The sacroiliac joint or SI joint (SIJ) is the joint in the bony pelvis between the sacrum and the ilium. It is joined by strong ligaments. In humans, the sacrum supports the spine; the forces of the upper body are transmitted through these joints to the pelvis, down to the legs and onto the ground. It acts as shock-absorbing structure and allows only small amount of movement. It is well innervated. The information it provides is very important for the brain to determine the weight distribution at upright postures and the timing of the gait cycle.

Sacroiliac joint dysfunction is improper movement of the joints. It affects the balance and movement of the whole body.

Sacroiliac Joint: Blausen.com, Wikimedia Commons

Facet Joints: Blausen.com, Wikimedia Commons

Nearly every flexible level of the spine (with the exception of upper cervical) is comprised of a pair of small facet joints, between and behind adjacent vertebrae. These joints are in constant motion, even the movement of breathing requires the participation of these joints. The sliding surfaces of the facet joints are normally coated with a very low friction, moist cartilage. They are enclosed individually in small capsules which secrete thin film of fluid for lubrication. These capsules are richly supplied with nerve fibres to provide a warning when irritated. Vertebral joint dysfunction is abnormal movement of the joint. It causes pain and affects nerve functioning.

The vestibular apparatus is the part of the inner ear that detects the position and motion of the head in space. Together with the neural circuitry in the brain and output, they are called the vestibular system. When the vestibular organs on both sides of the head are functioning properly, they send symmetrical impulses to the brain.

If the system is damaged by disease, aging, or injury, vestibular disorder occurs with some of the following symptoms:

• Vertigo and dizziness
• Imbalance and spatial disorientation
• Vision disturbance
• Hearing changes
• Cognitive and/or psychological changes

The Internal Ear: Blausen.com staff. "Blausen gallery 2014". Wikiversity Journal of Medicine.

Lateral eye and orbit anatomy with nerves: Patrick J. Lynch, Wikimedia Commons

Strabismus: Montrealais, Wikimedia Commons

Vision plays a significant role in balance.

With binocular vision, the 6 extra-ocular muscles on each side direct the eye in capturing image on the retina. The two images are fused in the brain to form a single percept that provides the depth dimension for maintaining body balance. When fusion is ineffective, the signals from one eye will be suppressed to avoid error, vision will become monocular.

Principle of binocular vision: Vlcekmi3, Wikimedia Commons

The lower jaw is suspended evenly underneath the skull by ligaments and muscles on both sides, like a swing hanging on a horizontal bar. When the skull is level, both sides of the jaw are subjected to the same amount of traction.

Left temporal muscle: via Wikimedia Commons

Left masseter muscle in red: via Wikimedia Commons

Left internal and external pteryoid muscles: via Wikimedia Commons

When upper cervical misalignment occurs, the head tilts sideways and offsets the axis of swinging of the jaw. So even just hanging there without any motion, the two joints (TMJ) are subjected to unequal stretch due to gravity. Muscles contract corresponding to the amount of stretch they receive.

So tilting of the axis through these two joints causes muscles that run in various direction to tense up creating force imbalance within the joints. In turn, it causes overwork of some muscles, and puts stress on some ligaments even at rest, and more so upon eating, swallowing and talking. The imbalance also leads to jerky movement of the joints.

In addition, malocclusion, missing teeth, prolonged opening of the mouth, singing, gum chewing, mouth breathing, etc also put strain on the TMJ.

As there is a strong correlation between position of the cervical spine and the jaw, and cervical nerves and the trigeminal nerves which supply the TMJ, aberrant sensory signals produced by the TMJ and muscles inevitable affects the equilibrium of the body long before pain arises.

Tilting of head/neck/TMJ: Maya2008, Shutterstock.com

Forward head carriage, poking chin: Maya2008, Shutterstock.com

Tactile Receptors in the Skin: Blausen.com, Wikimedia Commons

Supination and pronation of the foot: Braus, Hermann, Wikimedia Commons

The skin is the largest organ of the body, with coverage of 20 square feet. It serves as direct contact with the environment.

The feet adapt to the asymmetrical muscle tone by assuming abnormal positions like high arch, low arch, toe-in, toe-out, etc. The area and location of contact surface of the feet also changed, giving rise to dissimilar pattern of signals.

Corns, callus, ill- fitted shoes, and orthotics with high bumps irritate the sole and trigger the output of aberrant sensory signals.

Derangement of sensory input for equilibrium causes the brain to bring on patterns of asymmetrical muscle tone throughout the body to achieve balance. A hypotonic muscle is less bulky, slower to activate and weaker than a normal muscle. The converse happens to a hypertonic muscle. With incomplete contractions, the sensory information supplied by a hypotonic muscle is less than that of its opposite counterpart. The brain is misled that the evenly erected body is tilted if an antigravity muscle signals less contraction than the identical muscle on the other side. In this way, muscular dysfunction becomes the secondary source of aberrant sensory input for equilibrium.

Muscles work together in interlacing chains. A hypotonic muscle is a weak link that forces other muscles up-and-down the chain to work harder. These substitution muscles strain easily as they are recruited to do the work that they are not designed for.

Muscle weakness of the lower extremities, jeopardizing the stability of an upright body, is the key in most musculoskeletal dysfunction.

Muscular system (front): Bouglé, Wikimedia Commons

Muscular system (back): Bouglé, Wikimedia Commons