The purpose of this study is to investigate how the application of a Thorax belt affects the stability of the spine in subjects with a wide infra-sternal angle. A total of 15 subject with wide infra-sternal angle participated in the experiment. Active Double leg lowering and active one-leg raising were performed with or without a thorax belt. Two spinal stability tests (active double lowering and active one-leg raising) performed with or without thorax belt application showed significant differences between each condition. Based on the results of present study, the application of a thorax belt is considered to be an effective therapeutic tool that can stabilize the spine to subjects with abnormally increased chest cage and spinal or trunk instability.