VERTICAL VIBRATION TRANSMISSION THROUGH THE LUMBAR SPINE OF THE SEATED SUBJECT—FIRST RESULTS

Abstract Seven fresh, not embalmed, cadavers (58·1±6·6 years, 73±10·3 kg, 170·7±6·5 cm) were submitted, in the week following their death (7·1±3·1 days), to a whole-body vertical broad-band white random vibration in the bandwidth 0·8 to 25 Hz of about 1·5 m/s 2 r.m.s. Two postures were tested using the same rigid seat, each one with and without a lumbar support: seated erect and seated as in a car. Vibration was monitored on the floor, the seating in the vertical direction (buttocks-to-head), the five lumbar vertebrae and the sternum: vertical (buttocks to head) and longitudinal (back to chest). Biaxial accelerometers were mounted rigidly on the anterior face of the vertebral body, after the removal of the abdominal viscera. Analogue recordings of each channel were passed through an antialising filter (Fc=40 Hz) then sampled at 80 Hz (4096 samples/channel). The inclination of each accelerometer (α) was measured on the lateral X-ray taken for every trial, then the data were set in order to be in the same reference ( Z = z /cos α, X = x cos α). Spectral analysis was performed with a frequency resolution of 0·3 Hz, on the basis of Welch's method. Thirty one overlapping sections (256 samples per section using a Hanning window with an overlap rate of 128 samples) of the estimated periodograms were averaged. Transfer and coherence functions were than estimated between the vertical seating acceleration and the measured accelerations at the upper levels. The first results showed that the vertical vibration transmission was constant throughout the lumbar spine. Inter-subject variability was the major source of disparity. Resonance phenomena were observed between 4 and 9 Hz and depended on posture.