balance

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.

See also posture; proprioception; sensory receptors; walking.