Balance is maintained as a result of the interaction of three systems: the visual, the vestibular, and the proprioceptive systems.
Vision plays a significant role in balance. Your eyes give you a picture of the world and where you are in relation to other things in it. Approximately 20 percent of the nerve fibers in the eyes interact with the vestibular system.
The vestibular apparatus, an organ located in the inner ear, is responsible for maintaining general equilibrium. The receptors contained within the vestibular apparatus are sensitive to any changes in head position or movement direction. These receptors provide information regarding linear acceleration (being able to sense forward and backward, as well as upward and downward, movement) and angular acceleration, which enables you to detect rotation of the head while keeping the eyes still. The vestibular apparatus exerts direct control over the eyes so they can directly compensate for head movements. This is crucial in sports where tracking moving objects or an opponent with head and eye movement is a constant necessity. With head movement, receptors in the vestibular apparatus transmit neural information to the cerebellum and the vestibular nuclei located in the brain stem. When the brain receives the message, often reinforced by visual feedback, it sends a signal to the muscles; this tells them to react to the loss of balance. Even standing still is an exercise in dynamic equilibrium. A person is swaying very slightly all the time to all four sides, and balance is maintained by alternate contraction and relaxation of the leg muscles.
The proprioceptive system includes both muscle and joint proprioceptors. Muscle proprioceptors include muscle spindles (which detect changes in muscle length) and Golgi-tendon organs (which detect changes in muscle tension). The joint receptors include free nerve endings, pacinian corpuscles, and Golgi-type receptors. They are responsible for detecting changes in joint angle and pressure that compress and distort the joint capsule. Information from these receptors is relayed to the central nervous system, which then facilitates neuromuscular coordination to provide stability and maintenance of balance.
This is an excerpt from Training for Speed, Agility and Quickness.