Muscle forceThe source of the necessary forces is the muscular tissue forming the separable muscle masses that are given individual anatomical names. These recognizable ‘muscles’ each contain a large number of individual muscle fibres, each in turn dependent for its activity on the arrival of impulses in the motor nerve fibre to which it is connected. When a muscle fibre is made active, it will do one or more of three things, depending on the conditions: it will shorten if it can; it will develop a tension against a resistance; it will show an increased resistance to extension. Since each motor nerve fibre serves a number of muscle fibres, all of these will be active together as a motor unit when the nerve fibre is active. The results depend on what else is going on at the time. The control of the forces needed for postural adjustment thus involves organizing the precise timing of the activities of each of the various motor units in the body — a quite formidable task, there being something like a hundred thousand of them in all.
Pulling and pushingThe only type of force that a muscle cell can exert is a tension — whereas, for many postural purposes, what is required is a thrust. The necessary transformation is provided by the leverage of muscles pulling on the jointed skeleton. Force can be transmitted from one place to another through a triangulated lattice structure where each triangle consists either of two ties and a strut or of two struts and a tie. (A strut pushes, and a tie pulls.) The muscles and tendons can act only as ties, while a bone can act either as a strut or as a tie. Since the combined action of a tension and a thrust applied at different places at one end of a bone can cause that bone to exert transverse thrust at its other end, a bone can also be made to act as a lever. These two mechanisms, the triangle and the lever, serve to supply the necessary forces to produce all the required movements of the parts as well as to maintain the jointed body in its various postures when apparently at rest. The relevant patterns of motor unit activation are idiosyncratic as well as habitual. In consequence, the resulting posture, even at rest, is often highly characteristic of the individual. A ‘good’ posture is not only elegant and aesthetically pleasing to the onlooker; it is one in which the body is resilient, ready to undertake any movement that may be called for. This implies that none of the joints, especially those in the back and neck, is at the limit of its range of free movement. As a result, none of the ligaments that restrict joint movement are put under continuous strain.
MovementIn principle, the relative motion between any two objects has six degrees of freedom, three directions of linear motion, and three directions of rotation. In the body, however, the relative movement at any joint between two bones is limited in several respects by the ligaments. There are, nevertheless, few joints in the body that act as simple hinges, with only a single degree of freedom. In consequence, when an extremity such as a finger is to be moved from one place to another, there are several different ways in which the movement can be executed even without much change in the trajectory of the extremity itself. Any relative movement of a body part, even of a finger, calls for readjustments at all the joints in the body. All are affected because the change in the distribution of the weight alters the relationship between the position of the centre of gravity of the body as a whole and the disposition of the available supports, and to maintain balance a different pattern of force generation is now required.
SwayMuscle force is required, not only to support the weight of the body and its parts, but also to set them into motion and to bring them to rest thereafter. Small adjustments in posture are required all the time, to avoid the body being overbalanced by the continual movements of the ribcage in breathing and by the pumping action of the heart. These adjustments affect the position and direction of the resultant supporting thrust (see balance), and result in a continual postural swaying movement as successive incipient topplings in one direction or another are arrested and corrected. Larger changes in muscular activity are required for the voluntary movements of locomotion and for reaching, catching, throwing, and striking.
Where a linear movement is involved, the amount of force that has to be developed by the muscles depends on the linear inertia (or mass) of the part that is to be moved. For a rotation, a different measure, the moment of inertia, is involved. This measure takes account not only of the mass that is to be moved, but also of the distances of each part of that mass from the axis of rotation.
Preparation for actionMany voluntary actions are performed in two phases. There is a preliminary setting up, followed by the main part of the action. The success of the action is often greatly influenced by the nature of the preparation, e.g. the take-back or back-swing before the strike, or the enhanced sway required to move the weight off one foot to allow the other to be lifted ready to perform a step. Because the moving parts gain momentum in this preparatory direction and this momentum has to be absorbed before the main movement can start, the activation of the muscles that are to produce the main movement occurs when they are being stretched. The force that a muscle can develop depends on what is happening to its length. It develops more force if it is being stretched than it can while it is shortening. There is thus a significant advantage in the preparatory movement in what appears, at first sight, to be in the wrong direction.
Aiming, throwing, and catchingAny form of voluntary action involves aiming — that is to say, some sort of goal to be achieved by the action — together with some mechanism for indicating a successful outcome. The various modes of locomotion involve throwing and catching the body as a whole by adjusting the forces between the limbs and the available supports, in magnitude, in direction, and in point of application. Too small a supporting thrust will allow the body to fall, while a thrust against the ground greater than that needed for support at rest will throw the body upwards. A horizontal component of thrust at take-off allows the body to move over the ground during the free-fall phase before landing. The body acquires momentum from gravity during the fall and this extra momentum has to be absorbed by extra force at the point of landing, with careful adjustment of the direction of thrust to avoid overbalancing. In some cases the unwanted momentum is absorbed by the stretching of ligaments and tendons. These are elastic structures that can store strain energy, like a spring. The stored energy can be recovered later, during the recoil phase, to generate new momentum in a different direction. This mechanism plays an especially important role in the bounding progression of the kangaroo.
Moving vehiclesWhen a person is travelling in a moving vehicle, which is changing its momentum under the action of stress forces — as in starting, changing speed, stopping, or being steered into a curved path — appropriate stress forces have also to be applied to the person's body if it is to keep its station within the vehicle.
This requirement does not apply if the vehicle is a spacecraft in orbit. The curvature of the orbital path is attributable to the unopposed action of gravity. The spacecraft together with all its contents, including the passengers, is effectively in free fall. Everything in the spacecraft is accelerating in the same way under the action of gravity, so all retain their relative positions without any support force against the floor or walls of the spacecraft being called for. The condition is referred to as weightlessness. It is not correct to refer to it as a ‘condition of zero gravity’, since the orbital path itself is produced by the action of gravity on the tangential speed of the spacecraft acquired during its launch.
Skills and gesturesIn sporting activities the appropriate adjustment of the muscle forces required is clearly a matter of acquired skill, depending on assiduous practice. It is not usually realized that the more everyday activities of sitting up, standing, and moving about are equally dependent on acquired skill, as are also the small movements of the face, larynx, and chest-wall involved in the production of speech. We start learning the necessary skills in early infancy and continue the process throughout life, adjusting our behaviour to suit the various new tasks that we encounter each day. Some of the resulting changes in habitual posture can be interpreted as the body language indicating changes in mood. Other changes are used as gestures in deliberate communication with other people, as in adding emphasis to the spoken word. All changes in posture depend ultimately on the same general pattern of neural organization, namely the control of the detailed timing of the separate activations of each one of a large number of motor units in muscles distributed throughout the body.
Motor controlAttempts to account for the control signals in terms of engineering models usually depend upon the notion that the nervous system detects the sort of variables that an engineer designing a robot would use transducers to provide. However, none of the sense organs in the body deliver signals corresponding to the simple continuous variables that are required for such models. All sensory messages consist only of irregular sequences of nerve impulses, and any coding that could be devised to correspond to the transformation at the receptor, from stimulus condition to successions of impulses, is unlikely to survive transmission across the synapses of the central nervous system. These engineering schemata must therefore be regarded as not directly relevant to the way the body actually works. A better understanding of the way the nervous system actually organizes its control of the body musculature should have profound implications, both for the management of clinical disabilities, and for the design of self-adapting machines to take over, in hazardous conditions, certain skilled tasks that at present can be carried out only by a human operator.
T. D. M. Roberts
Roberts, T. D. M. (1995) Understanding Balance. Chapman and Hall, London.
See also balance; movement, control of; walking.
The modern history of children's posture involves an important episode of intense concern, followed by an equally interesting relaxation. Around 1900 American parents were told to devote a great deal of attention to the posture training of their children, but roughly fifty years later the campaign was receding dramatically. Posture history in other societies remains to be traced, but the beginnings of modern posture standards in Western Europe emerged in the seventeenth and eighteenth centuries, as part of the growing concern about precise manners and body discipline in the middle and upper classes. Children, and particularly boys, were urged to hold themselves erect. While most boys did not serve in the military, growing attention to formal military training may have played a role in the wider posture concern.
Books about manners dealing with children's posture spread to North America by the 1760s and 1770s. Middle-class Americans such as John Adams began to write about their concern for proper carriage of the body, so that social relationships would not be troubled by slouching or twitching. These new posture standards became a regular part of child-rearing advice through the nineteenth century, as a means to help children grow up to be respectable. Proper posture began to denote self-discipline. Doctors supported the movement, arguing that good posture was essential for proper health. At the same time, there is no indication that many people worried greatly about posture training, except perhaps in urging children to sit up straight at the dinner table. Rigid furniture and stiff clothing for formal occasions, including corseting of young women, helped maintain posture without too much effort.
This situation changed at the end of the nineteenth century, as a flurry of posture advice emerged. Doctors stepped up their campaign, arguing that a number of modern conditions, including cramped school desks, were leading to widespread physical deformity. More important still was the emergence of posture testing and training in the schools, backed by a growing body of physical education instructors. An American Posture League was formed early in the twentieth century, staffed mainly by the physical education group. Posture kits allowed teachers to evaluate children's posture. A number of school districts set up active posture programs, involving thousands of children. Children identified with bad posture were sent to a variety of remedial lessons, and in severe cases physical devices were imposed to straighten the body. More informally, habits such as walking with a book on the head gained in popularity as a means of acquiring attractive posture. The posture movement spread to American colleges in the 1920s, particularly for women. Schools like Vassar actively tested the posture of all entrants, often photographing them, and mandatory courses included posture training.
Several factors propelled this striking new concern. First, clothing and furniture became looser and more relaxed, and the posture programs were deliberately set in this context, seeking more generally to compensate for the growing indulgence of modern life. Second, the professional self-interest of doctors and physical education instructors supported what was a sincere but obviously advantageous interest in convincing parents and children that most young people suffered from posture defects. Third, the posture program served as an expression of anxiety about a number of more general features of modern society, including compulsory schooling and the lures of consumerism. Posture standards were now imposed quite widely, and not just for the respectable middle classes, though elite colleges developed a special concern. The democratization of standards was juxtaposed with concerns about immigrants and the need to use school programs to help bring their children into line.
By the 1940s the posture movement was past its peak. National associations disappeared. School programs were dropped or diluted. By the 1960s, doctors began to attack the old posture anxieties as false and misleading; few children had posture problems, according to the new wisdom, and those that did could be helped through medical treatments. Posture interests did not fade entirely, however. Interview manuals of college students still included reminders about good posture, and conservatives continued to lament the slouching of modern young people as part of their claims that character standards were deteriorating. In general, however, presenting oneself in a relaxed, informal mode replaced stiff posture as the expression of choice for American young people.
See also: Child-Rearing Advice Literature.
Kasson, John F. 1990. Rudeness and Civility: Manners in Nineteenth- Century Urban America. New York: Hill and Wang.
Roodenburg, Herman. 1997. "How to Sit, Stand or Walk: Toward a Historical Anthropology of Dutch Paintings." In Looking at Seventeenth-Century Dutch Art: Realism Reconsidered, ed. Wayne Franits. New York: Cambridge University Press.
Yosifon, Davis, and Peter N. Stearns. 1996. "The Rise and Fall of American Posture." American Historical Review 103: 317-344.
Peter N. Stearns
pos·ture / ˈpäschər/ • n. a position of a person's body when standing or sitting: he stood in a flamboyant posture with his hands on his hips | good posture will protect your spine. ∎ Zool. a particular pose adopted by a bird or other animal, interpreted as a signal of a specific pattern of behavior. ∎ fig. a particular way of dealing with or considering something; an approach or attitude: labor unions adopted a more militant posture in wage negotiations. ∎ fig. a particular way of behaving that is intended to convey a false impression; a pose: despite pulling back its missiles, the government maintained a defiant posture for home consumption. • v. 1. [intr.] [often as n.] (posturing) behave in a way that is intended to impress or mislead others: a masking of fear with macho posturing. ∎ [tr.] adopt (a certain attitude) so as to impress or mislead: the companies may posture regret, but they have a vested interest in increasing Third World sales. 2. [tr.] archaic place (someone) in a particular attitude or pose: and still these two were postured motionless. DERIVATIVES: pos·tur·al adj. pos·tur·er n.
Posture (Body Position)
Posture (Body Position)
To provide the longest possible casting arcs the caster’s body is angled sideward toward the target. The foot opposite the casting arm is placed in the forward position pointed at the target. The other foot is placed behind and turned outwards nearly at a right angle from the line of cast. (See Diagram 11) On the forward cast the body weight is shifted to the forward foot; likewise on the backcast the weight is shifted to the back foot. In this stance the caster can easily view the backcast by turning the head. This body position allows for maximum rod arc movement needed for maximum distance casting. This stance allows the rod hand to drift backwards and upwards on the backcast.
This drift occurs after the backcast is completed and allows for a longer forward cast stroke. (See Diagram 12)