Roberts book, Understanding Balance:The Mechanics of Posture and LocomotionI took the liberty of posting a few paragraphs from an essay about this book.
Balancing the body's loosely-jointed assemblage of heavy segments on one another during standing and locomotion calls for separate and continuous regulation of each of a very large number of independent motor units. Success depends on triggering appropriate anticipatory pre-emptive actions by gestalt recognition of specific developing trends in proprioceptive, and other, neural signals.
1. Balance is maintained by developing, against the available supports, forces suitably adjusted in timing, magnitude and direction. The oversimplified schoolroom physics of rigid bodies is not appropriate to body segments made up of soft tissue as well as bone, and the application of Newton's Laws has to be re-examined, particularly in relation to the distinction between stress and gravity and to D'Alembert's procedure for analysing problems in dynamics by the methods of statics.
2. Stress forces are the bulk manifestations of the intermolecular forces responsible for the cohesion and relative rigidity of solid objects. The convention of representing the forces in a structure in terms of an equivalent triangulated lattice demands that forces be regarded as acting at a point and that the struts and ties forming the lattice be linked by frictionless pin joints. These assumptions are unrealistic in that the relevant stresses are distributed over areas of significant magnitude. The locus of the resultant of the stresses at the area of contact with the supports can be determined with a force-plate of suitable design but analysis of the way forces are transmitted from one part of the body to another involves many uncertainties. Nevertheless, a number of interesting mechanisms can be identified.
15. It turns out that there are a great many instances in ordinary living where anticipatory pre-emptive actions intervene to provide very rapid smooth corrections to cope with the varying environment. They have the advantage of avoiding the delays and hunting oscillations associated with conventional servomechanisms.
20. The problems of maintaining balance when the supports are moving, as in a moving vehicle, make it clear that, in spite of what is generally believed, the direction of the gravitational vertical is irrelevant. The reactions of the body are, in contrast, organised around a direction referred to as the "behavioural vertical", which may be thought of as the "best direction in which to push against the supports to avoid falling over". This direction is very much dependent on circumstances, and changes when the platform undergoes linear acceleration. The choice of what is currently to count as the direction of the behavioural vertical is a matter of gestalt recognition, as the trigger for initiating "anticipatory pre-emptive actions" when there is imminent risk of overbalancing. Locomotion is achieved by deliberately resetting the behavioural vertical to initiate forced toppling, which then calls in a succession of rescue reactions which carry the body forward along the chosen path.
24. Special types of locomotor patterns are required in certain conditions, like those involving the wearing of specialised footwear such as skates or skis, and in the riding of a bicycle. Clearly, such new locomotor patterns must be learned, rather than innate. In considering the learning process, a distinction should be made between "conditioning" and "true learning". In classical conditioning, an alteration is induced in the gestalt forming the trigger for a specific reflex response. The response itself is not changed. In true learning, on the other hand, it is the motor performance that is altered, and this may take a quite new form after the relevance of certain features of the environmental situation has been recognised by the subject. The process of learning is reinforced when the resulting new behaviour triggers some form of reward-recognition process.
37. It may be noted that the direction of the behavioural vertical itself is not something that can be directly sensed. It has the status of a concept of the limiting condition in which the imminence of overbalancing is reduced to zero. It is built up from experience of the effectiveness of various thrusts that have been exerted in the past. Its direction, in relation to the trunk, is continually changing, for reasons that are explained. For stability, we aim to reduce the risk of overbalancing, while to initiate locomotion, we may develop a thrust that deliberately leads to overbalancing, so that a horizontal acceleration in a desired direction can be obtained from the interaction with gravity.
38. From what we now know, from computer simulations, of how interconnected groups of neurons can be expected to behave, we appear to be approaching a position in which we can account for the way the central nervous system organizes the strategic choice of sequences of offset thrusts by which we maintain our balance and perform acts of locomotion.
Sorry if I used too much band width. Thought it might be of interest. Later RicB.