Shock absorption by the spinal column in normals and in ankylosing spondylitis.

The transmission of heel-strike vibration using skin-mounted accelerometers was measured in normal subjects and subjects with ankylosing spondylitis. In normal subjects transmissibility was enhanced between 5 and 13 Hz and attenuated at frequencies above 15 Hz. In ankylosing spondylitis transmissibility was enhanced at 4 Hz but less so between 5 and 13 Hz and little attenuation was observed at the higher frequencies. This difference is expected in view of the pathological changes occurring in the spinal column in ankylosing spondylitis. The results support the hypothesis that the normal spinal column has to bend in order to absorb vibrations with a frequency greater than 15 Hz.