The objective of this study was to define the reliability and internal consistency of the accelerometric measurements of static equilibrium and gait in children aged between 6 and 12 years old. In order to the data, this method of assessment obtained positive results as an instrument for the quantitative assessment of balance in school-aged children. In the existing literature, we found no previous studies that performed accelerometric assessment in the lumbopelvic region to identify any difference between: (a) values recorded using different evaluation tests and (b) their different measurement reliabilities. Using a single accelerometer is common in studies of adult populations, however this study represents the first time that the reliability of a single device for assessing balance in children has been experimentally verified.
The results for the internal consistency and reliability of the instrument obtained in this study are positive. Nevertheless, it should be noticed that the results for children aged 6 and 7 years were only moderately positive, especially in the walking test. The normative description of the development of equilibrium and gait patterns throughout childhood maturation is complex; and it is closely related to the age of the individual, especially during the first years of independent walking [21].
By school age, children have already strengthened their gait and their ability to maintain static equilibrium. Thus, accelerometry could be used to study large groups of children, therefore future studies should establish the normative values of acceleration in: (a) static, (b) dynamic equilibrium and (c) walking.
Previous studies have provided reference databases for gait in children including: (a) temporal distance, kinematic and dynamic gait parameters of 10 toddlers aged 13.5 to 18.5 months old [22]; (b) ground reaction force patterns of more than 7000 children aged 1 to 13 years old [23]; and (c) the kinematic and dynamic parameters of 20 Chinese children aged 7 to 12 years old [24]. All of these three studies were based on the study of pressure centre with force platforms. These three studies were based on the study of the pressure center with force platforms. The pressure center is an indirect measure of the equilibrium reactions of the human body. However, the displacement of the center of gravity (CG) is a direct measure of biomechanical reactions against gravity [25]. This paradigm shift occurred after the definition of the multisegmental concept of equilibrium that defines the body as a system of rigid bodies, whose CG is the average of all the centers of mass of said segments [26].
Therefore, for a person to have a healthy control of balance (that is, to avoid falls) the determining aspect is keeping the CG under control. Such CG control can be automatic (involuntary) during activities of daily living, including activities such as walking, climbing and descending stairs, bending over or performing transfers sitting and standing, and vice versa; or voluntary, in the face of disturbances such as tripping and slipping [27].
In the field of research, the balance is usually evaluated by using force platforms. These instruments record the displacement of the pressure center, which, as mentioned, is an independent parameter of the CG and of the overall behavior of the body in the three planes of space. This parameter is subject to the inverted equilibrium pendulum theory [28], which is valid for movements that do not imply changes in the support area, it is inadequate for a holistic evaluation of the postural control system and all the strategies that this system uses to maintain balance [29, 30].
Alternatively, kinematic instruments, such as accelerometers, allow equilibrium to be objectively studied through GC analysis without great financial expenses on measurement devices, or complex data analysis processes [31, 32].
Acceleration results were increasing for all equilibrium tests as the three attempts of each test were performed. A plausible explanation for these accelerometric values is the appearance of fatigue [33], which occurs mainly in the stabilising muscles of the lower extremities (especially the hip abductors and stabilising ankle muscles), which alters the base of support and forces a readjustment of the trunk stabilising muscles (the abdominal muscles and the paravertebral musculature).
Despite the aforementioned, this phenomenon was not observed in the gait test. While NG fatigue does not appear as quick as it appears during equilibrium tests, notwithstanding it is a more complex activity than the static monopodal. In addition, it is a dynamic activity in which biomechanical actions are sequenced and coordinated between different muscle groups.
In relation to the accelerometric analysis of gait, the values for the RMS and the accelerations produced in the sagittal plane stand out as being particularly important. Both of them are in agreement with the existing literature.
Accelerations in the mid-lateral axis and the magnitude of the RMS of the accelerations have been strongly associated with the risk of falling in adults [14, 34]. This is relevant because it has been determined that falls are the most common injury mechanism in all age groups during childhood; and the origin of these falls: (a) the lack of sleep, (b) lack of concentration and (c) the deficit in the development of motor skills [35].
Studying the acceleration module is a constant in studies based on accelerometery, and measuring the magnitude of the movement has been used in almost all studies based on accelerometric analysis since this method was first introduced as a tool for assessing balance, both static and dynamic [10, 20, 36,37,38].
We should point out that the sample size is not sufficient to generalise the results obtained in the current study to the child population; however, it does confirm the reliability and consistency of static balance assessment instrument to carry out future studies that include normative values of acceleration and their evaluation percentiles according to age.
An important limitation of this study is that the results obtained do not allow us to describe how postural control systems work to maintain balance from a physiological point of view. Accelerometry is an indirect measure of the efficiency with which the central nervous system integrates information from the environment and from the subject themselves in order to maintain balance.
In the future, the possibility of expanding the sample to more specialised populations should be explored, including patients with neurological diseases such as cerebral palsy and muscular dystrophy.
The possibility of designing specific tests with accelerometric variables that would display the great deterioration in these populations should be considered. This would bring us the identification of patients in the early stages of these pathologies, as well as quantitatively evaluate specific interventions for early treatment.
It would also be of great interest to carry out a longitudinal study that relates variations in body composition with gait stability, and how these variables change as psychomotor maturation progresses. Such research would allow us to determine and compare the parallel evolution of body fat and muscle percentages with the kinematic parameters of balance and gait. In addition, the potential compenses of children for maintaining the balance should also be studied: with the use of a second Actigraph placed, for example, on the ankle or on upper limb.