Apparent mass of the human body in the vertical direction: Inter-subject variability

Abstract

The biodynamic responses of the seated human body to whole-body vibration vary considerably between people, but the reasons for the variability are not well understood. This study was designed to determine how the physical characteristics of people affect their apparent mass and whether inter-subject variability is influenced by the magnitude of vibration and the support of a seat backrest. The vertical apparent masses of 80 seated adults (41 males and 39 females aged 18 to 65) were measured at frequencies between 0.6 and 20 Hz with four backrest conditions (no backrest, upright rigid backrest, reclined rigid backrest, reclined foam backrest) and with three magnitudes of random vibration (0.5, 1.0 and 1.5 ms -2 r.m.s.). Relationships between subject physical characteristics (age, gender, weight, and anthropometry) and subject apparent mass were investigated with multiple regression models. The strongest predictor of the modulus of the vertical apparent mass at 0.6 Hz, at resonance, and at 12 Hz was bodyweight, with other factors having only a marginal effect. After correction for other variables, the principal resonance frequency was most consistently associated with age and body mass index. As age increased from 18 to 65 years, the resonance frequency increased by up to 1.7Hz, and when the body mass index was increased from 18 to 34 kgm -2 the resonance frequency decreased by up to 1.7 Hz. These changes were greater than the 0.9-Hz increase in resonance frequency between sitting without a backrest and sitting with a reclined rigid backrest, and greater than the 1.0-Hz reduction in resonance frequency when the magnitude of vibration increased from 0.5 to 1.5 ms -2 r.m.s. It is concluded that the effects of age, body mass index, posture, vibration magnitude, and weight should be taken into account when defining the vertical apparent mass of the seated human body.

The transmission of vertical vibration through seats: Influence of the characteristics of the human body

“…For example, variations in the mass of the body cause large inter-subject variability in vertical apparent mass at low frequencies, but after normalisation (dividing the modulus of the apparent mass by the static mass supported on the seat) the variability is much reduced [5]. The effect of subject characteristics on the vertical apparent mass of the body has been reported for the 80 subjects (41 men and 39) used in the present study [10]. With four backrest conditions (no backrest, upright rigid, reclined rigid, reclined foam) and three vibration magnitudes (0.5, 1.0 and 1.5 ms -2 r.m.s.…”

“…vibration. The resonance frequency in the apparent masses of the same subjects increased by 1.7 Hz over the range 18 to 65 years [10]. Subject age was the only subject characteristic to be significantly associated with seat transmissibility at resonance, with the mean transmissibility at resonance of the seat with backrest increasing by 0.52 with 0.5 ms -2 r.m.s.…”

“…vibration over the 18 to 65 year age range. Age was not a significant predictor of the apparent masses of the subjects at resonance [10]. Because the seat dynamic properties did not vary with subject age (Table 9), it seems that the effects of subject age on the seat resonance frequency were largely due to increased resonance frequency in the apparent mass with increased age.…”

“…This non-linearity has been observed with no backrest (e.g. [5,9]), with an upright backrest [10], and when sitting with a reclined backrest [8]. The dynamic properties of seat foam have also been shown to be non-linear, with the stiffness and damping decreasing with increases in the vibration magnitude [11,12].…”

“…The objective of this study was to determine the manner in which the principal factors affecting the vertical apparent mass of the human body (i.e., age, weight, body mass index, gender, backrest contact, and magnitude of vibration [10]) affect the transmissibility of a seat and its dynamic stiffness. It was hypothesized that the transmissibility of a seat would be influenced by factors that influence the apparent mass of the human body and that factors that increase the compression of the seat or the area of contact with the seat (e.g.…”