.
But, if the muscles do not return to a more relaxed state, that is not good.I notice how us senior citizens tend to walk with stiffness while youth do not. That speaks for itself, does it not? Of course, the limberness of the body is an indicator of health.