Part I: The Science of Balance; Balance in Dance
Science of Balance:
Equilibrioception or sense of balance is one of the physiological senses. It helps prevent humans and animals from falling over when walking or standing still. In humans, equilibrioception is mainly sensed by the detection of acceleration, which occurs in the vestibular system. Other senses play roles as well, e.g. the visual system and proprioception. The importance of visual input for balance is illustrated by it being harder to stand on one foot with eyes closed than with eyes open.
In the vestibular system (or balance system), equilibrioception is determined by the level of fluid properly called endolymph in the labyrinth - a complex set of tubing in the inner ear.
When the sense of balance is interrupted it causes dizziness, disorientation and nausea. Balance can be upset by Meniere's disease, superior canal dehiscence syndrome, an inner ear infection, by a bad common cold affecting the head or a number of other medical conditions. It can also be temporarily disturbed by rapid and vigorous movement, for example riding on a merry-go-round. See also vertigo.
Most astronauts find that their sense of balance is impaired when in orbit, because they are in a constant state of free-fall while their rockets are off. This causes a form of motion sickness called space sickness.
The vestibular system, or balance system, is the sensory system that provides the dominant input about movement and equilibrioception. Together with the cochlea, a part of the auditory system, it constitutes the labyrinth of the inner ear, situated in the vestibulum in the inner ear.
As our movements consist of rotations and translations, the vestibular system comprises two components: the semicircular canal system, which indicate rotational movements; and the otoliths, which indicate linear translations. The vestibular system sends signals primarily to the neural structures that control our eye movements, and to the muscles that keep us upright. The projections to the former provide the anatomical basis of the vestibulo-ocular reflex, which is required for clear vision; and the projections to the muscles that control our posture are necessary to keep us upright.
The semicircular canal system detects rotational movements. More precisely, it detects change in rotational movements. The semicircular canals are its main tools to achieve this detection. As the basis of our perception of a three-dimensional world, our vestibular system contains three semicircular canals in each labyrinth. They are approximately orthogonal to each other, and are called the horizontal (or lateral), the anterior semicircular canal (or superior) and the posterior (or inferior) semicircular canal. Anterior and posterior canals may be collectively called vertical semicircular canals.
Movement of fluid within the horizontal semicircular canal corresponds to rotation of the head around a vertical axis (i.e. the neck), as when doing a pirouette.
The anterior and posterior semicircular canal detect rotations of the head in the sagittal plane (as when nodding), and in the frontal plane, as when cartwheeling. Both anterior and posterior canals are oriented at approximately 45° between frontal and sagittal planes. The movement of fluid pushes on a structure called cupula, which contains hair cells that transducts the mechanical movement to electrical signals.
The canals are arranged in such a way that each canal on the left side has an almost parallel counterpart on the right side. Each of these three pairs works in a push-pull fashion: when one canal is stimulated, its corresponding partner on the other side is inhibited, and vice versa.
This push-pull system allows us to sense all directions of rotation: while the right horizontal canal gets stimulated during head rotations to the right (below), the left horizontal canal gets stimulated (and thus predominantly signals) by head rotations to the left.
Vertical canals are coupled in a crossed fashion, i.e. stimulations that are excitatory for an anterior canal are also inhibitory for the contralateral posterior, and vice versa.
The vestibulo-ocular reflex (VOR).
The vestibulo-ocular reflex (VOR) is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. For example, when the head moves to the right, the eyes move to the left, and vice versa. Since slight head movements are present all the time, the VOR is very important for stabilizing vision: patients whose VOR is impaired find it difficult to read, because they cannot stabilize the eyes during small head tremors. The VOR reflex does not depend on visual input and works even in total darkness or when the eyes are closed.
This reflex, combined with the push-pull principle described above, forms the physiological basis of the Rapid head impulse test or Halmagyi-Curthoys-test, in which the head is rapidly and forcefully moved to the side, while controlling if the eyes keep looking in the same direction.
The mechanics of the semicircular canals can be described by a damped oscillator. If we designate the deflection of the cupula with ?, and the head velocity with , the cupula deflection is approximately
a is a proportionality factor, and s corresponds to the frequency. For humans, the time constants T1 and T2 are approximately 3 ms and 5 s, respectively. As a result, for typical head movements, which cover the frequency range of 0.1 Hz and 10 Hz, the deflection of the cupula is approximately proportional to the head-velocity (!). This is very useful, since the velocity of the eyes must be opposite to the velocity of the head in order to have clear vision.
Signals from the vestibular system also project to the Cerebellum (where they are used to keep the VOR effective, a task usually referred to as Learning or Adaptation) and to different areas in the cortex. The projections to the cortex are spread out over different areas, and their implications are currently not clearly understood.
While the semicircular canals respond to rotations, the otolithic organs sense linear accelerations. We have two on each side, one called utricle, the other Saccule. Therefore they get displaced during linear acceleration, which in turn deflects the ciliary bundles of the Hair cells and thus produces a sensory signal. Most of the utricular signals elicit eye movements, while the majority of the saccular signals projects to muscles that control our posture. While the interpretation of the rotation signals from the semicircular canals is straightforward, the interpretation of otolith signals is more difficult: since gravity is equivalent to a constant linear acceleration, we somehow have to distinguish otolith signals that are caused by linear movements from such that are caused by gravity. We can do that quite well, but the neural mechanisms underlying this separation are not yet fully understood.
Experience from the vestibular system is called equilibrioception. It is mainly used for the sense of balance and for spatial orientation. When the vestibular system is stimulated without any other inputs, one experiences a sense of self motion. For example, a person in complete darkness and sitting in a chair will feel that he or she has turned to the left if the chair is turned to the left. A person in an elevator, with essentially constant visual input, will feel she is descending as the elevator starts to descend. Of more importance are illusions of the vestibular system. For example, a person in a descending elevator does not feel it is descending once its initial acceleration has ceased. Illusions include:
Diseases of the vestibular system can take different forms, and usually induce vertigo and instability, often accompanied by nausea. The most common ones are Vestibular neuritis, a related condition called Labyrinthitis, and BPPV. In addition, the function of the vestibular system can be affected by tumors on the cochleo-vestibular nerve, an infarct in the brain stem or in cortical regions related to the processing of vestibular signals, and cerebellar atrophy.
Alcohol can also cause alterations in the vestibular system for short periods of time and will result in vertigo and possibly nystagmus. This is due to the variable viscosity of the blood and the endolymph during the consumption of alcohol. The common term for this type of sensation is the "Bed Spins".
Balance in Dance
Balance, in dance as in life, is one of the most important skills, yet it is notoriously difficult to achieve. The body balances naturally in a variety of positions with every step we take. Centered alignment helps the body to use less muscle activity, thus aligned balance feels like less effort.
To improve balance, you first need to become aware of the way within which you are performing a movement. If you move the body in parts instead of as one complete, aligned, unit, the body uses tension in some body parts to compensate for a lack of balance in others. Compensation is complex and much more difficult than experiencing the whole body as one.
Primitive reflexes underlie all movement. Righting reflexes reorient us toward our center so that we know where we are in relation to our axis -- righting reactions are controlled by numerous sensory organs in the neck muscles (muscle spindles) and the balance organs of the inner ear (vestibulum) and the eyes (optical righting). Try to selectively eliminate the optical righting reflex (close your eyes) while performing a task and you will "see" how much you rely on this mechanism. For dancers it is especially important to train the vestibular and the neck righting mechanisms.
Equilibrium responses are complex reactions to difficult balancing situations and it is hard to not react. E.g. let yourself fall forward -- your foot will automatically move forward to stop your fall, meaning you can turn your thinking off and just let your equilibrium responses take over.
You can train your eyes to help improve your sense of balance and space. Generally speaking, women have better peripheral vision, while men's eyes are better at seeing things in a narrow focus (the theory behind this is that prehistoric women needed to have an eye on everything while men needed to observe animals moving in the distance).
- While you look in front of you, can you observe what is going on next to you?
- Hold the arms out to the sides and move the fingers. How far back can you hold the arms and still see the movement.
- Balance on one leg and notice your eye movement
Muscles are sensory organs that contribute greatly to your overall balance. The human body consists mostly of muscles created to move the body against resistance, with muscles working in groups or pairs that balance each other's function (agonists and antagonists).
When a muscle is stretched, its natural tendency is to protect itself with a shortening reflex called a "stretch reflex". Without which (the so-called reciprocal inhibition) movement would not be possible. Tightening also happens because of changes in the muscle spindles through complex connections to our emotional centers in the brain. For example, when you are afraid the muscles tighten up and make movement difficult and irregular.
Less Stress, More Balance: Because muscle spindles are influenced by levels of stress, your state of mind greatly impacts your muscle coordination. The more you worry about your technique and ability to perform a step or spin, the more your spindles are in a state of oversensitivity. There is a link between stress and injury in dance, since often dancers are surrounded by stress concerning their technique relative to other dancers. If you feel calm, your balance is better.
The section 'Balance in Dance' is largely adapted from exercises and analysis given by Eric Franklin, and his books on Conditioning for Dance and Visualization.
Sources: Eric Franklin; Wikipedia; S.M. Highstein, The vestibular system; Kenneth Laws, Physics and Dance; Rick Allen, Dancing for Health