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.
