The subject of this study was to examine the relationship between kinetic and kinesiological parameters of the trunk and postural deformities in the sagittal plane, as well as to determine the factors associated with these postural deformities and back pain in children aged 9 to 14 engaged in competitive sports. The participants in this study were 124 children aged 9-14, of both sexes, who came for regular preventive examinations at the sports doctor at the Novi Sad Healthcare Center and who trained in various competitive sports. Inertial sensors, a digital dynamometer and digital inclinometers were used in the work. The results of this study showed that pain in the lumbar spine during extension, as well as inclination curves of the spine, are mostly correlated with body mass index taking into account anthropometric characteristics. All observed inclination angles of the spinal segments differ statistically significantly in the group with lumbar back pain at extension in relation to the control group. There is a connection between back pain during extension and the position of inertial sensors in the extension, namely sensor S1 at the position of the second sacral vertebra and sensor S2 at the position of the third lumbar vertebra. It was determined that in most cases when back pain is present, there is no physiological extension of the lumbar segment of the spine, but dyskinesia or flexion, with the sacral segment of the spine going into nutation, ie bending forward. Further results are that there is a statistically significant difference in measures of isometric strength of extensor and abductor muscle groups, back extensor muscles, lower trapezius muscle and abdominal muscles in the group with lumbar spine pain compared to the control group. The results further showed that the angles of lumbar lordosis and thoracic kyphosis were most correlated with the strength of the hip extensors and the m. erector spinae.