**2. Structural and functional symmetry of the body**

Majority of living organisms that have evolved in the gravitational field of the Earth, except for a large part of protozoa, known as non-axial, developed at least one symmetry axis and usually a symmetrical arrangement of many internal organs and body parts. The symmetry of the body is, therefore, a basic and quite specific feature of the structure, defining the distribution of its parts in relation to the adopted axis or plane. Using the symmetry criterion, the organisms can be divided into:


Each of the organ pairs existing in the living organism (with bilateral symmetry), despite the high level of structural distinctiveness, functions in a substratecoupled manner with odd central organs (heart, liver) by means of mirror elements of the vascular system and information coupling with the brain via the peripheral nervous system, seeking to maintain their structural and functional symmetry. It has been proved that information and ion-molecular regulatory phenomena that penetrate from the microscopic to macroscopic scale, by maintaining functional symmetry, also tend to maintain structural symmetry that can be found in shape geometry at all levels of twin organs (e.g., limbs). These features can be described by the laws of formal logic and reflected in a model as structural-functional symmetries of parameters, not only amplitude-angle but also thermo-emissive, rheological, magnetometric, resistive, and electrometric. As mentioned above, one example can be balance in a seemingly static standing posture, which is a characteristic feature of human motor activity, defined as the body's ability to maintain body position without assistance, allowing to maintain this state during and after specific activities [20]. Balance in its essence is therefore a dynamic act of mobility, whose stability is conditioned by the efficiency of spatial neuromuscular coordination in the field of statokinetic microreflexes. Its expected result is the projection of the body's gravity center into a closed critical curve of the supporting plane, thus maintaining the standing posture. This feature is one of the external manifestations of

#### *Recent Advances in Biomechanics*

the homeostasis of the body, which tends to keep symmetry of function in the twin body organs. Thus, both the statokinetic micromovements that condition maintaining a relaxed standing posture and more easily discernible features of the symmetry of gait phases indicate the efficiency of the osteoarticular, muscular, and nervous systems. Another variable characteristic of balance is stability, that is, the ability to restore the position of the body in space, that was lost as a result of external forces or one's own physical activity.

## **3. Anatomical and functional outline of balance phenomena**

There are three main groups of receptors adapted to the perception of the data necessary for the smooth functioning of the regulator in control of body balance (seen as an irregular body), namely, the vestibular system, the proprioceptor system, and eyesight and hearing under certain conditions. *Signals generated by these structures are a source of information about the position of the body and its orientation relative to external and internal reference systems* [21].

In the process of postural balance control, human brain creates an internal reference system in the form of a symmetric, proprioceptive simulation, which is a dynamic, spatially symmetric strain gauge load distribution model in the body's schemes of postural and motor habits, calibrated by an external reference system and engrams of acquired variants of gravity deviations [22]. This model is referred to as an external visual reference system, which is a dynamic, optical model of the external space, which surrounds the subject [23–25]. Under specific conditions, acoustic model can also be considered [26]. One of the elements of the subject's interaction with an external reference system is the need to determine the relationship of its own dimensions in relation to the marked, repetitive components of the environment. Archaic measurement systems (where, e.g., foot and cubit serve as a unit) were created in this mechanism. Internal and external markers allow to monitor the deviations of the center of gravity and inertia of the body and even its individual parts from the state of balance. Anatomy-wise, eyesight and the vestibular system are organs that monitor the position of the head in space, whereas proprioreceptors and exteroceptors form a network of sensors covering the whole body. Each of these recorders collects a different type of data that individually affects the operation of the regulator and systemic balance control in a specific way [27].

In this context, it is worth to mention the structure of the vestibular system, which is composed of three semicircular canals located in mutually perpendicular planes. On the inner walls of the channels, there are receptors (hair cells) transmitting information on the direction and speed of movement of the head. Other parts of the vestibular system, saccule and utricle, are located at the base of the semicircular canals. From a structural point of view, they are two chambers filled with endolymph, in which the bottom walls have concentrations of hair cells—their cilia are located between the otoliths submerged in endolymph, which are built of crystals of calcium salt crystals. Their function is similar to the seismic mass in accelerometers. Signals from these organs transmit information about the static position of the head in space, first to the spinal cord via the vestibular nuclei. This is followed by the integration of signals from the otoliths, cerebellum, and spinal cord in the lateral vestibular nuclei (Deiter's nuclei), where efferent signal sequences that stimulate α- and β-motor neurons of the spinal cord (in control of muscles) are being generated [28].

As already mentioned, the regulation of spatial orientation, of balance in particular, is also based on the analysis of the image registered by the eye. In this case, the role of the external reference system is performed by the geometrical relations

**51**

*Biomechanics as an Element of the Motion Clinimetry System*

and movement speed of the body modules [35–38].

of the spatial location of other objects on the proximal and distal set. In the neural network of the retina, there are specialized groups of neurons that perform loss conversion of the image fragments in order to obtain the vertical and horizontal gradients. These data have a similar role to the signals from the vestibular system

The network of proprioception receptors (proprioceptors) is located mainly in

collagen bonds connecting rigid modules (bones), i.e., in joint capsules, ligaments, and tendons, as well as in viscoelastic modules (muscles). A network of sensors sensitive to mechanical deformation generates data to create a projectionproprioceptive simulation, and after the confrontation with the model's pattern database—a proprioceptive-cognitive simulation of spatial distribution of the load in the individual body modules, the rigidity of viscoelastic elements and intermodular stress. The conversion and transmission of this data to the dimension of conscious feeling give practical information about the mutual orientation

Type I mechanoreceptors are usually composed of 3–8 recording structures (40–100 μm in size) and are usually located in the outer (fibrous) layer of the joint capsule. Their characteristic feature is low excitability threshold, enabling the generation of positional information about the angular position in the joint.

Type II mechanoreceptors are usually built of 1–2 recording structures (100– 280 μm in size), located in the inner layer of the joint capsule. Their characteristic feature is low excitability threshold, as well as generating information about the

Type III mechanoreceptors are composed of 1–2 registering structures (100– 600 μm in size) with high excitability threshold. Being located in enthesis, they

Cutaneous mechanoreceptors respond directly to the movement (as a change in shape) or indirectly, through skin contact with clothing and other objects. Type I cutaneous mechanoreceptors, known as the Meissner bodies (detectors of motion speed) and Merkel bodies, are most commonly located on the palms, feet, and lips. They are characterized by a lack of spontaneous activity and high sensitivity to skin movements, allowing for surface type and shape cognition. Type II cutaneous mechanoreceptors, known as Ruffini and Pacini bodies, are most often located on palms, feet, and trunk. They display spontaneous activity for skin deformations and stretching resulting from mechanical stimuli in areas distant from the receptor. In their action, they have a static component, dependent on the strength of the stimulus, and a dynamic component, dependent on the rate of parameter change of the stimulus [39]. The cerebellum receives and triggers information from the vestibular receptors, proprioreceptors, exteroceptors, and telereceptors in the functional buffer, forming model equivalents of the state of balance at T1, T2, etc., Tn, causing the evolution in the center of gravity's motion toward the central area of the supporting plane's critical curve. When the pyramidal tract is activated by the control sequence of conscious movement, especially when there is a significant shift of the center of gravity, it activates an involuntary, multi-muscle sequence of movement, resulting

The simplest model describing standing posture is the inverted pendulum model. According to it, stability in standing position requires data regarding the position of the upper end of the pendulum (head), as well as monitoring the rake angle relative to the supporting plane. The main mechanism is the proprioception of

generate alarm information about muscle and tendon overtension.

*DOI: http://dx.doi.org/10.5772/intechopen.92757*

[29–34].

direction of motion.

in balance correction [27].

**4. Model and outline of balance mechanisms**

*Recent Advances in Biomechanics*

or one's own physical activity.

the homeostasis of the body, which tends to keep symmetry of function in the twin body organs. Thus, both the statokinetic micromovements that condition maintaining a relaxed standing posture and more easily discernible features of the symmetry of gait phases indicate the efficiency of the osteoarticular, muscular, and nervous systems. Another variable characteristic of balance is stability, that is, the ability to restore the position of the body in space, that was lost as a result of external forces

There are three main groups of receptors adapted to the perception of the data necessary for the smooth functioning of the regulator in control of body balance (seen as an irregular body), namely, the vestibular system, the proprioceptor system, and eyesight and hearing under certain conditions. *Signals generated by these structures are a source of information about the position of the body and its orientation* 

In the process of postural balance control, human brain creates an internal reference system in the form of a symmetric, proprioceptive simulation, which is a dynamic, spatially symmetric strain gauge load distribution model in the body's schemes of postural and motor habits, calibrated by an external reference system and engrams of acquired variants of gravity deviations [22]. This model is referred to as an external visual reference system, which is a dynamic, optical model of the external space, which surrounds the subject [23–25]. Under specific conditions, acoustic model can also be considered [26]. One of the elements of the subject's interaction with an external reference system is the need to determine the relationship of its own dimensions in relation to the marked, repetitive components of the environment. Archaic measurement systems (where, e.g., foot and cubit serve as a unit) were created in this mechanism. Internal and external markers allow to monitor the deviations of the center of gravity and inertia of the body and even its individual parts from the state of balance. Anatomy-wise, eyesight and the vestibular system are organs that monitor the position of the head in space, whereas proprioreceptors and exteroceptors form a network of sensors covering the whole body. Each of these recorders collects a different type of data that individually affects the operation of the regulator and systemic balance control in a specific way [27]. In this context, it is worth to mention the structure of the vestibular system, which is composed of three semicircular canals located in mutually perpendicular planes. On the inner walls of the channels, there are receptors (hair cells) transmitting information on the direction and speed of movement of the head. Other parts of the vestibular system, saccule and utricle, are located at the base of the semicircular canals. From a structural point of view, they are two chambers filled with endolymph, in which the bottom walls have concentrations of hair cells—their cilia are located between the otoliths submerged in endolymph, which are built of crystals of calcium salt crystals. Their function is similar to the seismic mass in accelerometers. Signals from these organs transmit information about the static position of the head in space, first to the spinal cord via the vestibular nuclei. This is followed by the integration of signals from the otoliths, cerebellum, and spinal cord in the lateral vestibular nuclei (Deiter's nuclei), where efferent signal sequences that stimulate α- and β-motor neurons of the spinal cord (in control of muscles) are

As already mentioned, the regulation of spatial orientation, of balance in particular, is also based on the analysis of the image registered by the eye. In this case, the role of the external reference system is performed by the geometrical relations

**3. Anatomical and functional outline of balance phenomena**

*relative to external and internal reference systems* [21].

**50**

being generated [28].

of the spatial location of other objects on the proximal and distal set. In the neural network of the retina, there are specialized groups of neurons that perform loss conversion of the image fragments in order to obtain the vertical and horizontal gradients. These data have a similar role to the signals from the vestibular system [29–34].

The network of proprioception receptors (proprioceptors) is located mainly in collagen bonds connecting rigid modules (bones), i.e., in joint capsules, ligaments, and tendons, as well as in viscoelastic modules (muscles). A network of sensors sensitive to mechanical deformation generates data to create a projectionproprioceptive simulation, and after the confrontation with the model's pattern database—a proprioceptive-cognitive simulation of spatial distribution of the load in the individual body modules, the rigidity of viscoelastic elements and intermodular stress. The conversion and transmission of this data to the dimension of conscious feeling give practical information about the mutual orientation and movement speed of the body modules [35–38].

Type I mechanoreceptors are usually composed of 3–8 recording structures (40–100 μm in size) and are usually located in the outer (fibrous) layer of the joint capsule. Their characteristic feature is low excitability threshold, enabling the generation of positional information about the angular position in the joint.

Type II mechanoreceptors are usually built of 1–2 recording structures (100– 280 μm in size), located in the inner layer of the joint capsule. Their characteristic feature is low excitability threshold, as well as generating information about the direction of motion.

Type III mechanoreceptors are composed of 1–2 registering structures (100– 600 μm in size) with high excitability threshold. Being located in enthesis, they generate alarm information about muscle and tendon overtension.

Cutaneous mechanoreceptors respond directly to the movement (as a change in shape) or indirectly, through skin contact with clothing and other objects. Type I cutaneous mechanoreceptors, known as the Meissner bodies (detectors of motion speed) and Merkel bodies, are most commonly located on the palms, feet, and lips. They are characterized by a lack of spontaneous activity and high sensitivity to skin movements, allowing for surface type and shape cognition. Type II cutaneous mechanoreceptors, known as Ruffini and Pacini bodies, are most often located on palms, feet, and trunk. They display spontaneous activity for skin deformations and stretching resulting from mechanical stimuli in areas distant from the receptor. In their action, they have a static component, dependent on the strength of the stimulus, and a dynamic component, dependent on the rate of parameter change of the stimulus [39]. The cerebellum receives and triggers information from the vestibular receptors, proprioreceptors, exteroceptors, and telereceptors in the functional buffer, forming model equivalents of the state of balance at T1, T2, etc., Tn, causing the evolution in the center of gravity's motion toward the central area of the supporting plane's critical curve. When the pyramidal tract is activated by the control sequence of conscious movement, especially when there is a significant shift of the center of gravity, it activates an involuntary, multi-muscle sequence of movement, resulting in balance correction [27].

## **4. Model and outline of balance mechanisms**

The simplest model describing standing posture is the inverted pendulum model. According to it, stability in standing position requires data regarding the position of the upper end of the pendulum (head), as well as monitoring the rake angle relative to the supporting plane. The main mechanism is the proprioception of pressure distribution of the ankle joint surfaces, changes in lower leg muscle's length and tension, as well as angle changes between axis of the foot and lower leg [40].

Balance loss prevention is effective provided that the nervous system is able to recognize in less than 70–100 ms, the characteristics (mainly direction and force) of a destabilizing stimulus, and raise a competent engram containing a set of drivers for adequate muscle synergy (rapid coordinated movements compensating instability), which would restore synergy [41]. The speed of the motor reaction (which restores the balance) to the stimulus decreases in proportion to the number of alternative patterns of motion behavior, existing in subject's memory. Hence, the adoption of a specific position (bending body forward) limits the choice of a large number of possible alternative movement patterns, reducing time of a proper coordination scheme to access the motoneurons of the pyramidal and extrapyramidal tracts [42]. It reduces the time to generate a motion sequence, which corrects the displacement of the center of gravity outside the critical curve of the supporting plane, increasing the stability. This behavior can be seen in young people, walking on an unstable surface (adaptation), or in the elderly, even on a stable surface (involution). A strategy for (slightly disturbed) balance recovery in subjects standing on a stable surface has been described, where corrective sequence begins with the contraction of the ankle joint muscles (ankle joint strategy), as well as another strategy, in people standing on a narrow ground, which begins with thigh and trunk muscle contractions, and further includes lower limb muscles (hip joint strategy) (Horak, Neshner). The third way for balance recoFect the body from falling down (step strategy) (Nesher) [20].
