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Balance is an ability to maintain the line of gravity (vertical line from centre of mass) of a body within the base of support with minimal postural sway. Sway is the horizontal movement of the centre of gravity even when a person is standing still. A certain amount of sway is essential and inevitable due to small perturbations within the body (e.g., breathing, shifting body weight from one foot to the other or from forefoot to rearfoot) or from external triggers (e.g., visual distortions, floor translations). An increase in sway is not necessarily an indicator of dysfunctional balance so much as it is an indicator of decreased sensorimotor control.
Maintaining balance requires coordination of input from multiple sensory systems including the vestibular, somatosensory, and visual systems.
Vestibular system: sense organs that regulate equilibrium (equilibrioception); directional information as it relates to head position (internal gravitational, linear, and angular acceleration)
Somatosensory system: senses of proprioception and kinesthesia of joints; information from skin and joints (pressure and vibratory senses); spatial position and movement relative to the support surface; movement and position of different body parts relative to each other
Visual system: Reference to verticality of body and head motion; spatial location relative to objects
The senses must detect changes of spatial orientation with respect to the base of support, regardless of whether the body moves or the base is altered. There are environmental factors that can affect balance such as light conditions, floor surface changes, alcohol, drugs, and ear infection.
There are balance impairments associated with aging. Age-related decline in the ability of the above systems to receive and integrate sensory information contributes to poor balance in older adults. As a result, the elderly are at an increased risk of falls. In fact, one in three adults aged 65 and over will fall each year.
In the case of an individual standing quietly upright, the limit of stability is defined as the amount of postural sway at which balance is lost and corrective action is required.
Body sway can occur in all planes of motion, which make it an increasingly difficult ability to rehabilitate. There is strong evidence in research showing that deficits in postural balance is related to the control of medial-lateral stability and an increased risk of falling. To remain balanced, a person standing must be able to keep the vertical projection of their center of mass within their base of support, resulting in little medial-lateral or anterior-posterior sway. Ankle sprains are one of the most frequently occurring injuries among athletes and physically active people. The most common residual disability post ankle sprain is instability along with body sway. Mechanical instability includes insufficient stabilizing structures and mobility that exceed physiological limits. Functional instability involves recurrent sprains or a feeling of giving way of the ankle. Nearly 40% of patients with ankle sprains suffer from instability and an increase in body sway. Injury to the ankle causes a proprioceptive deficit and impaired postural control. Individuals with muscular weakness, occult instability, and decreased postural control are more susceptible to ankle injury than those with better postural control.
Balance can be severely affected in individuals with neurological conditions. People who suffer a stroke or spinal cord injury for example, can struggle with this ability. Impaired balance is strongly associated with future function and recovery after a stroke, and is the strongest predictor of falls.
Another population where balance is severely affected is Parkinson’s disease patients. A study done by Nardone and Schieppati (2006) showed that individuals with Parkinson’s disease problems in balance have been related to a reduced limit of stability and an impaired production of anticipatory motor strategies and abnormal calibration.
Balance can also be negatively affected in a normal population through fatigue in the musculature surrounding the ankles, knees, and hips. Studies have found, however, that muscle fatigue around the hips (gluteals and lumbar extensors) and knees have a greater effect on postural stability (sway). It is thought that muscle fatigue leads to a decreased ability to contract with the correct amount of force or accuracy. As a result, proprioception and kinesthetic feedback from joints are altered so that conscious joint awareness may be negatively effected