Proprioception and proprioceptive sensitivity

Edited by Dr. Davide Sganzerla

The posture

The posture is a function of stabilization, consolidation and maintenance of balance. The balance is reached when the perpendicular passing through the center of gravity of the body falls inside the support plane, delimited by the outer edge of the feet. A correct posture is maintained. through a constant re-elaboration of the parameters of muscular activity, essential to keep the center of gravity inside the support base. volunteer.

The external forces they have a "constant destabilizing action, but changing in intensity and direction, therefore unpredictable. The response to these forces can only occur in the short period of time that follows their action and precedes the loss of balance; it must" be absolutely immediate and effective, under penalty of the fall to the ground. Not being able to be programmed and having to be immediate, this postural reaction must occur at the same time as the loss of balance.
THE voluntary movements instead they are the cause of other types of postural reactions. These reactions are programmed (together with the voluntary movement itself) before the motor execution. The voluntary postural reactions are simultaneous to the movement and begin a fraction of a second before the loss of balance, which is why they are called anticipatory reactions.
Postural reactions are triggered by various sensory mechanisms:

- proprioceptive afferents;
- mechanoceptive afferents;
- exteroceptive afferents;
- labyrinthine afferents;
- visual afferents.

The neurophysiological control of posture can be articulated in three distinct moments: postural control in the standing position, postural control when external mechanical forces are applied to the body sufficiently intense to give rise to a loss of balance, and postural control during the execution of voluntary movements.

Read also: The best postural correctors

However, it must be remembered that the adaptation of posture cannot always be achieved with precision and in an integrated way. The vestibule that records the variations in balance, in fact, is a lower center than the cerebellum and when a reflection of it starts in dangerous conditions it is more rapid cerebellar adaptation. It is for this reason that with a sudden variation of the support base, everything in one's hand is dropped to restore a very old reflex of searching for grip with the upper limbs. Only by knowing in advance that one will be at that juncture can the cortex prepare the body to withstand the situation and master the defense reflex. Another element that can undermine the adaptation of posture is the nociceptive reflex present on the sensory pathways such as sight and hearing. However, this is more easily controlled, as it starts from centers at least at the same level as the cerebellum.

1) Postural control in standing.

The human postural system is able to respond to the oscillations of the center of gravity given by minimal forces, with an adequate and counter force exerted in a timely manner, which allows the maintenance of the upright position. This postural response is partly linked to the intrinsic mechanical properties of the muscle, in part to a set of nervous-type reflex mechanisms which, drawing information from different sensory channels, through the action of numerous circuits, are able to modulate muscle tone in order to allow the maintenance of an upright position. Muscle tone resulting from postural reactions is called postural reaction tone. It is the identical but opposite work performed by the entire musculoskeletal system to maintain the center of gravity in its ideal position and, in any case, within the support base. The choice of the movements through which to carry out the work is a complex function : there is not a single movement of reaction to an external force, but a series of equivalent possible movements. Among these, that part of the Central Nervous System that presides over the control of posture (the cerebellum), chooses the ideal reaction movement. of postural tone does not cause electromyographic muscle activity.

2) Postural control when external mechanical forces are applied to the body capable of causing a loss of balance

The picture of postural reactions is different if the forces applied to the body are able to move the center of gravity outside the support base. In this case, the postural tone of reaction is no longer able to maintain the position of equilibrium by itself: it is necessary to introduce a real muscular work. This work leads to the reconstruction of a new position of equilibrium that maintains account of the intensity of the external force applied, of the duration, of the gravitational acceleration and of the body parameters. The postural reaction will be of the motor type. The necessary coordination for this pursuit is given by the cerebellum: its functional exclusion, in fact, leads to disturbances The circuits that govern the postural motor reactions are the same ones that control the postural reaction tone: they recalibrate themselves to the new needs, giving rise to motor reactions, rather than variations in muscle tone.

3) Postural control during voluntary movement (anticipation reactions)

The muscular reactions caused by voluntary movements are highly variable and depend on the relationship between the type of voluntary movement and the state of equilibrium. This type of postural reactions are called anticipatory, since they intervene first in the voluntary movement. In the execution of a voluntary movement, the center of gravity moves outside the support base, exactly as it happens for the intervention of forces external to the organism. In this case, however, unlike postural reactions to external forces, before the movement postural strategies are put in place to avoid imbalance and the consequent fall. Two difficult situations can be encountered: a voluntary movement that does not cause the loss of balance and a voluntary movement that causes it. If the voluntary movement does not jeopardize the state of equilibrium, the consequent reactions only have a compensating effect on the displacement of the center of gravity, are very weak and not strictly linked, in a temporal sense, to the execution of the voluntary movement. If voluntary movement jeopardizes the state of equilibrium, then anticipatory postural reactions are observed. The anticipatory postural reactions are strongly affected by motor learning phenomena: if the conditions of equilibrium change, two or three repetitions of the voluntary gesture are sufficient to adapt the postural reaction.