In biomechanics, balance is an ability to maintain the line of gravity (vertical line from centre of gravity) 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 for one foot to the other or from forefoot to rearfoot) or from external sources (e.g., air currents, floor vibration). An increase in sway is not necessarily an indicator of poorer balance so much as it is an indicator of decreased neuromuscular control.
- Vestibular system: sense organs that regulate equilibrium; 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 body position with respect to the base of support, regardless of whether the body moves or the base moves or changes size. 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.
The two types of sway are anterior-posterior sway and medial-lateral sway. 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 their center of pressure within their base of support, resulting in little medial-lateral or anterior-posterior sway.
Balance can be severely affected in individuals with neurological conditions. Patients who suffer a stroke or a spinal cord injury for example, can struggle with this ability. It has also been determined that impaired balance is strongly associated with future function and recovery in some cases, particularly in stroke patients. Additionally, balance problems have been identified as the strongest predictor of falls.
Another population where balance is serverely affected is Parkinson's disease patients. A study done by Nardone and Schieppati (2006) showed that individuals with Parkinson's disesase 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.
Balance training 
Since balance is a key predictor of recovery and is required in so many of our activities of daily living, it is often introduced into treatment plans by physiotherapists and occupational therapists when dealing with geriatrics, patients with neurological conditions, or others whom they have determined it to be beneficial.
Balance training in stroke patients has been supported in the literature. Methods commonly used and proven to be effective for this population include sitting or standing balance practice with various progressions including reaching, variations in base of support, use of tilt boards, gait training varying speed, and stair climbing exercises. The type of training should be determined by a physiotherapist and will depend on the nature and severity of the stroke, stage of recovery, and the patient’s abilities and impairments after the stroke.
Populations such as the elderly, children with neuromuscular diseases, and those with motor deficits such as chronic ankle instability have all been studied and balance training has been shown to result in improvements in postural sway and improved “one-legged stance balance” in these groups. The effects of balance training can be measured by more varied means, but typical quantitative outcomes are centre of pressure (COP), postural sway, and static/dynamic balance, which are measured by the subject’s ability to maintain a set body position while undergoing some type of instability.
Functional balance tests 
Functional tests of balance focus on maintenance of static balance, balance during weight shifting or voluntary movement (dynamic balance), balance responses to manual perturbations, and functional mobility. Standardized tests of balance are available to allow physiotherapists and other health care professionals to assess an individual’s functional performance. Some functional balance tests that are available are:
- Romberg Test: used to determine proprioceptive contributions to upright balance. Subject remains in quiet standing while eyes are open. Once excelling in this test subjects can go through the Sharpened Romberg Test which is a harder version of the Romberg Test. Subjects are to have their arms crossed, feet together and eyes closed. This decreases the base of support in the medial and lateral direction, raises the subjects centre of mass and now allowing arms for balance. This really tests the subjects ability to balance. 
- Functional Reach Test: measures the maximal distance one can reach forward beyond arm’s length while maintaining feet planted in a standing position.
- Berg Balance Scale: measures static and dynamic balance abilities using functional tasks commonly performed in everyday life. One study reports that the Berg Balance Scale is the most commonly used assessment tool throughout stroke rehabilitation, and found it to be a sound measure of balance impairment in patients following a stroke.
- Performance-Oriented Mobility Assessment (POMA): measures both static and dynamic balance using tasks testing balance and gait.
- Timed Get Up and Go Test: measures dynamic balance and mobility.
- Balance Efficacy Scale: self-report measure that examines an individual’s confidence while performing daily tasks with or without assistance.
- Star Excursion Test: A dynamic balance test that measures single stance maximal reach in multiple directions.
Static balance is the ability to maintain the line of gravity (vertical line from centre of gravity) within the base of support with minimal movement. The most commonly used test to measure static balance is monitoring centre of pressure (COP) motion for a specified duration as an athlete attempts to stand motionless on a force plate, double or single stance with eyes open or closed. It has been noted that COP is not the same as centre of gravity; however, little COP motion is suggestive of good balance and COP measured from a force plate is considered the gold standard measure of balance. Some studies suggest that balance training could be effective at increasing muscular strength, power output and jump height though enhanced neuromuscular efficiency and stretch reflexes.
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