Many types of skilled exercise require an accurate sense of balance and a rapid restoration of the body to its normal position when balance has been disturbed. This ability depends on nerve impulses originating in the labyrinth of the inner ear (the otolith organs and the semicircular canals).
There are two main groups of labyrinthine reactions: the acceleratory reflexes and the positional reflexes. Acceleratory reflexes are evoked by movements of the head and the effective stimulus is acceleration (a change in velocity). The response to linear acceleration is useful to the jumper in effecting a landing on the feet. Upon angular acceleration the responses are evoked in the muscles of the eyes, neck, limbs and trunk.
Rotary acceleration produces eye movements known as nystagmus, which is a quick swing of eyes in the direction of the rotation alternating with a slow deviation in the opposite direction. If the rotation continues at a constant rate the nystagmus gradually dies away, an example of the fact that this type of reaction depends upon acceleration, not velocity. Cessation of the rotation may cause turning of the head, body and arms in the direction of the previously experienced movement, with the result that the individual tends to fall to that side.
An example of the positional reflex is the righting reaction, which acts upon the neck muscles to keep the head in a normal position regardless of the position of the body. Another example of a positional reflex is the compensatory deviations of the eyes which are evoked by changes in the position of the head. When the body is placed in an abnormal position, there is an immediate effort to restore it to its normal position.
A normal, blindfolded person standing on a platform readily adjusts the position of his body when the platform is gradually tilted, but deaf-mutes (in whom the inner ear is defective) immediately fall. Impressions from the labyrinth, together with the visual and proprioceptive impressions from the muscles control the process by which physical equilibrium is gained or maintained. Impressions from the labyrinth are especially concerned with maintaining the normal position of the head.
Overstimulation of the labyrinthine receptors, as in whirling or rapid tumbling, may produce dizziness and even nausea. Seasickness results from a certain type of overstimulation of these receptors. Dizziness reduces skill and accuracy of movement. A common practice of dancers and figure skaters, whose routines include rapid spins, is to fix the eyes on a distant point and to watch that point until the head has turned as far as is compatible with comfort.
The head is then shifted quickly in the direction of the spin until another point is sighted. The eyes rest on this object until the limit of the turn of the head is reached and the shift is repeated. This momentary pause in the rotation of the head provides a brief resting period which reduces the dizziness of overstimulation. The susceptibility to dizziness on continued turning is diminished during training so that skill and accuracy during spinning or tumbling movements is increased.
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