Characterization of apparent mass of human body seated on rigid and elastic seats under vertical vibration

Characterization of biodynamic responses of seated body exposed to whole-body vibration forms an essential basis for understanding of mechanical-equivalent properties of the body and potential injury mechanisms, and developments in frequency-weightings and enhanced design tools for the coupled human-seat system. Such responses are strongly dependent upon human anthropometric, gender, sitting posture and vibration condition in a highly complex and coupled manner, while only limited knowledge exists on effects of these factors. Furthermore, such responses are mostly evaluated for body on a rigid seat due to complexities associated with measurement of forces developed at an elastic human-seat interface under vibration. An elastic seat greatly alters human-seat interface contact force and contact area. The biodynamic responses with an elastic seat are thus expected to differ. This dissertation research concerns with development of a methodology for measurement of apparent mass (APMS) responses of human body seated on an elastic seat and exposed to vertical vibration. A force-sensing resistive pressure measurement system was initially used to capture responses of 58 human subjects (31 male and 27 female) seated on a rigid seat with and without a vertical back support, and exposed to three different magnitudes of broad band random vibration in the 0.5 to 20 Hz range (overall rms acceleration = 0.25, 0.50 and 0.75 m/s2). The APMS responses were also obtained using the conventional force plate. The responses acquired from the force plate were thoroughly analyzed to study effects of gender, and mass-related (body mass, body mass index, body fat, lean body mass), stature-related (standing height, sitting height, C7-height) and build-related (buttock circumference, contact area) anthropometric dimensions. The results showed strong coupling between the gender and the body mass, while a strong correlation of the peak APMS was evident with body mass, body mass index, body fat and hip circumference. The data were subsequently grouped within three different body mass ranges in order to decouple the effect. The gender effect was observed in the vicinity of secondary peak where female subjects revealed higher APMS magnitude, while the male subjects showed relatively higher primary peak frequency than females. Comparisons of APMS responses with those derived from the pressure sensing mat revealed large differences. APMS magnitudes derived from the pressure sensing mat were considerably lower than those obtained from the conventionally used force plate in the entire frequency range. The differences were attributed to low resolution of the sensor and limited acquisition rate of the hardware. A correction function was subsequently derived from the ratio of response functions obtained from the two measurement systems, which revealed nearly linear decreasing trend with frequency. The application of correction functions resulted in comparable responses from the two measurement systems. It was then hypothesized that the proposed correction function, mostly attributed to limited acquisition rate, would be equally applicable for cushion seats. Subsequent measurements were performed to derive APMS of subjects seated on a cushion seat. Comparisons of APMS magnitudes obtained for the cushion seat with those obtained with the rigid seat revealed that response magnitudes and the primary resonance frequency of subjects when seated on a cushion seat are generally lower. The effects of selected anthropometric factors, sitting posture and vibration magnitudes, however, were very similar to those observed for the rigid seat.