The apparent mass of the human body exposed to non-orthogonal horizontal vibration

Apparent masses of 15 male and 15 female subjects have been measured during exposure to various directions of horizontal vibration. Twenty vibration conditions were used in the experiment. In each of five directions (0, 22.5, 45, 67.5 and 90° to the mid-sagittal plane) subjects were exposed to random vibration in the frequency range of 1.5–20 Hz at 0.25, 0.5 and 1.0 m s −2 r.m.s. The five remaining conditions were selected to give measurements whereby the magnitude of the x-component of the vibration was fixed and the y-component changed and vice-versa. Two peaks were observed in the apparent masses. The first peak occurred at about 3 Hz and reduced in frequency with increases in vibration magnitude. The frequency of the first peak also reduced as the direction of vibration changed from 0 to 90°. The magnitude of the peak increased as the vibration magnitude and direction increased. The second peak occurred at about 5 Hz and decreased in both frequency and magnitude with increases in vibration magnitude. There was no change in the frequency of the second peak with vibration direction, although the magnitude of the peak decreased as the angle of vibration to the mid-sagittal plane increased. Increasing the magnitude of the x-component of vibration whilst using a fixed y-component changed the magnitude of the first peak but did not change the frequency of the first or any characteristics of the second peak. In contrast, increasing the y-component of vibration whilst using a fixed x-component changed the frequencies and magnitudes of both peaks. Predictions of the response at 45° by applying the principle of superposition to data measured at 0 and 90° showed that the response of the body with direction was not linear. This implies that the apparent mass in non-orthogonal axes cannot be predicted from the apparent masses measured in orthogonal directions.