balance or equilibrium, is a state in which opposing tendencies are equal. To balance an object means to position it with its centre of gravity above its supports in such a way that there is no tendency for it to topple over to one side rather than to another. Toppling is not the same as falling. It is the toppling motion that gives rise to the ‘sensation of loss of balance’ and one feels ‘balanced’ when such sensations do not occur. We say that someone has a ‘good sense of balance’ when they appear able to move freely in all sorts of circumstances without obvious signs of accidental toppling. An object topples when the resultant of the stress forces acting on it does not pass through its centre of gravity (c of g). Stress forces are the forces of common experience—pushing and pulling—which are always associated with deformation of the molecular architecture of objects in contact. Gravity, on the other hand, is something quite different. It is the force to which Newton attributed the observed accelerations of objects in free fall. It acts at a distance, without contact. It is not gravity that breaks an egg when you drop it, but the stress forces on impact. The egg remains perfectly intact while it is in free fall under the action of gravity. An object can be prevented from falling if it is supported by stress forces exerted at contact with other objects, which are themselves supported in turn on the solid crust of the planet.

An object is said to be in ‘stable equilibrium’ if any small perturbation generates a force to oppose the displacement. This will be the case if the projection of the c of g falls within an ‘area of support’, defined as that polygon, with no re-entrant angles, that just encloses the projections of all the available points of support. Balance is maintained by moving the resultant of the supporting forces about in such a way as to resist perturbations. A piece of furniture, such as a table standing on its legs, is stable because, if any attempt is made to tilt it, the support thrusts in the legs alter and their resultant consequently shifts to resist the tilting. If you stand on one leg and pay attention to your standing foot, you will be able to feel changes in the foot as muscular forces alter the position of the thrust exerted between the foot and the ground to compensate for and resist the inevitable swaying arising from movements of the heart and chest.

Role of proprioception

sensory receptors of several kinds are involved in the complex process of maintaining uprightness, as well as in the recognition of the imminence of toppling. There are no ‘gravity receptors’ as such, in spite of what is generally believed. The parts of the inner ear commonly associated with this function turn out to be accelerometers; i.e. they are detectors of stress gradient, not of gravity. Proprioceptors elsewhere in the body can also act as accelerometers and thus make a contribution to indicating the direction of the resultant support thrust. The actual position of the thrust line is indicated by deformation of the soft tissues of the feet and hands at the areas of contact with the supports. Movements of the head during overbalancing are indicated by the streaming of details in the images of the environment on the peripheral retina.

Stability

Restricting the area of support diminishes the available range through which the support thrust can be moved to resist perturbations, unless the position of the support is itself appropriately moved by the perturbation. When an egg is placed on a hard surface, the area of support is restricted to the very small area of contact. It is, accordingly, very hard to balance an egg on one end, because any accidental tilting produces more movement of the c of g than of the point of support, the centre of curvature of the shell at the ends being below the c of g of the egg. The shift of the thrust line, which necessarily passes through the area of support, is thus not sufficient to correct the tilt. With the egg on its side, however, a brief push in the direction of the long axis of the egg produces temporary rocking, followed by a return to the original position. The centre of curvature in the plane of the long axis is above the c of g, so the shift of the thrust line exceeds that of the c of g. For a sideways perturbation, the centre of curvature is coincident with the c of g, and the egg just rolls away from the perturbation, with the thrust line continuing to pass through the c of g. This is what happens to a wheel: the balance is neither stable nor unstable.

If the body of an animal or of a person is to stay in more or less the same place, any accidental displacement in a particular direction will have to be corrected by a corresponding displacement in the opposite direction. This is achieved by adjusting, by muscular forces, the thrust forces exerted by the limbs against the supports — in magnitude, in direction, in timing, and in point of application.

Anticipatory pre-emptive actions

A number of reflex reactions have been identified that produce the appropriate changes in the musculature, by swaying, hopping, and stepping. In the intact subject, however, many of these reflexes are effectively replaced by ‘anticipatory pre-emptive actions’. These are voluntary actions, based on the underlying reflexes, but initiated in response to the detection that the incoming sensory information is changing in a way that might lead to a need for corrective action. Appropriate action is initiated early, before the reflex responses themselves are triggered into action. Frequent rehearsal, from a very early age, leads to these voluntary actions being performed without the subject being aware of what is going on—that is to say, they become habits. Their promptness plays an important role in maintaining smoothness of control, since they are not subject to the delays inevitable in reflex responses.

Overbalancing

The erect posture of man, particularly when standing on one leg, is a condition of precarious equilibrium, because the area of support is small compared with the height of the c of g above the feet. The strategies for avoiding falling over are related to what happens to an egg placed on its side. Small perturbations are met by shifting the centre of pressure at the foot and thus developing an inclined thrust to oppose the perturbation, as in the egg displaced in the direction of its long axis. This strategy will fail when the thrust line reaches the edge of the area of support, because further displacement will cause the body to topple. The imminence of such toppling is detected by the proprioceptive system and a different strategy is brought into play. If another limb is available, it will be thrust out in the direction of the impending fall in a ‘rescue reaction’ that attempts to find a firm obstacle against which to develop force and thus to extend the effective area of support. This is the basis of stepping. A succession of steps, in locomotion, brings the legs into action in turn, like the spokes of a wheel, so that the body may be moved through an indefinite distance without falling over—like the egg being rolled sideways. The legs do not provide the same continuous support as a wheel because, when one leg is being swung forward in a step, the body topples forward over the stance leg and acquires some downward momentum. This toppling movement has to be corrected when the swing leg eventually touches down, so this leg then at first gives, to absorb the unwanted momentum, and later straightens again to restore the c of g to its earlier height above the ground. As the body continues to move forward over the new stance foot, that leg extends to provide extra thrust, which propels the body forward into the next step. If this thrust is strong enough, the body can be launched into a free fall phase while the free leg is still swinging. This extends the step length, as in running or jumping.

Uprightness

When an object is at rest on a stationary support, the thrust line is parallel to a radius of the planet, i.e. it lies in the gravitational vertical. Experiments with moving platforms reveal, however, that the direction of the thrust line appropriate to the avoidance of falling over is dependent on the accelerations associated with the movement of the platform. A person standing in a vehicle that is moving in a curved path has to lean inwards, to develop a horizontal component of thrust to accelerate his body into an equally curved path, as well as developing an upward thrust to prevent falling. The best direction for the thrust line is thus not the same as the gravitational vertical.

The thrust developed against the supports, both on moving platforms and on firm ground, is under continuous readjustment by the nervous system to suit the needs of the moment, be it to remain in one place or to move about in locomotion or athletic activity. The successful control of the necessary muscular activity is a matter of skill; the basis of this is first acquired in infancy and it is continually being revised and rehearsed throughout life as different types of activity are undertaken.

T. D. M. Roberts

Bibliography

Roberts, T. D. M. (1995) Understanding Balance, Chapman and Hall, London.

Balance and Coordination Tests

Definition

Balance is the ability to maintain a position. Coordination is the capacity to move through a complex set of movements. Balance and coordination depend on the interaction of multiple body organs and systems including the eyes, ears, brain and nervous system, cardiovascular system, and muscles. Tests or examination of any or all of these organs or systems may be necessary to determine the causes of loss of balance, dizziness, or the inability to coordinate movement or activities.

Purpose

Tests of balance and coordination, and the examination of the organs and systems that influence balance and coordination, can help to identify causes of dizziness, fainting, falling, or incoordination.

Precautions

Tests for balance and coordination should be conducted in a safe and controlled area where patients will not experience injury if they become dizzy or fall.

Description

Assessment of balance and coordination can include discussion of the patient's medical history and a complete physical examination including evaluation of the heart, head, eyes, and ears. A slow pulse or heart rate, or very low blood pressure may indicate a circulatory system problem, which can cause dizziness or fainting. During the examination, the patient may be asked to rotate the head from side to side while sitting up or while lying down with the head and neck extended over the edge of the examination table. If these tests produce dizziness or a rapid twitching of the eyeballs (nystagmus ), the patient may have a disorder of the inner ear, which is responsible for maintaining balance.

An examination of the eyes and ears may also give clues to episodes of dizziness or incoordination. The patient may be asked to focus on a light or on a distant point or object, and to look up, down, left, and right moving only the eyes while the eyes are examined. Problems with vision may, in themselves, contribute to balance and coordination disturbances, or may indicate more serious problems of the nervous system or brain function. Hearing loss, fluid in the inner ear, or ear infection might indicate the cause of balance and coordination problems.

Various physical tests may also be used. A patient may be asked to walk a straight line, stand on one foot, or touch a finger to the nose to help assess balance. The patient may be asked to squeeze or push against the doctor's hands, to squat down, to bend over, stand on tiptoes or stand on their heels. Important aspects of these tests include holding positions for a certain number of seconds, successfully repeating movements a certain number of times, and repeating the test accurately with eyes closed. The patient's reflexes may also be tested. For example, the doctor may tap on the knees, ankles, and elbows with a small rubber mallet to test nervous system functioning. These tests may reveal muscle weakness or nervous system problems that could contribute to incoordination.

Preparation

No special preparation is required prior to administration of balance and coordination tests. The patient may be asked to disrobe and put on an examination gown to make it easier for the doctor to observe muscles and reflex responses.

Aftercare

No special aftercare is generally required, however, some of the tests may cause episodes of dizziness or incoordination. Patients may need to use caution in returning to normal activities if they are experiencing any symptoms of dizziness, lightheadedness, or weakness.

Risks

These simple tests of balance and coordination are generally harmless.

Normal results

Under normal conditions, these test will not cause dizziness, loss of balance, or incoordination.

Abnormal results

The presence of dizziness, lightheadedness, loss of coordination, unusual eye movements, muscle weakness, or impaired reflexes are abnormal results and may indicate the problem causing the loss of balance or incoordination. In some cases, additional testing may be needed to diagnose the cause of balance or coordination problems.

Resources

ORGANIZATIONS

American Academy of Otolaryngology-Head and Neck Surgery, Inc. One Prince St., Alexandria VA 22314-3357. (703) 836-4444. 〈http://www.entnet.org〉.

Ear Foundation. 1817 Patterson St., Nashville, TN 37203. (800) 545-4327. 〈http://www.earfoundation.org〉.

Vestibular Disorders Association (VEDA). P.O. Box 4467, Portland, OR 97208-4467. (800) 837-8428 or (503) 229-7705. Fax: (503) 229-8064.

KEY TERMS

Meniere's disease— An abnormality of the inner ear that causes dizziness, ringing in the ears, and hearing loss.

balance instrument used in laboratories and pharmacies to measure the mass or weight of a body. A balance functions by measuring the force of gravity that the earth exerts on an object, i.e., its weight. Since the mass of an object is directly proportional to its weight, a balance can also be used to measure mass.

Types of Balances

The Equal-Arm Balance

The simplest type of balance, the equal-arm, or beam, balance, is an application of a lever . A uniform bar, the beam, is suspended at its exact center on a knife-edge set at right angles to it. The point of support is called the fulcrum. Two pans of equal weight are suspended from the beam, one at each end, at points equidistant from the fulcrum. Since the center of gravity of a uniform bar is at its midpoint, the beam supporting the pans will be in equilibrium, i.e., will balance on the knife-edge. A long pointer attached at right angles to the beam at the fulcrum indicates zero on a scale when the beam is at rest parallel to a level surface. It shows also the extent of swing of the beam on one side or the other, acting somewhat as a pendulum, when the beam is coming to rest. The object to be weighed is placed on one pan, and standard weights are added to the other until the balance of the beam is established again. The unknown weight can then be determined by adding up the standard weights in the pan.

The platform balance is a form of equal-arm balance in which two flat platforms are attached to the top side of the beam, one at each end. Such a balance has a rider, or weight, mounted on a bar that has a calibrated scale, is parallel to the beam, and connects the supports of the two platforms. This rider is moved along the bar, its edge marking decimal fractions of the unit weight.

The Unequal-Arm Balance

On the unequal-arm balance the beam is suspended at a point a very short distance from one of its ends. The object to be weighed is placed on this end, and a small known weight is moved out along the longer arm until balance is obtained. The unknown weight is then determined by using a formula involving the known weight and the distance of each weight from the fulcrum. One example of this type of balance is the steelyard, an ancient device still used in underdeveloped nations because of its portability and low cost; since the difference in length of the arms can multiply the effect of the smaller weight by a factor of 100 or more, a small steelyard hung from a tree can be used, for example, to weigh a side of beef.

The Spring Balance

A spring balance consists of a coiled spring fixed to a support at one end, with a hook at the other to which the body to be weighed is applied. Within the spring's limit of elasticity, the distance through which it is stretched is directly proportional to the weight of the applied body. A pointer and graduated scale attached to the spring convert this distance into a weight reading.

Accuracy of Balances

Although extremely accurate results can be obtained in measuring the weights of minute objects, it is physically impossible to construct any balance perfect enough to yield absolutely accurate determinations. For example, the analytical balance, a type of equal-arm balance, is used for delicate weighing in quantitative chemical analysis and in preparing pharmaceutical prescriptions; it must be kept in a glass case, since its accuracy is easily affected by dust and moisture. A spring balance does not retain its accuracy permanently, for no matter how carefully it is handled, the spring very gradually uncoils even though its limit of elasticity has not been exceeded.

For ordinary purposes the errors are so small that they are considered insignificant, but in chemical analysis it has been necessary to develop methods by which they can be further minimized. A so-called torsion balance , which depends on the twisting of a wire or thread, is employed for weighing, but the term is commonly used to indicate a device for measuring minute electrical and magnetic forces.

See scale .

bal·ance / ˈbaləns/ • n. 1. an even distribution of weight enabling someone or something to remain upright and steady: slipping in the mud but keeping their balance. ∎  stability of one's mind or feelings: the way to some kind of peace and personal balance. 2. a condition in which different elements are equal or in the correct proportions: overseas investments can add balance to an investment portfolio. ∎  Art harmony of design and proportion. ∎  [in sing.] the relative volume of various sources of sound: the balance of the voices is good. 3. an apparatus for weighing, esp. one with a central pivot, beam, and a pair of scales. 4. a counteracting weight or force. ∎  (also balance wheel) the regulating device in a mechanical clock or watch. 5. a predominating weight or amount; the majority: the balance of opinion was that work was more important than leisure. 6. a figure representing the difference between credits and debits in an account; the amount of money held in an account: he accumulated a healthy balance with the savings bank. ∎  the difference between an amount due and an amount paid: unpaid credit-card balances. ∎  [in sing.] an amount left over. • v. [tr.] 1. keep or put (something) in a steady position so that it does not fall: a mug that she balanced on her knee. ∎  [intr.] remain in a steady position without falling: Richard balanced on the ball of one foot. 2. offset or compare the value of (one thing) with another: the cost of obtaining such information needs to be balanced against its benefits. ∎  counteract, equal, or neutralize the weight or importance of: he balanced his radical remarks with more familiar declarations. ∎  establish equal or appropriate proportions of elements in: balancing work and family life. 3. compare debits and credits in (an account), typically to ensure that they are equal: the law requires the council to balance its books each year. ∎  [intr.] (of an account) have credits and debits equal. PHRASES: balance of payments the difference in total value between payments into and out of a country over a period. balance of power 1. a situation in which nations of the world have roughly equal power. 2. the power held by a small group when larger groups are of equal strength. balance of trade the difference in value between a country's imports and exports. in the balance uncertain; at a critical stage: his survival hung in the balance for days. on balance with all things considered: but on balance he was pleased.DERIVATIVES: bal·anc·er n.

balance
1. With reference to diet, positive balance is a net gain to the body and negative balance a net loss from the body. When intake equals excretion the body is in equilibrium or balance with respect to the nutrient in question. Used in reference to nitrogen (protein), mineral salts, and energy.

2. A balanced diet is one containing all nutrients in appropriate amounts.

balance of a node in a binary tree. A measure of the relative size of the left and right subtrees of the node. Usually, the balance is defined to be the height of the left subtree minus the height of the right subtree (or the absolute value thereof). However, formulas are also used that measure balance in terms of the total number of nodes in the left and right subtrees.

balance balance of nature a state of equilibrium produced by the interaction of living organisms, ecological balance; the phrase is recorded from the early 20th century.
balance of power a situation in which states of the world have roughly equal power. The phrase in this sense is recorded from the early 18th century.

balance n. the ability of a boat to stay on course without adjustment of the rudder.
balance of power
1. a situation in which nations of the world have roughly equal power.

2. the power held by a small group when larger groups are of equal strength.

balance weighing-scales, uncertainty XIII; adjustment of accounts XVI; sum remaining over XVIII. — (O)F.:- Rom. *bilancia, f. late L. bilanx, bilanc-, f. BI- + lanx scale.
So balance vb. XVI. — (O)F. balancer.

balance
1. (in animal physiology) Equilibrium in the posture of the body. In vertebrates balance is sensed and maintained by the vestibular apparatus of the inner ear.

2. (in nutrition) See diet.

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