Thierry Pozzo – Gravitational Field, Bodily Equilibrium and Perceptive Activity – 2003

One of the mechanical principles used to describe balance is to consider the human body as a rigid system modeled in the form of an inverted pendulum in unstable balance. In a standing position, the pendulum is immobile when the center of mass [1] is projected on the vertical of the center of pressure [2], i.e., on the point located between the foot and the ground, on which the forces of reaction that are transmitted to the feet are felt.

At rest, the body, which is bent slightly forward (around 5 cm forward of the ankle’s axis of rotation) falls slightly under the action of gravity or the effect of internal troubles (e.g., breathing). It is then regained thanks to the contraction of, amongst others, the extensors of the ankle. These act as braces that brake and bring the bodily axis back to the vertical. This subtle game between gravitational force, endogenous disturbances and muscular actions represents a simple and effective solution: the nerve command results in control of a single muscle group (the extensors), which, since they are connected with the properties of muscular viscosity and elasticity, are enough to regulate imbalance. This classic scheme attributes particular status to the ankle, on which significant forces are exerted, compared with the other articulations, and it conceals the potential role of the trunk, the pelvis and the neck. The recent improvements in methods of exploring the posture bring to light the considerable role of proximal segments (pelvis, trunk, neck). Balance and postural transitions are not “mass” movements. They come from differentiated mobilization of segmental stages, which are associated with local postural adjustments that are impossible to design from the inverted pendulum model. Even though this model has its limitations, it does, nonetheless, simplify understanding of the mechanical conditions of a chain of postural sequences.

The inverted pendulum model offers a special arrangement of spatial exploration with advantages and drawbacks. Gravitational force, which operates on the body located near the earth, attracts it and holds it steadily in the same spot. Balance is stable if and only if the configuration of the mechanical system corresponds at least to the potential energy. Sitting in the hollow of the valley, the sleeper, with his body flattened on the ground, rests, immobile like the classic pendulum whose balance has stopped oscillating. This stillness has a price, and extracting oneself from the valley will require intense effort. On the other hand, sitting on the top of a crest, balance is a perpetual threat. The careless walker, just like the inverted pendulum in an unstable equilibrium, is furthermore strongly charged with potential energy that a fleeting disequilibrium will be transformed into movement almost effortlessly. What the chain of postural sequences actually does is use the gravitational force to the best advantage by placing the center of mass as high as possible above the supports and at the same time by exercising a fine control of the position of the pendulum, when that position has been weakened by its elevation.

To put itself into motion, the body does not passively experience the effects of gravitational force, like a falling stone, but produces active muscular contractions. Nonetheless, those contractions would remain ineffective if the body did not have contact with the ground or some other support. The body cannot actually be put into motion solely by muscular contraction. The astronaut in a state of weightlessness cannot plan the slightest movement of the body without using a handhold or foot straps attached to the space station’s floor. In the end, it is the forces of reaction to the muscular contractions that, via the feet, produce the voluntary motion of the body overall.

As a result, the feet, the major interface between the subject and the world, form a surface on which the exchanges with the terrestrial path are concentrated. This dialog with the world “via the feet” attains an extreme degree of sophistication in land-bound mammals for which the acquisition of bipedia was doubtless a decisive factor in humanization. On the contrary, in water, the whole body can resist. The feet, on the ground path, constitute the sole place of resistance to weight and represent a kind of reduced model of the body on which the motor orders sent by the nervous structures are concentrated. Thanks to nerve plasticity and learning mechanisms, other bodily interfaces may develop. The belly and the back in contact with the fabric of the trampoline, or even the hand of the acrobat will then take over from the feet.

Apart from their mechanical requirements, balance and motion also respond to physiological constraints, the understanding of which has increased greatly over the last 20 years. Like all motor activities oriented in space, balance can be described according to a succession of operations of sensorimotor transformation during which the various sensory signals must be transformed into muscular actions. This – theoretically simple – input-output system is in fact more complex than it appears, since it relies on the growing number of observation systems that were requested: first intrinsic ones, for the sensory organs located in different parts of the body (the retina, the tactile surface of the feet, or neck muscles, etc.), then extrinsic for the motor stage that is organized in a three-dimensional environment and which takes account of the gravitational vertical, an absolute measure from which the bodily movements must be defined.

The difficulty that the robot engineers have in developing machines equipped with as many muscle motors as are to be found in man and that represent so many degrees of freedom to control and changes of reference point to be specified in advance illustrate the mathematical complexity of the sensorimotor transformation operations. In developmental terms, phylogenetic evolution has selected biological solutions that simplify the coordination of the elements of a complex system, such as the human body. Hence, we now know that the nerve command is aimed at groups of muscles in the form of an activity plan in “synergy” instead of with the muscles separately. This control principle also seems valid when it comes to the articulations that are not commanded independently but that co-vary. Other solutions that facilitate motor coordination consist in finding postural configurations that facilitate the changes of viewpoint and the passage from a bodily benchmark to an external benchmark. For example, if the head is stabilized in rotation in the sagittal plan during walking or even during a pirouette around the longitudinal axis of the body, the interpretation of vestibular signals is simplified, since the measure of the accelerations from the head in a single benchmark is done both egocentric (the head stabilized and aligned on the gravity vertical) and allocentric (the gravity vertical).

For a long time the traditional approach to the balancing functions consisted of unbalancing the posture on the basis of random movements of the support base. The stimulation produced in these cases is a localized reaction that is principally at the level of the point of application of the disturbance, essentially the feet. The study of the role of other parts of the body, such as the cephalic segment, was therefore neglected. Even though the pathology had already shown the major effects of vestibular lesions on the cephalic posture and balance, or even the presence of asymmetrical postural hypertonia of the paravertebral muscles after dystonia of an oculomotor muscle, these studies were essentially focused on the postural reactions of the lower members. In effect, the simple displacement of the subject’s look to the left, when the subject is holding his head still leads to an increase of tone in the muscles of the right side of the neck and by irradiation to all the adjacent articulations. When standing, a displacement of the look may lead to postural reactions. In addition, in some experimental conditions, we can manipulate the tonic equilibrium of the ocular muscles and the propriocetive nerve messages (with tendon vibrations) and provoke significant postural effects.

Furthermore, the paradigm of the postural reaction, which is inspired by the model of reflexological physiology from the beginning of the 20th century, has favored the use of concepts such as “stabilization” and “postural compensation,” in which lack of balance is canceled thanks to reflex loops set off by specific sensory relations. In such a type of approach, which favors the model of static equilibrium, little room is made for the concepts of prediction and anticipation of the sensory and mechanic consequences of the motion. The principle is simple: by immobilizing the body by reducing – according to a homeostatic mechanism – the variance between the vertical and the bodily axis.

According to the current conception of the equilibration functions, the sensory organs would belong to a kind of tool box, whose instruments would assume an independent role in bodily stabilization. Hence, a simple addition of the specific information that comes from the treatment of specialized areas would result in an optimization of these equilibration processes. Several experimental facts contradict this point of view.

First of all, the study of the properties of the sensory organs shows how inadequate they are for postural efficiency and therefore the need for an additional and integrative use. The proprioception, for instance, that detects the displacements of the segments where one is compared with the others, measures angular excursions in a reference system that is the subject’s alone, without being able to locate the position of a segment in space. The vision that is sensitive to the speed of the objects or of the body in motion cannot distinguish the displacement of the body from that of the environment. Meanwhile, the vestibular system does not distinguish gravity (an inclination of the head) from inertia (an acceleration in translation). Furthermore, the sensory organs measure different sizes (mechanical or electromagnetic) at various places in the body (the retina, the articulations or the internal ear). Therefore, the equilibrium likely results from a multi-sensory merger, in which signals are put in correspondence with the signals that, taken separately, cannot supply coherent information. When a receptor fails, if the completion of a posture of control is weakened (by single-foot support or the eyes closing), this required an even more effective integration of the remaining signals and a greater contribution of the attention processes.

The signals coming from the sensory organs are, furthermore, included into nerve structures whose specialization is become more and more disputed. The results gained in neurophysiology show the existence of multi-potent nerve cells that respond without distinction to signals coming from the muscles, the skin and the retina. The cartography of the nervous system, which first appears topological, is now functional. In other words, the perception in terms of nerves is very close to the action and the work that shows the reciprocal links between action and perception confirms the intuition of Bergson for whom “There is no perception that is not extended in motion” (Bergson, Matière et mémoire [Matter and Memory], 1896).

Another argument in favor of the multimodal treatment of sensory signals is that if what balance does is control movement of the center of mass, then a convergence of the information is vital for the control of that virtual part of the body that is not equipped with sensory receptors.

When one receptor provides a position datum, whereas another detects motion, the nerve structure in charge of integrating the contradictory signals informs the subject that there is a sensory conflict by setting off nauseous reactions. These conflicts occur in astronauts moving in weightlessness. The space station that is equipped with a ceiling and a floor supplies a visual environment that is structured with a top and a bottom, whereas the astronauts’ vestibular and tactile systems no longer observe the gravity vertical. The astronauts often tell about how they cannot find their way around the space module with their eyes closed.

In summary, the use of sensory signals is possible only via an arrangement that works as a team. In a standing posture, the tactile signals of the soles of the feet are systematically coupled with a vestibular signal that detects the earth’s gravity. In situations of fall or weightlessness (equivalent situations of lack of bodily contact with a support), both these types of signals disappear. It is no doubt thanks to the coupling of the two sensory organs and the equivalence of the sizes that they measure (spatial orientation) that the patient with vestibular injuries is again capable of producing balanced walking.

The principle of cooperation and mapping of the sensory signals on which the perception of the oriented body seems to be built also leads the observer to consider the equilibration functions as a synthesis between various parameters that are useful for maintaining balance: the geometric arrangement of the segments, the acceleration of the head or even the orientation of the head with respect to gravity. It can be supposed that out of the linking of the various data comes the estimation of abstract sizes such as, for example, the position of the center of mass or what is often referred to as “body schema.”

Just like the alternation between day and night, the gravitational field created by the earth’s mass is part of the ethological constraints that have been acting since the dawn of days. Gravity, which provides and takes away body equilibrium, “envelops the body” and acts on all the body’s segments simultaneously. In weightlessness, the sensory stimulations are local and uncoupled. The feet become phantom members in an environment in which the subject must go looking for the strength (the support) that stabilizes or propels, whereas on earth this force is permanent. Gravity, unlike the other sensory entries, represents a permanent action of the environment form which one cannot extract oneself.

This continuity of effect likely causes a complex of sensorimotor solutions that are selected at the times of evolutionary leaps. Hence, the modes of locomotion are the product the successful adaptation in an earth environment that has been memorized in the form of primitive motricities. The postural reactions that are set off in the infant by which it inclines its body by comparison with the vertical may be these primitives of the perception of the gravitational field. All the reflex activity (vestibule-ocular reflex, automatic walking set off by pressure that simulates the forces of reaction to the weight –to gravity– under foot, etc.) illustrates the initial resources possessed by the new-born. The pre-existence and pre-cabling of these solutions is one form of recording of the mechanical effects of earth’s gravity that operates on the Stimulus mode (exogenous action) => Response (reaction in the physical sense of the word). On a planet with a different mass, this pre-equipping would lose all usefulness. It is the equivalent of the immune system faced from birth with lymphocyte/microbe interactions and the mechanism of precognition of the earth’s environment.

In ontogenetic terms, the first stages of perception-building are restricted to setting up an efficient resistance (postural tone) to the gravitational force that keeps the body on the ground, restricts displacements and action on things. Once the balanced vertical station is mastered, the child makes the phylogenetic voyage over again, at high speed. The hand becomes free and exploration becomes possible: gravity no longer acts as a constraint, but is used as the motor of intentionality (a deliberate order without support produces no displacement), of attitudes, an external force with no cost and from which one can benefit. Locomotion is in fact the optimal transformation of potential energy into kinetic energy of the inverted pendulum. Anti-gravitational orientation is set up first in a space marked off by the body’s envelope. As soon as I can predict the effect of this force acting upon me on which I am acting, displacement becomes possible, space grows and becomes more complex.

The physiological explanation of equilibration is therefore not reducible to a collection of localized reflex loops on the medullar level that responds only to the laws of cybernetics. Even if complex calculations can be done at certain stages of the spinal cord, new experimental approaches should be invented to verify that balance is indeed a cognitive activity that brings into play central internal models. The existence of illusions of motion, vertigo and out-of-body phenomena are all indirect behavioral proof of the integrative and central treatment of signals that are not classically considered to be objects of study for the neurosciences.

The peaceful posture of the body on a stable ground structures objects and the world of perception in which it is the constant reference thereof. Hence, keeping the body in balance is part of building the subjective vertical as an axis of reference of the perceived world. All our perceptual activities take place against a background of constant experience, which is that of being freely able to move from a position of rest, and of being able to stay at rest on the ground at will, which is itself apprehended as unchangeable. Balance is therefore a forerunner to the constitution of internal models of a world which is not “topsy-turvy,” but which possess a stable structure, and models of actions that one can exert on things. Furthermore, balance, which is a consequence of the struggle against gravity, expresses our active resistance to earth’s attraction, and the emotions and subjectivity show up via our bodily attitudes. For these reasons, the use of images or demonstration by the teacher is doubtless very useful in pedagogical terms, during phases of acquisition and reinforcement of postural sequences.

Muscular contraction (“postural tone” for physiologists) that erects the muscular-skeletal system against the mechanical action of gravity is a first act of consciousness that is then “put into parentheses.” The relationship between intention and consciousness to gravity, more than a biomechanical arrangement, comes out of a perceptual expectation, an attitude (that of a standing man preparing to climb stairs) or even out of a preparation for action towards a specific object of the environment. Action, perception and intentionality are thus significantly constrained by the gravitational field. Is it not the standing station that enables us to validate the world? It is actually from this position that the exploration of the world begins.