Balance (ability)

Balance (ability)

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.

Maintaining balance requires coordination of input from multiple sensory systems including the vestibular, somatosensory, and visual systems.

• 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.