Posture for a human is unique in comparison to any other animal. We are a species of mammal who walk only on their feet without the assistance of our upper limbs. Because of this our spine is oriented vertically. Also like other mammals our musculoskeletal system is structured symmetrically from a front-to-back point of view so when we stand on level ground our head should be level, our shoulders should be level, and the tops of our pelvis should be level. Our head should also be oriented over the center of our body. If we deviate away from this symmetrical posture, then we start to distribute our weight inefficiently throughout our body. This will build up stress in joints and muscles asymmetrically which can cause inflammation and potentially symptoms such as pain can become present.
From a side point of view our ears should line up with the midpoint of our shoulders, our shoulders should stack up over our hips, our hips should be over our knees and our knees over our ankles. This upright posture is the most efficient when standing still. If you are noticing that your posture is not quite lined up and oriented as described above then you are probably a good candidate for chiropractic care.