New technological tools for the evaluation and training of postural control and its validation with "gold standard" systems in biomechanics laboratory.

Presentation

This report is part of the project "BABEN - BALANCE BENCHMARK: New technological tools for the evaluation and training of postural control and its validation with "gold standard" systems in biomechanical laboratory", a project whose main objectives are: "BABEN - BALANCE BENCHMARK: New technological tools for the evaluation and training of postural control and its validation with "gold standard" systems in biomechanical laboratory": 

1. Project 1 - Development of a postural control evaluation and training system
2. Project 2 - Validate the functioning of the system developed in "Project 1", as well as its evaluation and training functions, in comparison with "gold standard" systems. 

In general, the final objective of the project is to provide a valid and reliable system, easy to use, highly portable and with controlled costs, which allows a more democratised and widespread evaluation and training of the capacity for balance and postural regulation of the elderly, minimising the risks of falls and the respective repercussions on morbidity and health costs. However, it is intended that the system can be used by other populations, healthy or not, in which it is recommended to evaluate and train this motor skill and others associated with it. 

The new system under development and evaluation, developed by TECNALIA, is called EQUIMETRIX and incorporates innovative solutions that give it great portability and consistency with the systems considered reference in laboratory biomechanics. 

The EQUIMETRIX system is based on the estimation of the subject's Center of Mass (CM) through a "fixed" anatomical point, based on the image, and on assuming its vertical projection as a valid representation of the Pressure Center (PC) -point of application of the force resulting from the reaction of the soil to the support- mainly in a bipedal orthostatic position. The PC must then remain within the support base, defined by the area between the limits of the support, which is measured through a contact mat, and the structure of its migration must be studied from a stabilometric, statokinesigraphic and upwelling perspective, as well as the definition of the stability limits. 

Validation in the biomechanics laboratory will be carried out by comparing the results provided by the EQUIMETRIX system and the systems considered as reference systems for the evaluation of equilibrium capacity and postural regulation, i.e. the stabilization of the generalist force platform (using the evaluation and treatment of the CP migration data in the soil reference plane, i.e. in relation to the migration zone, the amplitude and speed of migration, as well as the rambling and trembling velocity). A comparison will also be made with the results obtained with those provided by the dedicated systems available on the market for medical and biomechanical devices: the BIODEX BALANCE SYSTEM. 

Since EQUIMETRIX is based on the estimation of the specific kinematics of the CM - which is not of major importance in the assessment of postural balance and regulation, rather than that of the CP - this fundamental assumption will be further validated. For this, the data provided will be compared with the vertical projection of the CM determined by the optoelectronic MoCap of retro-reflected infrared light (Qualisys System, Sweden) with 12 cameras, going through the kinematic modeling of the whole body with the IOR model of markers and subsequent reconstruction in V3D (C-Motion, USA), defining the different segments with 6 degrees of freedom. This approach can be reinforced by a simultaneous comparison with an inertial whole-body biomechanical modelling system (XSENS, The Netherlands). 

As the biomechanical tools referred to in the previous paragraph are not normally used as reference tools in the assessment of the equilibrium, they will not be analysed in this report, although their inclusion in the "beta version" is foreseen. 

The ability of the EQUIMETRIX device to improve the training of the neuromotor capabilities in question will be assessed by pre and post tests, respectively, before and after the proprioceptive training intervention, performed on all the devices mentioned. 

Since these are procedures, methods and variables that have been developed, applied and selected over a long period of time, the volume of specialized scientific publications is very significant. However, the convergence in terms of the relevant variables seems to be very high. For this reason, we focus this literature review on these variables and evaluation domains, preferably using the most frequently cited literature. We do not consider the dedicated devices of the competition. 

From the total of more than nine hundred texts analyzed, it is expected to implement a "beta version" of the state of the art, extracting in "meta-analysis" reference and cut-off values that support a critical analysis of the records obtained in the sample groups considered in the validation process. 

State of the art in stabilometry and postural control: first results of a systematic review 

1. Introduction 

The ability to balance is a necessary condition for postural control, whether static or dynamic, and is considered an essential element in daily activities and sports (Almeida et al., 2016; Greve et al., 2013; Dawson et al., 2018). To maintain postural control, it is necessary to maintain the vertical projection of the center of mass (MC) of the body within its support base (the support base is defined as the surface in the reference plane of the ground limited by the extremes of the subject's support) during the erect posture (Duarte and Zatsiorsky, 1999), with the nervous, sensory, vestibular and locomotor systems involved in this regulation. These systems are responsible for transmitting information to the somatosensory cortex, interacting with the neuromuscular system and providing an adequate motor response (Delahunt et al., 2013; Almeida et al., 2016). 

The vertical projection of the CM is decisive in this context, since, respecting the principle of minimum energy, the systems responsible for regulation and postural balance will try to make it coincide with the Pressure Centre (PC), defined as the point of application resulting from the reaction force of the soil to the support. 

Normally and in most situations, the vertical one passing through the CM and the one passing through the CP are almost coincident. Given the relationship between the CP and the CM of an individual, the regulation of its position is determined, in the first place, by the stability of the CM (Duarte et al., 2000). This depends on the postural control related to the position of the body segments, as well as on the forces that act circumstantially on the body, without overloading or destabilizing the sensory systems. Therefore, the process of postural regulation and equilibrium is triggered by the integration of the sensory input and the motor process (Duarte and Zatsiorsky, 2002), that is, the postural control strategy is dependent on the task and the environment (Duarte and Zatsiorsky, 2002). 

Given the complexity of the systems involved in postural regulation and equilibrium, it is necessary to understand what methods are currently used to accurately measure and trust the processes and variables involved in this regulation. To this end, a systematic review of the literature has been initiated with a view to compiling the best practices of the scientific community, while identifying the variables, as well as the methods, processes and equipment used for this type of assessment. 

2. Methodology 

The systematic review was carried out by one of the researchers in the main databases of scientific publications: PubMed, Web of Science, SCOPUS. The key to the research used was limited to searching for titles, published before 2019, with the following structure: ("Postural Control" OR "Balance" OR "Stabilometry" OR "Stabilogram" OR "Statokinesiogram") AND ("Device" OR "System" OR "Method" OR "Assessment" OR "Evaluation").

The analysis of the results was carried out in accordance with the guidelines established by the PRISMA Procedure: Preferred Reporting Items for Systematic Reviews and Meta- Analysis (Moher et al. 2009, 2015). After the elimination of duplicates, articles that did not meet the inclusion criteria (IC) were also excluded: CI1 - original articles and CI2 - articles written in English. A title selection process was then carried out independently by two researchers in order to identify whether the titles were within the scope of this work. Articles meeting the rejection criteria were excluded: R1 - papers on subjects other than balance and postural control from a biomechanical point of view, or relating to physical principles, and/or balance chemistries and R2 - studies focusing on animals or robots. In cases of non-agreement between the elements that carried out this triage, a third researcher and his choice was used to determine the inclusion or exclusion of the article. 

The sequence of these procedures and the results obtained at each stage are described in Figure 1. 

3. Results

As no start date was set for the search, the number of results obtained was considerably high (n=17,118). Of these, only 6,351 records were duplicated due to multiple database searches. 

Of this set of documents, 1,763 were identified as being written in a language other than English, and Chinese, German and Russian were the most frequent languages in the results. Among the 9,004 documents written in English, those that were not published as original articles in a journal were also eliminated. This resulted in the exclusion of 3,237 documents, which included publications of various types at scientific conferences or meetings (n=2,584), reviews (n=258), books or chapters (n=131) and others (n=263) in which editorial publications, letters and notes were included. 

A total of 5,767 original articles published in English were found, after which the title selection process began. At this stage, 4,823 articles related to subjects not focused on the biomechanical study of equilibrium and postural control were identified, and 41 articles that, although possibly related to the scope of this work, were made with animals or robots. At this stage of the systematic review, a total of 903 illegible articles were found to proceed to the next stage. 
(Figure 1). 

4. Discussion

Given the volume of literature selected for analysis, which will be the subject of an exhaustive analysis in the beta version of this "State of the Art", this document will present a necessarily brief review of the most relevant information gathered from a selected set of articles resulting from this systematic review, which makes it possible to establish with high confidence the current state of knowledge on the systems of balance regulation and postural control, as well as their evaluation. 

4.1 Biological regulation systems 

Different biological systems are used to regulate postural stability: 

(i) the sensory system provides information on the position of body segments; 
(ii) the motor system activates the musculature to perform the intended movement
(iii) the nervous system coordinates the connection between the sensory and motor systems (Lestienne and Gurfinker; 1988; Duarte and Zatsiorsky, 2002). 

In general, children, adults and the elderly, whether athletes or not, may suffer from postural balance disorders, which may be related to perceptual problems with decreased muscle strength, joint disorders, medication use, and aging (Wegener et al., 1997; Ruwer et al., 2005; Duarte et al., 2000). In this context, imbalance can impair motor function, that is, impair the quality of sports gestures or increase the risk of falls and injuries (Almeida et al., 2016; Delahunt et al., 2013; Coughlan et al., 2012). Therefore, it is fundamental to establish strategies for the prevention of imbalance and the preservation of postural regulation mechanisms and their evaluation, in order to obtain information that allows understanding the relationships of each of the different variables and, in this way, to produce guidelines on the appropriate way to guide the evaluation of the regulation of balance and postural regulation. 

4.1.1 Neuro-motor system 

In order to maintain postural balance, information is required about body segments and the forces acting on them. For this, the somatosensory, visual and vestibular systems must act in a precise and integrated way; in short, synergistically (Ruthwell, 1994; Duarte and Zatsirosky, 1999). In addition, it is necessary that the nervous system and the motor system act concomitantly in this process (Chandler, Duncan, 1992), that is, the responses and strategies of body movement must be adjusted according to the feedback of the nervous system (Bankoff, 1996). 

4.1.2 Sensory system 

In relation to sensory systems, the visual system is considered one of the most important and the one that most seems to determine equilibrium (Latash, 1997). Its functioning implies mechanisms related to the capture of environmental information, that is, the position of the body and its members in relation to the surrounding environment (Guyton, 1986). The somatosensory system allows, together with the central nervous system, to understand the position of the body in the environment, mainly through touch and pressure (Guyton, 1992). The vestibular system coordinates the position of the head, the force of gravity and the forces of inertia resulting from the linear and rotational movements of the head. Duarte and Zatsiorsky (1999) emphasized that the vestibular system has two types of receptors to obtain head orientation and movement: receptors related to angular accelerations (semicircular channels) and receptors that detect linear accelerations (utricle and sacrum) (Figure 2). 
The literature shows that these three systems act in an integrated manner; however, the specificity of each of them for equilibrium has not yet been fully clarified (Duarte and Zatsirosky, 1999; McCollum et al., 1996). Sensory information is believed to be modulable and redundant, i.e., dependent and often conflicting, because if there is some deregulation of one system, another may increase its function to maintain postural control (McCollum et al., 1996). 

4.2 Measurement and evaluation of postural balance and regulation 

To measure equilibrium capacity, the literature has reported different types of solutions, study variables and equipment, such as force platforms, podobarometric platforms and templates, electromyography and clinical tests, such as the Star Excursion Balance Test and the Y Balance Test, in addition to the use of computer games, commonly known as Exergames (Duarte et al., 2000; Dawson et al., 2018; Mochizuki and Amadio, 2003; Hertel et al., 2006; Coughlan et al., 2012). 

The most commonly used variable to evaluate equilibrium is the migration of CP and its subvariables, which reflect the effect of the forces acting on the supporting surface and therefore represent the combination of postural stability and the gravitational force regulation system (King and Zatsiorsky, 1997; Duarte et al., 2000). Migration to CP is defined as a measure of displacement, dependent on the dynamics of CM (Duarte et al., 2000; Duarte and Zatsiorsky, 1999; Duarte and Zatsiorsky, 2002), and promoted by postural oscillations resulting from the circumstantial position of the individual. This type of sign can be analyzed in the field of time and frequency (Carpenter et al., 2001). 

The stabilogram consists (generally, but not exclusively) in recording the postural equilibrium of the whole body in an erect position (Terekhov, 1976), quantified in terms of anteroposterior and mid-lateral oscillations of the CP. Stabilomeric studies can be performed under two conditions: static and dynamic. Static analysis is normally performed in the erect orthostatic position, therefore, without the subject making any movement; on the other hand, dynamic analysis is performed by applying a specific perturbation, in accordance with the proposed objective (Duarte et al., 2000). Duarte et al (2000) suggest that none of the analyses is appropriate to describe the natural position, given that in the static condition no significant and intentional postural changes are allowed and in the dynamics external forces are applied that instabilize this base position. 

4.3 Means of evaluation and postural balance 

The instruments for assessing equilibrium are associated with the variables to be assessed, the task to be controlled and the environment (Delahunt et al., 2013; Dawson et al., 2018; Duarte, Harvey and Zatsiorsky, 2000). Therefore, understanding the adverse conditions that interfere with postural balance responses and knowing the effects of postural stability through instrumentation can generate enough information to allow better clinical decision making during rehabilitation and prevention. 

As already mentioned, equilibrium can be analyzed using different equipment, such as force platforms and podobarometrics (or podobarometric templates), electromyography and clinical tests. 

4.3.1 Force and podobarometric platforms 

Since the CP is the main variable used to evaluate postural stability (Duarte, Freitas, 2010), force platforms are the ideal equipment for its measurement. The force platforms are composed of force sensors capable of measuring the components of anteroposterior, mid-lateral and vertical force, as well as the moments around the corresponding axes (Figure 3). 

According to the specialized literature, the force platform is effectively the most used instrument to evaluate postural stability (Duarte and Freitas, 2010; Duarte and Zatsiorsky, 1999; Collins and de Luca, 1993; Baratto et al., 2002). The stabilometric analysis with this equipment can be divided into global evaluation and structural evaluation. The global analysis is related to the evaluation of the oscillations of the flat position of the CP in the domain of time and frequency, while the structural analysis is related, in turn, to the data of postural stability and motor control (Duarte and Freitas, 2010; Duarte and Zatsiorsky, 1999; Collins and de Luca, 1993). 

Regardless of the type of force platform used, it must meet a number of requirements, including accuracy greater than 0.1 mm, accuracy greater than 0.05 mm, a frequency band between 0.01 and 10 Hz, and linearity greater than 90% in the measurement range (Scoppa et al., 2013). 

Pressure platforms, or podobarometers, as pressure sensitive instruments (vertical force per contact surface unit), also allow the calculation of the vertical force resulting from the soil reaction and, therefore, also from its point of application: the CP. The podobarometric templates also make it possible to determine the CP of each one (on each foot), thus also allowing the CP of the force resulting from the contralateral forces to be calculated. 

4.3.2 Electromyography 

In a situation of imbalance, a neuromuscular reorganization is necessary to recover postural stability. A pattern of electromyographic activity, especially of the musculature passing through the ankle joint complex, is also recognisable (although less evident) in the simple maintenance of the orthostatic position. Therefore, electromyography can be a useful tool for understanding, qualifying, comparing and correlating muscle activity during instability, or as a guarantee of stability (de Luca, 1997; Duarte and Zatsiorsky, 1999). Although electromyography does not quantify the activation of propriocetors, it is a tool that allows the verification of muscular activation (in amplitude, time and frequency) and the possible postural changes resulting from this activation. Delahunt et al (2006), in a study that evaluated hip, knee and ankle joints during walking, observed an increase in electromyographic activity in the muscles necessary to maintain stability during this particular activity. Electromyography therefore appears as a useful complementary tool in the analysis of postural stability and can be considered in the analysis of the results provided by traditional stabilometry.

4.3.3 Clinical trials 

When instruments are not available to allow direct and objective evaluation of robust parameters that translate the ability to balance and regulate postures, clinical tests (sometimes also called physical ability tests) are often used. Among them, the Star Excursion Balance Test (SEBT) is considered reliable for assessing dynamic postural balance. The test consists of performing movements in a sequence of 8 predetermined directions, through signals on the ground, performed in unipodal support of the lower limb.

In a review of Gribble et al. (2012), it is reported that the SEBT has good reliability for the assessment of postural balance with adequate results to measure dynamic imbalances. However, Coughlan et al (2012) highlight the existence of several protocols and point out that, due to the high number of repetitions in the eight directions, the execution time makes SEBT unfeasible for clinical evaluation (Figure 4). 

The Y Balance Test (YBT) is an instrumented version of the SEBT, which allows evaluation in three directions: anterior, medial-posterior and lateroposterior. In addition, it is performed with a greater number of repetitions, thus reducing the number of errors and the variability of execution. The YBT allows the normalization of distances according to the length of the extremity of the individual (Plisky et al., 2009). The same authors found a good to excellent ICC for the test and considered it a good method for evaluating lower extremity imbalances. 

The concurrent use of clinical trials can be considered as a complementary option for the analysis of the relevance of new devices with a more parametric, objective and controlled vocation. 

4.3.4 Exergames 

Another tool that has been used to promote postural balance (and less to evaluate it) are exergames, or virtual reality games, which are combined with body movements. 
This is a low-cost and promising tool, and the literature has shown that exergames are beneficial for improving static balance and postural control. Gatica et al (2010) and Williams et al (2010) evaluated the elderly and found positive results in postural control through the use of exergames, which may represent a good strategy in the prevention and rehabilitation of postural control. 

Considering their potential in the field of proprioceptive formation, the exergames included in the systems dedicated to the proprioceptive formation of postural balance and regulatory capacity (such as the BIODEX BALANCE SYSTEM) can be considered as an intervention instrument between pre and post-test. 

4.4 Characterization of pressure center migration 

Most of the studies related to stabilometry have been carried out with force platforms, which allow the CP to be measured as the point of application of the forces resulting from each lower limb, as well as the movements of the CM of the whole body. 

The representation of the displacement of the CP in the antero-posterior and mid lateral directions as a function of time is generally called a stabilogram. It is also possible to represent these variables as a function of each of them, and the resulting graph is often referred to as a statoquinesigram (Kapteyn et al., 1983). Zatsiorsky and Duarte (1999) established the rambling-trembling hypothesis as a way of explaining the movement of two of the components of the statoquinesigram. In this model, equilibrium is achieved by the conservative but exploratory movement of the CP, which migrates along the support base (divagation), and by the oscillations of the CP around its migration trajectory (trembling). 

Of the various methods available to describe postural control, the most common are those that describe the properties of the COP as a function of time and frequency (Prieto et al., 1996; Rocchi et al., 2004). Time parameters include the mean migration of the mean CP position, root-mean-square of the mean CP position migration, the total CP migration distance, the peak to peak CP migration, the mean CP migration speed, the circle or ellipse area that defines the migration area with 95% confidence, or the area "swept" by the CP. The frequency parameters describe the total frequency intensity of the CP signal, the mean frequency, the 95% frequency range, the centroid frequency and the frequency dispersion. 

4.5. Support base or support base 

Balance and postural control tests can be performed in bipedal or unipedal conditions (Paillard and Noah, 2015). In the bipedal condition it is possible to position the lower extremities in various positions, from the feet together to the feet apart, generally with an angle between 15 and 30 degrees and a distance between 5 and 15 cm between the medial malleolus (Mehdikhani et al, 2014; Kim, Kwon and Jeon, 2014; Paillard and Noah, 2006). Depending on the mode of positioning, a differentiated support zone is obtained, which corresponds to the zone of the ground in which the feet are in contact and to the zone between the two feet (Figure 6). 
To maintain a stable and balanced position, the CP must be within the support base (Kerr, 2010; Meyer and Ayalon, 2006), so the position of the feet affects the stability of the erect position (Mouzat et al., 2004).

Nam et al (2017) demonstrated the existence of significant differences in the activity of the trunk muscle for the maintenance of postural control, as well as the characteristics of CP according to the size of the support base. Meyer and Avalon (2006) reiterated this fact, stating that a smaller support base provides a smaller area of CP alignment, which induces less stability. 

5. Conclusions 

The biological systems for maintaining balance and postural regulation are complex and interconnected. The scientific literature is detailed in publications dedicated to the study of these relationships, which are carried out taking into account clinical trials, the electromyographic activity of the muscles involved in postural regulation, or using more robust measurement systems such as force platforms. 

However, there seems to be a consensus on the need to guarantee adequate precision with respect to the observation of the kinematics of personal computers, such as their spatial location, their ambling and trembling movement and their space-time and frequency characteristics. 

The support base is also decisive for maintaining balance and postural control, as it defines the area available for the CP to move - stability limits. 

In summary, the study of this topic is possible and should preferably be carried out through the use of appropriate equipment and the analysis of the parameters that describe the movement of the CP within the support base. 

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