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.