The strength and length of muscles involved in joint motion must be balanced. The balance is based on force couple (two or more translatory forces that in combination produce rotation) principle among muscles involved in the three cardinal planes of motion. When a force couple is out of balance, the segment moves off its axis of rotation and there is faulty joint motion. The head, trunk, shoulders and pelvic girdle serve as the foundations, from which forces are directed to the limbs.
Postural faults can be used as guidelines for identifying alterations in muscle and ligament length. This may occur when one muscle groups becomes tight and the antagonist elongated. Synergistic muscles around a joint may be unbalanced as well as the agonists.
Minor alignment faults in posture limit motion and lead to tightness of muscles and other soft tissues. Muscles that are elongated often develop their maximal force in the stretched position and are weak in the normal physiological position.
Alignment of body segments should be observed while the person is standing still and during such movements as walking, to detect faulty patterns of muscle activity and joint mobility. The better the quality of movement and the better the alignment of gravitational forces through joints axes, the better is the sequence of motion. When postural alignment improves, imbalances are minimized.