Seat belts in vehicles, for many years, have the task to reduce the level of injuries that can occur during traffic accidents. Although the number of children who die in traffic accidents decreasing, there are still some parents who incorrectly associated children to their seats. The most common injuries that occur in children are cervical spine fractures and lumbar spinal dislocations. We studied the correct and incorrect way binding belt child in the back seat. The three-dimensional model of lumbar spine (segment L4-L5) has been developed and analysed. The model consisted of two vertebral bodies and one intervertebral disc. Numerical solutions of von Misses stresses and displacements, occur as a result of vehicle crash, are calculated and analysed. With computer manipulation it is possible to simulate the behaviour of lumbar spine, and this may help to better understand the mechanical and physical characteristics this part of the body.