Difference thresholds of multi-axis whole-body vibration

A laboratory study was conducted to investigate the effects of lateral and horizontal vibration on the difference threshold of vertical vibration. Twelve male subjects sat on a rigid seat and were exposed to four different vibration conditions (pure vertical vibration; combination of horizontal and vertical vibration; combination of lateral and vertical vibration; combination of horizontal, lateral and vertical vibration). Vertical vibration for four conditions was 5 Hz sinusoidal with a magnitude of 1 ms-2 r.m.s. Horizontal and lateral vibration for the last three conditions were sinusoidal with magnitude 0.5 ms-2 r.m.s. and contained ten frequencies (1 to 8 Hz in third-octave band step). The frequency-weighted acceleration of the ten frequencies was equal. Results showed that horizontal and lateral vibrations have different effects on the difference threshold of vertical vibration. The combination of vertical and horizontal vibration gave a significantly lower difference threshold of vertical vibration than the combination of vertical and lateral vibration.