Postural muscles are also referred to as core stability muscles, are found deep in the body.
The first muscles that typically come to mind with posture are the muscles along the spine. The erector spinae muscles run along the bones of your spine, the vertebrae, from your neck all the way to your tailbone. These muscles hold the spine in extension or upright, counteracting the pull of gravity. Abdominal muscles support the structures at the front of your trunk, balancing the pull of your back extensors. The quadratus luborum muscles at the top of your pelvis help stabilize the pelvis and spine in the lower back in an upright position.
The purpose of postural muscles is to keep good posture in the field of gravity. These muscles consist mostly of slow-twitch muscle fibres and have a greater capacity for prolonged use but are prone to over working. This means if you have poor posture or you have long periods of sitting immobile your postural muscles get tired and they become weak and less efficient. The longer the situation continues the more likely you are to develop postural imbalances and they finally you get knots and pain!