Sedentary lifestyles slow down the child's naturally predetermined motor development, which is why the development of a child's skills and motor skills is lacking, which can impair his or her functional status. Considering that the basic postural characteristics change periodically through the stages of growth and development of the child, and the backbone of these changes are changes related to mobility of the spine and chest, then the question arises of the connection between the mobility of the trunk through all three planes with the respiratory index in today's children. The purpose of this study is to determine whether trunk mobility influences chest expansion and therefore respiratory index in primary school children, and if so, what is the level of correlation. The correlation between carcass mobility and obesity has also been questioned. The study found that trunk mobility does not affect the breathing index because the correlation between these two variables is indicated by poor statistical connectivity for trunk mobility through the transverse plane, and a slight statistical association with motility through the sagittal plane. We also found that there was no statistical association between obesity and carcass mobility. Since scoliosis of up to 5% according to Cobb was found in a large number of subjects, we additionally tried to determine the level of correlation between the presence of scoliosis and spinal mobility. With such young subjects and such a low degree of scoliosis, no statistical correlation exists.