Lumbar braces, cast jackets and lumbar corsets are commonly prescribed for the relief of low back pain or the prevention of instability of lumbar or lumbosacral vertebrae. These devices have been presumed to achive their effect by (1) decreasing movement at the involved intervertebral joints, (2) shifting a portion of the load from the spine to the rest of the trunk by compressing the abdomen, (3) decreasing lumbar lordosis and therefore, mechanical stress on the spine and (4) providing sufficient support to allow relaxation of the muscles of the trunk. But it is doubtful that lumbar braces or corsets significantly decrease movement between lumbar vertebrae. To maintain the stability of the lumbar and lumbosacral spine, abdominal muscles and intrinsic muscles of the back play an important role. When these muscles weaken the lumbar lordosis becomes exaggerated possibly bringing about degenerative changes in the articular processes and approximation of the spinous processes. In severe cases results in the so-called kissing spine which causes low back pain. The pain may be relieved by use of lumbo-sacral corsets or knight braces, which diminish lordosis. However, there has been no scientific report on adequate types of braces, duration of use or muscle weakness resulting from its use as yet. The prupose of this study is to provide a theoretical braces for, planning adequate prevention and early treatment, so that labour loss from retirement or change of job due to low back pain could be minimized, permitting early return to employment and resumption of activities of daily living. In other words, this study concentrates on the indications for use and selection of which will produce optimum results in the treatment of patients with low back pain. The EMG was taken immediately after admission, and after 3 weeks of bed rest accompanied by pelvic traction with or without heating, and also it was taken after 6 weeks of use of brace. The results obtained from this study were as follows; 1. After bed rest the EMG activities were increased in back muscles, such as longissimus dorsi, multifidus and iliocostalis lumborum and in abdominal muscles of internus abdominis while decreased in rectus abdominis. 2. There has been no significant change after use of corset in acute cases while the muscle activities. were increased after use of lumbar corset in chronic cases. 3. Rectus abdominis was most senstively influenced by bed rest or use of brace both in acute and chronic cases. And it was the last to recover probably because it does not contract in ordinary alow walking. 4. Knight brace was considered to be preferable to lumbar corset in acute cases with evidence of weakness in abdominal muscles by EMG. 5. There was no difference between bed rest and use of brace in patients who had radiculopathy with functional difficulties or not.