Proper posture simply refers to maintaining the body in a position which protects against excessive stresses which cause injury, while requiring a minimal amount of muscular effort to maintain. In other words, a comfortable position which will not irritate your spinal tissues.
Posture plays a significant role in the development of chronic conditions such as chronic back and neck pain. Poor posture is responsible for overstretching ligaments and other supportive spinal structures as well as exhausting spinal musculature, all of which quickly leads to the development of chronic spinal pain.
Learning and practicing proper postural habits will help reduce the likelihood of acquiring back and neck injuries, reduce spinal degeneration, and help keep your spine and body happy and healthy.