MECHANICAL IMPEDANCE OF THE HUMAN-BODY IN THE HORIZONTAL DIRECTION

The mechanical impedance of the seated human body in horizontal direct ions (fore-and-aft and lateral) was measured during different experime ntal conditions, such as vibration level (0.25-1.4 m/s(2) r.m.s.), fre quency (1.13-80 Hz), body weight (54-93 kg), upper body posture (relax ed and erect) and gender. The outcome showed that impedance, normalize d by the sitting weight, varies with direction, level, posture and gen der. Generally the impedance spectra show one peak for the fore-and-af t (X) direction while two peaks are found in the lateral(Y) direction. Males showed a lower normalized impedance than females. Increasing fo re-and-aft vibration decreases the frequency at which maximum impedanc e occurs but also reduces the overall magnitude. For the lateral direc tion a more complex pattern was found. The frequency of impedance peak s are constant with increasing vibration level. The magnitude of the s econd peak decreases when changing posture from erect to relaxed. Male s showed a higher impedance magnitude than females and a greater dip b etween the two peaks. The impedance spectra for the two horizontal dir ections have different shapes. This supports the idea of treating them differently; such as with respect to risk assessments and development of preventative measures. (C) 1998 Academic Press.