Poor posture is caused by many things. Reasons include hereditary physical disease, inner ear infections, nervous system disorders, eye problems, muscle and skeletal problems and psychological problems such as low self-esteem, causing slouching. If not corrected, all of these causes of bad posture can lead to serious problems with the skeletal system.
Rib Constriction and Tilted Pelvis
Slouching and/or slanted posture causes the ribs to compress and your spine to curve. The cage of the ribs becomes rigged and inflexible and keeps your lungs from expanding fully. This creates a lack of oxygen that impacts your entire system from your skin to your heart and other organs. Poor posture aggravated by lack of exercise--and, in some cases, obesity--cause weakened stomach muscles that normally hold the pelvis in the proper position. Weak muscles mean the pelvis is tilted forward, which further exaggerates poor posture and actually can make it painful to stand erect.
Knee Alignment Problems and Knee Pain
Often called knock-knees, or the opposite, bow-leggedness, these conditions are created by poor posture that leans toward one side, and places more weight on the instep or out-step of the foot. Overtime the knees weaken and twist. Slouching pushes the shoulders back, and focuses all of the supporting weight that the hips normally bear on the front of the knees. The strain creates painful knee joints.
Foot Pain, Neck Pain and Hunchback
Poor posture puts the entire skeletal system out of position and creates stress all the way down to the base of the structure---the foot. At the top of the skeletal ladder the first place to often experience problems from bad posture is the neck. Pain in the neck also causes tension that radiates down to the shoulders, and also up to the head to cause headaches. Slouching causes a rotation of the shoulders. Prolonged time in this position causes the muscles and tendons to adapt and shorten, making it a "natural" position that moving out of to stand straight actually causes pain. However, being in the hunched position makes it harder to breathe, move or accomplish simple actions.
Long-term poor posture degenerates past pain in the joints to the formation of osteoarthritis. The cartridge wears out between the joints from the constant friction and abnormal pressure, causing the bones to rub together when moving and creating pain.