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Posture can be best described as the position your body hold when in a sitting or standing position. Muscles are responsible for maintaining posture and stability.  Muscle keep up standing up straight, and prevent any slumping or falling over, Hence sometimes being called “anti gravity muscles” . A variety of muscle groups such as the hamstring, large muscles in one’s back are vital in one’s posture.  The “neutral spine” is he most common term when speaking of posture. This standing with a straight neck, level shoulder and feet directly below the hips. 
Inactive posture is when someone is sleeping or resting, this usually requires little to no muscle activity.When just standing straight up the intrinsic muscles in the feet are not really active because other ligaments are there for support. The soleus,which is a a strong muscle that goes from the back of the knee down to the ankle keeps the body sturdy as gravity pulls the body over the feet. The iliopsoas is also always engaged as  this is a joint that connects the lower part of the body and the trunk of the body. The gluteus medius and erector spinae is also active in counteracting gravity. 
In contrast, active posture requires the action of a variety of muscles in order to work.  Active posture can be broken down into two categories:
Static Posture: Is any form of standing, sitting, lying, or kneeling. In this the the body is aligned and in a standard position and muscle groups are working to counteract the forces of gravity.
Dynamic Postures: Is any form of movement such as walking, running, jumping, throwing, and lifting. 
 Stability of the body starts with the spine. The spine is fully equipped with support structures and muscles that include joints to relieve the tension and stiffness o the spine. The muscles are also equipped with a nervous systems that signals the necessary information needed to provide stability and support. So when we practice good posture we are giving our joints the chance to be aligned which gives us good structure an stability. 
References
McGill, S.M. (2002). Low back disorders: Evidence based prevention and rehabilitation, Second Edition, Human Kinetics Publishers, Champaign, IL.