Orthograde posture in man is possible only with the cervical and lumbar curves of the spine steadied by the powerful sacrospinalis muscle groups. In man's transition from the more primitive postures only the rectus abdominis muscles remain to flex the lumbar spine and counter those forces tending toward further extension. In present-day sedentary activities, the abdominal muscles are seldom used for function other than to retain abdominal viscera; yet in sitting, in standing, and in work and play the lumbar spine and lumbosacral joint are constantly subjected to hyperextension. Williams, Thieme, and others point to hyperextension as one of the important precipitating factors in lumbar breakdown in man, including protruded intervertebral disk, spondylolisthesis, and the poorly understood syndrome often called "lumbago," "sacroiliac strain," or, for want of a better term, the lumbosacral syndrome. Of the latter group we have had particularly encouraging results with conservative treatment and feel its clinical
Kelly RP, Johnson JT. ACUTE LOW BACK PAIN. JAMA. 1955;158(17):1520–1521. doi:10.1001/jama.1955.02960170036010b
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