The e!ect of variations in posture and vibration magnitude on apparent mass and seat-to-pelvis pitch transmissibility have been studied with vertical random vibration over the frequency range 1)0}20 Hz. Each of 12 subjects was exposed to 27 combinations of three vibration magnitudes (0)2, 1)0 and 2)0 m/s r.m.s.) and nine sitting postures (&&upright'', &&anterior lean'', &&posterior lean'', &&kyphotic'', &&back-on'', &&pelvis support'', &&inverted SIT-BAR'' (increased pressure beneath ischial tuberosities), &&bead cushion'' (decreased pressure beneath ischial tuberosities) and &&belt'' (wearing an elasticated belt)).
Peaks in the apparent masses were observed at about 5 and 10 Hz, and in the seat-to-pelvis pitch transmissibilities at about 12 Hz. In all postures, the resonance frequencies in the apparent mass and transmissibility decreased with increased vibration magnitude, indicating a non-linear softening system. There were only small changes in apparent mass or transmissibility with posture, although peaks were lower for the apparent mass in the &&kyphotic'' posture and were lower for the transmissibility in the &&belt'' posture. The changes in apparent mass and transmissibility caused by changes in vibration magnitude were greater than the changes caused by variation in posture.