Effect of frequency, magnitude and direction of translational and rotational oscillation on the postural stability of standing people

Oscillatory motions can cause injury in transport when standing passengers or crew lose balance and fall. To predict the loss of balance of standing people, a model is required of the relationship between the input motion and the stability of the human body. This experimental study investigated the effect of frequency, magnitude and direction of oscillation on the postural stability of standing subjects and whether response to rotational oscillation can be predicted from knowledge of response to translational oscillation.Twelve male subjects stood on a floor that oscillated in either horizontal (fore-and-aft or lateral) or rotational (pitch or roll) directions. The oscillations were one-third octave bands of random motion centred on five preferred octave centre frequencies (0.125, 0.25, 0.5, 1.0, and 2.0 Hz). The horizontal motions were presented at each of four velocities (0.04, 0.062, 0.099, and 0.16 ms⁻¹ rms) and the rotational motions were presented at each of four rotational angles (0.73, 1.46, 2.92, and 5.85° rms) corresponding to four accelerations (0.125, 0.25, 0.5, and 1.0 ms⁻² rms), where the acceleration is that caused by rotation through the gravitational vector. Postural stability was determined by subjective methods and by measuring the displacement of the centre of pressure at the feet during horizontal oscillation.During horizontal oscillation, increases in motion magnitude increased instability and, with the same velocity at all frequencies from 0.125 to 2.0 Hz, most instability occurred in the region of 0.5 Hz. Fore-and-aft oscillation produced more instability than lateral oscillation, although displacements of the centre of pressure were similar in both directions. With the same angular displacement at all frequencies from 0.125 to 2.0 Hz, pitch oscillation caused more instability than roll oscillation, but in both directions instability increased with increased frequency of oscillation. Frequency weightings for acceleration in the plane of the floor during translational and rotational excitation show the significance of low-frequency translational oscillation and high-frequency rotational motion, and show that it is necessary to know whether the measured acceleration is caused by translation or rotation through gravity.