Balance is the ability to perceive the force of gravity and to adapt our body accordingly to it and to other external forces.

From this definition, it emerges that equilibrium is a continuous search, a continuous process of change to which our body is constantly subjected.

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Let us see what this continuous research consists of and why is that possible.

First of all, the position of a body in space depends on the centre of gravity.

The centre of gravity is the point of application of all the “weighted” forces on a body. Its position changes respecting the shape and position of all the parts that form the body.

In the upright position, the centre of gravity is located in front of the upper third of the sacrum, about three fingers below the navel.

The vertical “ideal” line that passes through the centre of gravity is the line of gravity.

The centre of gravity is projected onto the ground in the base of support

  • If the projection of the centre of gravity remains in the base of support we are in equilibrium. 
  • When it comes out of the base of support we lose stability and to maintain balance we have to do muscle work or to change the base of support quickly.

There are different types of equilibrium depending on the position of our centre of gravity. 

Stable: the centre of gravity is under the base of support. For example, if we cling to an iron bar with our arms, our centre of gravity will be located under the base that in this case, is precisely the bar. 

Unstable: the centre of gravity is above the base of support, e. g. when we are standing.

Indifferent: the centre of gravity is at the same level as the support base like when we lie down on the ground. 

The equilibrium can also be static or dynamic. 

Static: when a body is not subject to forces that change its state of calm

Dynamic: when the body is subjected to a uniform straight motion

So when we stand we are in a dynamic and unstable equilibrium. 

In addition to the centre of gravity, the following are necessary for our balance:

  1. The foot;
  2. The eyes; 
  3. The ears;
  4. The brainstem.

The foot

A perfect structure expressly built to withstand the weight of the body thanks to its 3 arches which, for its wonderful functionality, can be compared with the Gothic vault.

The foot has three arches:

  • Medial between the 1st metatarsus and the heel;
  • Lateral between the 5th metatarsus and the heel;
  • Cross between the 1st and the 5th metatarsus.

And three points of support:

  • 1st metatarsus;
  • 5th metatarsus;
  • Heel.

The muscles that outline the arcs are the flexors of the fingers and the long flexor of the toe.

The medial arch or even the internal arch is the part most involved in dynamic equilibrium, while the 1st metatarsus is essential for walking.

The eye 

Our perception depends for 75% on the eyes.

The ability to move is due to the possibility of perfectly representing the world around us and the perception of our position in the world. 

The ocular system includes: 

  • the oculus motor nerves: they originate from the oldest part of the brain or from the midbrain; 
  • the spinal cord: it allows motor control. 

They constitute the black box that serves for balance, posture, gaze, body perception.

Setting aside the desire, the propensity for an object, we help to stop our body. Focusing our attention on one point allows us to decrease the distractions, the stresses that lead us to move for achieving a more or less static state of equilibrium.

The ear

The organ of equilibrium is formed by the three semicircular canals, arranged perpendicularly to each other and also by the cavities of the utricule and the saccule. All these bone cavities are covered with membranes with ciliated cells and containing endolymph. The three channels are in communication with each other and each has a dilated end forming an ampulla, inside which hair cells are immersed in the endolymph and in a gelatinous structure called cupula. It is also necessary to remember the otoliths, tiny granules of limestone covered with gelatinous substance, contained in the endolymph.

Depending on the movement of the head, the vestibular apparatus responds differently.

Information about the features of acceleration and speed of head’s movement in space are sent from the labyrinth to the nervous system.

Our posture is upright and rests on a relatively narrow base (the feet) while the head is a heavy structure at one end of a lever system, that’s why information about its movement in space are essential in maintaining balance.

The brainstem and the balance system 

Foot, eyes and ears are essential but nothing without the brainstem.

The information reaches the vestibular nuclei, located in the brainstem, which represent the true organ of balance.

They receive the information of all postural sensory receptors (vestibule, skin, proprioceptors and visual exteroceptors, they processed these information together with the reticular substance and under the control of the cerebellum and of the cerebral cortex, allowing the balance system (postural tonic system) to perform its task, that is to ensure the correct postural arrangement both static and dynamic.

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