The muscles involved in the postural system are muscles "tonic" located primarily in deeper location than the "phasic" muscles.
The "tonic" muscles have a structure making them suitable to a long contraction, protracted, while the "phasic" have the task of carrying out the rapid movements, typical in daily activities.
The phasic muscles are what we "see" and that is so important to our aesthetic image but the tonic ones, which you do not "see" are even more important because they keep us standing.