The electrical activity of the muscles of the trunk was studied in ten normal young adult subjects standing at rest, walking on the level at 4.39 and 5.29 kilometers per hour, and walking up a 5-degree incline at 4.39 kilometers per hour. Each activity was studied with and without a chairback brace and a lumbosacral corset.

With the subject at rest, both the chairback brace and the lumbosacral corset either decreased or had no effect on the electrical activity of the back muscles in the majority of subjects. When the subject walked at the so-called "comfortable" speed, neither support had a significant effect on the activity. When the subjects

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walked at a fast pace, wearing the chairback brace, the activity of the back muscles was increased in the majority of subjects in comparison with the activity of those muscles when no support was worn.

We suspect that the different effect on electrical activity produced by the chairback brace and the lumbosacral corset is paradoxically caused by the former's more rigid construction. Transverse rotation of the trunk is an inherent feature of ambulation. Electromyographic studies suggest that the back muscles play an important role in this motion and in the stabilization of the trunk^6,13. At the level at which the thoracic band of the chairback brace is attached to the trunk, transverse rotation is slight³. At the level of the pelvic attachment, on the other hand, there is an average transverse rotation of 8.7 degrees at 4.39 kilometers per hour and of 13.2 degrees at 5.86 kilometers per hour¹. The wearing of a chairback brace or a lumbosacral support does not significantly alter these rotations: They occur within the enclosure of the support¹³. The extent of restriction of the transverse rotations of the trunk by a support depends on its rigidity and the adequacy of its fixation to the trunk. The greater electrical activity recorded in the subjects walking at 5.29 kilometers per hour while wearing chairback braces must reflect the increased muscular exertion of the back muscles as they attempt to overcome the immobilizing effect of the chairback brace. Since persons with low-back pain do not ordinarily walk fast, this finding is presumably of little clinical significance. The flexible lumbosacral support at both walking speeds and the chairback brace at 4.39 kilometers per hour apparently do not exert sufficient restrictive forces to alter muscle effort and electrical activity.

In both the internal and external oblique muscles, both supports either decreased or did not affect the amount of electrical activity recorded at rest and during ambulation at 4.39 kilometers per hour. We suspect that the single instance of increased activity (Subject 3) was a manifestation of reflex muscle contraction related to irritation produced by the electrodes. The increased activity occurred in all three abdominal muscles in both chairback brace and lumbosacral support. We believe that the similar effect on the abdominal muscles of back supports is explained by the fact that both have flexible corset fronts and exert similar forces on the anterior aspect of the trunk during ambulation. Hence the requirement for oblique muscle activity to support the abdominal contents and maintain intra-abdominal pressure is lessened. Finally, it is noteworthy that step frequency and duration of left heel contact were characteristically altered by the speed of ambulation but were not appreciably affected by the wearing of a spinal support. This observation therefore suggests that spinal supports do not interfere with the normal function of the lower extremities or that, if they do, there is complete compensation for whatever interference with their function is produced.