Bidimensional accelerations of lumbar vertebrae and estimation of internal spinal load during sinusoidal vertical whole-body vibration: a pilot study

Accelerations, a, in z- and x-directions were measured on the skin over spinous processes L₃ and L₄ in three subjects during sinusoidal whole-body vibration (WBV) at 4·5 and 8 Hz and 1·5 and 3·0 ms⁻² r.m.s. A method for the prediction of bone accelerations was applied using measurements on the skin. Relative accelerations were calculated by subtracting a_L4 from a_L3. The phase relations between relative accelerations in the z-direction indicating compression and the absolute maximum a_z of L₄ exhibited marked between-subject variability. One subject was selected for a detailed analysis in the time domain of head, shoulder and upper trunk accelerations, and for comparison with an invasive study. Bidimensional acceleration data confirmed the suggestion that relative motions in the z-direction are combined with angular motions. The results indicate complex internal loads with coupled bending, compression and shear forces.