The human body, like that of any other vertebrate animal, is a very flexible structure, the bones of the skeleton being linked by almost frictionless joints. In all conditions except that of free fall, a live body can be distinguished from an inert structure by the relative disposition of the body parts, its ‘posture’. This is because, during waking life, most of the body is held clear of the ground. Because the body is flexible, appropriate forces have to be developed across each of its joints to support the weight of those parts that are not themselves directly in contact either with the ground or with some other structure supported on the ground. It is these forces that are responsible for the arrangement of the body parts that constitutes the live posture.
Muscle force
The 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 pushing
The 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.
Movement
In 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.
Sway
Muscle 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 action
Many 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 catching
Any 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 vehicles
When 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 gestures
In 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 control
Attempts 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
Bibliography
- Roberts, T. D. M. (1995) Understanding Balance. Chapman and Hall, London
See also balance; movement, control of; walking.
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