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.
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.