Balance (ability)

In biomechanics, 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.

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 the physiological limits.Functional instability involves recurrent sprains or a feeling of giving way of the ankle. It is found that 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.

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