Kyphosis is an exaggerated curvature of the thoracic spine, where the head and shoulders move forward from neutral posture, The cause can be genetic, occupational for example sitting slouching forward at a computer and over training the muscle of the chest.
The cervical spine can also often develop an excessive curve to compensate and try to bring the head back to a position so the eyes are facing straight forward and not toward the floor.
This type of posture will make the thoracic (chest) cavity smaller, making breathing less efficient inhibiting how aerobically fit a person can become, which can have an impact on weight loss.
It also effects the position of the shoulder blades by pulling them apart and weakens the shoulder joint making the joint more unstable. This type of posture is the result of excessive shortening and lengthening of muscles in the front and back of the upper body, mainly shortening of the chest muscles (pectorals) and lengthening of the mid back muscles (trapezius and rhomboids).
Our back is strongest is the position of neutral spine and as our muscles pull our skeleton out of that position we increase our chances of back pain. This then redefines our back pain as feedback to let us know we have an imbalance to be corrected.