Purpose. In this single subject case study, the objective is to identify cerebral palsy trunk muscles activity in sitting position and to assess differences in muscle activity between cerebral palsy and normal conditions. Modeling. an opensim model of upper body is needed Experiment. A quadriplegia spastic cerebral palsy and a normal control subject without any neuromusculoskeletal disorders participated in the experiment. Subjects performed a voluntary trunk flexion from neutral upright position with a self-defined speed followed by a back extension return to neutral position. Three markers were placed on the bony landmarks of S1, T10, and C7. Data representing the electromyographic activity of the muscles along with the kinematic results of motion analysis were recorded simultaneously.