Vertical and Fore-Aft Seat-to-Head Transmissibility Response to Vertical Whole Body Vibration: Gender and Anthropometric Effects

JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL Pages 11 40 and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects Krishna N. Dewangan 1,2, Arman Shahmir 1, Subhash Rakheja 1,* and Pierre Marcotte 2 1CONCAVE Research Centre, Concordia University, Montreal, Canada, 2Institut de recherche Robert-Sauvé en santé et en sécurité du travail, Montreal, Canada * Corresponding author s email: subhash.rakheja@concordia.ca; phone: 514-848- 2424 ext 3162 ABSTRACT In this study, the biodynamic responses to vertical vibration are investigated in terms of seat-to-head vibration transmissibility (STHT) to study the effects of gender and eleven different anthropometric parameters on the STHT responses in the vertical and fore-aft directions. The STHT response of 31 male and 27 female human subjects were measured under three levels of vertical vibration (0.25, 0 and 0.75 m/s 2 rms acceleration) in the 0 to 20 Hz range, while seated without a back support and against a vertical back support with hands on a steering wheel. The results showed that the vertical and fore-aft STHT responses of the two genders were distinctly different. The primary resonance frequency of the male subjects was higher than the female subjects, while the peak magnitudes were comparable. The male subjects showed relatively greater softening effect, i.e. decrease in the primary resonance frequency with increase in excitation magnitude, as compared to the female subjects, irrespective of the sitting condition. The body mass revealed strong effect on both the male and female STHT responses. The primary resonance frequency of heavier subjects was lower than that of the lighter subjects, while the peak magnitude was higher for the heavier subjects. The male subjects showed significantly higher primary resonance frequency than the female subjects, even when comparable body mass, BMI and lean body mass were considered. The vertical STHT response of the two genders with same body fat mass was very similar for the sitting and excitation conditions considered in the study, particularly up to 10 Hz. Keywords: whole-body vibration, seated body biodynamic response, vibration transmissibility, gender effect, body mass effect, body fat, sitting condition 1. INTRODUCTION The biodynamic responses of the seated human body exposed to whole-body vibration (WBV) are invariably expressed in terms of two functions: (i) to-thebody response function (mechanical impedance, apparent mass and absorbed power); and (ii) through-the-body response function (vibration transmissibility). The to-the-body response functions describe the force-motion relationship at the body-seat interface and have been extensively reported under a variety of test conditions involving wide differences in subjects mass, sitting posture and excitation magnitude [1]. The through-the-body response function describes the transmission of seat vibration to different body segments and could yield better information on modes of vibration of the seated body and potential adverse health Vol. 32 No. 1+2 2013 11

and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects effects of WBV exposure [2]. Furthermore, the contributions of the low inertia subsystems, in particular, are better understood from the through-the-body responses compared to the to-the-body response functions. However, owing to the complexity of vibration measurements of different segments and the errors associated with skin-mounted accelerometers due to relative movements of the skin over the bones [3], the vast majority of the studies have considered the transmission of vibration to the head as an adequate measure of the seated body biodynamics. Wang et al. [4] stated that seat-to-head transmissibility (STHT) may be more representative of multiple vibration modes of the upper body than the apparent mass driving-point response. However, owing to the ease of measurement, the to-thebody response function has been the focus of substantially greater number of studies compared to the STHT. Transmission of seat vibration to the head has been widely studied under different back supports, types and magnitudes of whole-body vibration, and hands and feet positions, which have been summarized in [1,5]. Considerable differences among the reported datasets, however, have been observed [1,5,6]. These are attributable to differences in the measurement systems, vibration type and magnitude, posture, involuntary changes in muscle tension and the physical attributes of subjects used in different studies [2,7,8]. Moreover, the reported STHT responses exhibit substantially large inter-subject variability, which may be partly caused by differences in measurement systems and physical characteristics of the subjects [9]. The physical characteristics among the humans are known to vary widely and the differences are far more important between the male and female populations. On average, stature, sitting height and body mass of females are lower than those of the males [10]. Furthermore, females possess relatively less muscle mass than males, but greater fat mass in proportion to the total body mass. Because of the lower muscle mass, the isometric strengths of females in general are approximately two-thirds of those of the males [11]. It is thus expected that the characteristics of whole-body vibration transmission in male and female subjects groups would be different, although only a few studies have explored the gender effect on the STHT. Paddan and Griffin [5] conducted a review of studies reporting vibration transmissibility, while Rakheja et al. [1] performed a synthesis of selected datasets on apparent mass and vertical STHT. These suggest that the vast majority of the studies have either considered only male subjects or reported the overall mean or median responses of subjects within both gender groups. The effect of gender on the vibration transmission, therefore, could not be clearly established. Hagena et al. [12] measured the STHT responses of the male and female subjects but reported only the mean response of the entire subject population. Another study by Griffin et al. [13] suggested higher magnitudes of vibration transmitted to the head of females compared to the males at frequencies above 5 Hz, and an opposite trend was observed at frequencies up to 4 Hz. Griffin et al. [14] further reported only slight differences in the STHT responses of the two genders. On the basis of STHT responses measured under sinusoidal vibration, Wilder et al. [15] reported lower primary resonance frequency of female subjects compared to the male subjects, while the second and third resonant frequencies were identical for both gender groups. Laurent [16] measured the vibration transmissibility of cushioned seats with male and female subjects, and concluded a strong gender effect on the vibration transmission properties of seats. From the synthesis of reported data on transmission of seat vibration to the head, it has been shown that seat-to-head vibration transmissibility is most significantly affected by the sitting posture, particularly the backrest contact [1]. The study proposed different ranges of seat-to-head vibration transmissibility for back supported and back unsupported sitting postures. Apart from the sitting posture, the transmission of seat vibration may also be affected by various mass-, stature- and build-related anthropometric parameters. The effects of anthropometric parameters on the seated body STHT response to vibration have been investigated in even fewer 12 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL

Krishna N. Dewangan, Arman Shahmir, Subhash Rakheja and Pierre Marcotte studies. Donati and Bonthoux [9] studied vibration transmissibility between the thorax and the pelvis, and its correlations with different anthropometric dimensions such as the standing body mass, body mass supported by the seat pan, stature, trunk height, sitting height and chest circumference. The study reported higher transmissibility magnitudes at frequencies up to 4 Hz with increasing stature and negative correlation of the higher frequency magnitudes with body weight and some thorax dimensions. The reported studies have invariably considered male and female subjects of different body mass and anthropometric parameters. The studies reporting to-thebody biodynamic responses have shown coupled effects of body mass and various anthropometric parameters on the measured apparent mass [17-19]. The effects of various anthropometric parameters like hip circumference, buttock-seat contact area, sitting height, body fat mass and lean body mass on the STHT responses have not yet been thoroughly studied for both the genders. The ISO 5982 [20] standard provides reference values of, which can be considered representative of only male subjects [6]. Owing to the differences in anthropometry of the two genders, the characterization of the STHT responses of the female and male subjects are desirable not only to establish coupled effects of mass-, statureand build-related factors but also to establish reliable target response functions for deriving effective biodynamic models and frequency weightings. In the present study, the vertical and fore-aft STHT responses of 31 male and 27 female subjects are measured under three different levels of vertical vibration in the to 20 Hz frequency range to study the effects of gender and selected anthropometric parameters. These include the body mass-related (body mass, BMI, body fat, lean body mass), build-related (hip circumference, seat pan contact area and mean contact pressure) and stature-related factors (standing height, seated height, and C7 height). The data were acquired with subjects seated with and without a back support, and hands supported on a steering wheel. Owing to the coupled effects of various anthropometric dimensions, the STHT responses of the two genders are studied for comparable ranges of each anthropometric parameter. 2. MEASUREMENT AND ANALYSIS METHODS 2.1. Subjects A total of 58 subjects, 31 males and 27 females, were recruited for the experiments. All the recruited subjects were healthy with no signs of back injuries or low back pain. Prior to the test, each subject was informed about the purpose of the study, experimental setup and usage of hand held emergency stop, which could suppress the vibration platform motion in a ramp-down manner, when activated. Each subject was also given written information about the experiment and was requested to sign a consent form that was previously approved by the Human Research Ethics Committee of Concordia University. For each subject, various anthropometric body parameters were measured prior to the experiments. These included: the standing body mass; stature; sitting and C7 heights; and hip, neck and waist circumferences. The body fat and lean body mass were estimated using the US Navy formula [21]. Table 1 summarizes the physical characteristics of the subjects for both genders in terms of mean, standard deviation, minimum and maximum values, grouped under mass-, stature- and build-related parameters. 2.2. Experimental Setup and Measurement Methods The seat-to-head vibration transmissibility (STHT) responses of seated subjects were measured using a Whole Body Vibration Simulator (WBVVS), which comprises a platform supported on two servo-hydraulic actuators, as described in [3]. A rigid seat with a horizontal seat pan and a vertical backrest was mounted on the WBVVS platform. The height of the seat pan with respect to the WBVVS platform was 445 mm. The WBVVS was also equipped with a steering column with a steering wheel fixed to the platform that served as the hands support. The seat and the platform structure provided nearly flat frequency response up to 30 Hz with a Vol. 32 No. 1+2 2013 13

and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects Table 1 Mean, standard deviation, minimum and maximum values of the selected anthropometric dimensions of the participants. Particulars Minimum, maximum, mean (standard deviation) Ranges: Mean (n,standard deviation) Male (n=31) Female (n=28) Male Female Age (years) 23.0, 58.0, 31.2 (7.2) 19.0, 49.0, 19.0 (7.1) - - mass-related Body mass (kg) 55.0, 106.0, 79.8 (15.7) 45.5, 7, 60.1 (8.3) 6(9,4.3), 81.6(9,4.1); 96.7(9,6.4) 50.4(9, 3.3), 6(9, 2.8); 69.1(9, 2.7) Body mass index (kg/m 2 ) 19.96, 34.99, 26.12 (4.24) 15.78, 26.31, 22 (2.73) 21.6(11, );25.4(11, 1.6);31.4(8, ) 19.4(9, );22.6(8, 0.8);25.3(10, 0.7) Body fat (%) 16.10, 37.72, 23.59 (5.93) 19.26, 39.06, 33 (4.83) 16.6(9, 2.4);21.9(10, );28.9(6, 1.6) 25.9(9, 0.8);30.8(9, 2.1);35.7(9, 1.7) Body fat (kg) 1, 39.0, 19.8 (8.2) 8.8, 25.3, 18.6 (4.7) 1(11, 1.6);16.6(10, 2.2);26.2(7, 3.3) 13.5(8, 1.1);19.1(9, );23.7(9, 1.3) Lean body mass (kg) 43.3, 77.5, 61.6 (9.0) 34.1, 49.5, 41.6 (4.8) 50.1(9, 4.7); 6(10, 2.4); 68.8(11, 3.8) 36.0(8, 1.2);41.4(11, 2.1);47.3(8, 1.8) build-related Hip circumference (cm) 88.0,116.0, 103.6 (7.4) 89.5, 109.0, 99.9 (5.5) 95.5(9, );102.8(11, 2.8); 110.7(9, 4.0) 92.6(8, 1.9);100.4(9, 2.2);105.2(10, 1.6) Seat pan contact area (cm2) 211, 1050, 575 (195) 250, 890, 515 (175) 362(10, 69);556(8, 34);666(8, 30) 350(9, 63);510(9, 48);682(6, 68) Mean contact pressure (kpa) 8.1, 26.7, 13.5 (4.6) 5.9, 14.0, 9.5 (2.3) 9.1(11, 0.9);13.2(10, );17.1(8, 1.9) 7.2(9, );9.3(9, );12.3(8, 1.4) stature-related Stature (m) 9, 1.92, 1.75 (8) 1.48, 1.73, 1.63 (7) 1.70(9, 2); 1.75(10, 2); 1.81(8, 2) 6(9, 5);1.64(9, 2);1.71(9, 1) Sitting height (cm) 81.3, 96.7, 88.8 (6.2) 63.2, 88.3, 8 (7.7) 86.1(8, );91.2(10, 1.6);95.2(8, 1.1) 80.4(8, 1.7);84.4(8, );88.3(8, 1.2) C-7 height (cm) 59.4, 74.4, 66.2 (4.6) 56.5, 70.4, 61.4 (4.2) 61.9(9, 1.7);67.4(10, );7(8, ) 58.0(8, 1.2);61.4(8, 0.9);65.1(8, 1.7) n: number of subjects 14 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL

Krishna N. Dewangan, Arman Shahmir, Subhash Rakheja and Pierre Marcotte resonant frequency of 63 Hz. A single-axis accelerometer (B&K 4370) was mounted on the seat pan to measure vertical acceleration at the seat. An adjustable light-weight helmet-strap mounting system integrating a three-axis accelerometer (Analog Devices ADXL05), described by Wang et al. [4], was used to acquire the head vibration along the three translational axes. The helmet-strap comprised a ratchet mechanism so as to adjust its tension. Three different levels of white noise random vertical vibration in the to 20 Hz frequency range were synthesized using a vibration controller (Vibration Research Corporation) to realize overall rms accelerations of the platform of 0.25, 0 and 0.75 m/s 2. Each subject was advised to sit on the seat with (WB) and without (NB) the back support with hands placed on the steering wheel. Consideration of only vertical back support, which is likely not representative of the vehicle seats, may be a limitation of this study. An adjustable support for the feet was placed on the WBVVS platform to realize nearly vertical lower leg and horizontal thighs of the subject. The static body-seat contact area and mean contact pressure of each subject were measured for both back support conditions (NB and WB) using a seat pressure sensing mat (Tekscan), as described in [19,22]. The sensing mat comprised a grid of 42 rows and 48 columns of sensels encased between two Mylar sheets with density of 1 sensel/cm 2. The sensing area of the mat was 487.7 mm long and 426.7 mm wide, while the pitch of the columns and rows was 10.2 mm. The pressure sensing mat could measure contact pressure up to 207 kpa with a resolution of 0.83 kpa. The distributed body-seat interface pressure was acquired using a sampling frequency of 128 Hz for a duration of 10 s for both back support conditions (NB and WB), and the data were analyzed to derive mean contact pressure. The I-scan software permitted the analyses of pressure distributions and the contact force through integration of the pressure data over the contact area. The static mean and peak contact pressures together with the contact area were recorded for each subject and sitting condition prior to application of vibration. The minimum, maximum and mean values of the measured mean contact pressure and area are also summarized in Table 1. Subsequently, each subject was asked to wear the helmet-strap headaccelerometer and adjust its tension to ensure a tight but comfortable fit. The experimenter made the necessary adjustments to ensure appropriate orientation of the head accelerometer. The subject was advised to sit upright with hands on the steering wheel. The mid back of the subjects was in contact with the back rest of the seat while sitting with WB condition. The subjects adopted similar posture except mid back of the subject was not in contact with back rest of the seat while sitting without back support. Before start of the experiment, one of the experimenter showed require posture for the experiment to the subject. The subject was also advised to stare at an object fixed on the wall, around 4 meters away from the WBVVS, at the level of eye height to ensure a steady head position. The subject s posture and head orientation were also visually monitored by the experimenter during the trial. The WBVVS was operated to realize a desired level of vibration. The seat and head acceleration signals were acquired in a multi-channel spectral analysis system (B&K PULSE 1) using a bandwidth of 50 Hz and frequency resolution of 625 Hz. The head acceleration measurements were limited to vertical and fore-aft axis, since the lateral head vibration is known to be relatively small under vertical seat vibration [23]. Subsequently, the measurements were performed under different levels of vertical vibration and back support conditions. Each measurement was repeated twice. The duration of each measurement was 60 s (12 averages), while the participants were asked to relax for up to 5 minutes between the successive trials. 2.3. Data Analysis The acquired acceleration signals were analyzed to derive the vertical and fore-aft STHT responses (magnitude and phase) using the H 1 frequency response function method involving the complex ratio of the cross-spectral density of the seat Vol. 32 No. 1+2 2013 15

and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects (vertical) with the head (vertical and fore-aft) accelerations, and the auto-spectral density of the seat vertical acceleration. The data analysis corresponding to each trial involved Hanning-windowed time averages with an overlap of 75%. The data was analyzed considering the moving coordinate system and it was not converted to global coordinate system for comparison of seat accelerometer as given in DeShaw and Rahmatalla [24], which may be a limiting factor. The vertical and fore-aft STHT magnitude and phase responses of each subject were obtained for the selected back support and excitation conditions. In order to study the gender effect on the STHT response, the data acquired for the male and female subjects were analyzed separately for each sitting and excitation condition. Owing to the possible coupled effects of body mass and gender, the body mass dependence of the STHT responses is investigated by grouping the data sets according to three different body mass ranges of each gender group: 55-65, 75-85 and 90-106 kg for the male subjects; and 45-55, 55-65 and 66-7 kg for the female subjects. Each body mass group included 9 subjects, while the respective mean masses were 6±4.3, 81.6±4.1 and 96.7±6.4 kg for the male subjects, and 50.4±3.3, 6±2.8 and 69.1±2.7 kg for the female subjects (Table 1). The effects of selected anthropometric parameters on the STHT responses were also investigated for the two genders by grouping the datasets within three ranges of each of the selected anthropometric dimension. Table 1 also lists the mean and standard deviations of the selected dimensions together with the subjects sample corresponding to the three ranges. The number of subjects within each range varied from a minimum of 6 to a maximum of 11. Considering the widely different body mass of the male and female subjects, the influence of gender on the STHT response was further investigated considering comparable body mass of the male and female subjects. In a similar manner, owing to the possible coupled effects of other selected anthropometric parameters, the datasets were also grouped so as to achieve comparable target values for the male as well as female subjects. This task, however, was quite challenging considering relatively higher body mass range of the male subjects (55 to 106 kg) compared to the female subjects (45.5 to 7 kg), and large differences in the selected anthropometric dimensions of the two gender groups. Table 2 summarizes the ranges of comparable anthropometric dimensions for the two genders together with the corresponding number of datasets. Only the data for body mass in the vicinity of 60 and 70 kg could be used to study gender effect considering similar body mass. Table 2 The ranges of selected anthropometric factors used to compare STHT responses of male and female subjects of comparable anthropometric dimensions. Gender Male Female Anthropometric parameters Range n Range n mass-related Body mass (kg) G60 55.0-65.0 7 57.0-65.0 7 Body mass (kg) G70 66.0-75.0 7 66.0-7 7 BMI (kg/m 2 ) 23.3-27.5 11 24.4-26.3 10 Body fat (kg) 19.0-29.0 7 2-25.3 9 Body fat (%) 26.9-31.2 6 27.9-33.8 9 Lean body mass (kg) 43.3-54.5 6 45.4-49.5 8 build-related Hip circumference (cm) 98.3-106.4 11 97.0-103.0 9 Contact area (cm 2 ) 615-695 8 600-760 6 Mean peak pressure (kpa) 8.1-10.4 11 8.7-10.2 9 stature-related Stature (m) 1.60-1.72 10 1.61-1.67 9 Sitting height (cm) 83.0-87.5 8 83.0-85.5 8 C7 height (cm) 65.8-68.7 10 63-67.6 8 n: number of subjects 16 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL

Krishna N. Dewangan, Arman Shahmir, Subhash Rakheja and Pierre Marcotte These included 7 female and 7 male subjects for each of the two mass ranges, denoted as G60 (male: 60.4±4.2 kg; female: 6±2.6 kg) and G70 (male: 70.3±3.7 kg; female: 69.6±2.7 kg). In a similar manner, 6 to 11 subjects representing comparable values for each gender group could be extracted for the other parameters as summarized in Table 2. The data for these subjects were subsequently analyzed to derive the respective mean magnitude responses to identify gender effect, if any, considering comparable anthropometric parameters including the body mass. Three factor analyses of variance (ANOVA) were performed to identify the statistical significance levels of the main factors, the gender, the back support and the excitation magnitude, on the STHT response. The gender effect was further evaluated from a paired t-test considering the data of the two gender groups with comparable anthropometric dimensions (Table 2) for both the back support conditions and the three levels of excitation. 3. RESULTS 3.1 STHT Response Characteristics Figure 1, as an example, compares the measured vertical and fore-aft STHT responses of 58 subjects seated without the back support (NB) and exposed to 0 m/s 2 rms acceleration excitation. The figure shows the mean responses of each subject, obtained from the two trials, and the phase responses of the vertical STHT. The comparisons suggest large inter-subject variations in the as well as the primary resonance frequency. For vertical STHT responses, the primary resonance frequency varied from 4.13 to 6.00 Hz for the NB support, and was observed in the 3.94 to 6.50 Hz range for the WB sitting condition. The coefficient of variation (CoV) of the magnitude data ranged from 15% to 29% for the NB support and 12% to 27% for the WB support in the vicinity of the primary resonance frequency. The vertical STHT responses of many subjects also revealed a distinct secondary magnitude peak in the 8 to 14 Hz range for both the back support conditions. (c) Figure 1. Comparisons of STHT responses of 58 (31 male and 27 female) subjects seated without back support (NB) and exposed to 0 m/s 2 rms acceleration excitation: vertical ; fore-aft ; and (c) vertical STHT phase. The frequency corresponding to the peak fore-aft magnitudes varied from 3.88 to 6.31 Hz for the NB support and 3.56 to 6.06 Hz with the WB support. The fore-aft STHT responses revealed relatively higher scatter with CoV of the magnitude data ranging from 35% to 44% for the NB support, and 27% to 31% for the WB support condition, respectively around the primary resonance frequency. The measured Vol. 32 No. 1+2 2013 17

and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects vertical and fore-aft magnitude data revealed relatively lesser degree of scatter when sitting with a back support. The fore-aft magnitude responses of most of the subjects also revealed a peak in the -2 Hz range for both the back support condition, while some of the subjects data revealed a secondary peak in a relatively higher frequency range (9 to 16 Hz). The magnitude of this secondary peak was substantially smaller compared to that in the vertical direction. The light-weight male subjects data also showed an additional peak in the 3 to 4 Hz range for the NB sitting condition. Similar degree of scatter and trends were also observed in the responses obtained under other test conditions involving different excitation magnitudes. The mean responses of the male and female subjects were subsequently obtained for each test condition. As an example, Figure 2 compares mean STHT responses of the 31 male and 27 female subjects for two sitting conditions, i.e. NB and WB, and 0 m/s 2 rms acceleration excitation. The comparisons show slightly higher peak of the male subjects than the female subjects, when seated with NB support. The mean STHT response of the female subjects, however, is higher than that of the male subjects at frequencies above 7 Hz for both the NB and WB supports. The mean primary resonance frequency of the female subjects was lower compared to the male subjects, irrespective of the back support and excitation conditions, while the frequency corresponding to the secondary peak was higher for the female subjects. The vertical STHT phase responses of the male and female subjects, however, were comparable for both the sitting conditions. The mean foreaft of the male subjects was also higher than the female subjects in the 3 to 15 Hz frequency range for the NB support, as seen in Figure 2. The mean fore-aft STHT responses of the two genders, however, are comparable for the WB support. Similar trends in both the STHT responses were also observed under other test conditions involving different excitation magnitudes. Figure 2. Mean responses of 31 male and 27 female subjects exposed to 0 m/s 2 rms acceleration excitation: without back support; and with back support. 18 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL

Krishna N. Dewangan, Arman Shahmir, Subhash Rakheja and Pierre Marcotte The effect of the back support condition on the mean vertical and fore-aft STHT responses of the male and female subjects under 0 m/s 2 excitation are shown in Figure 3. The results show substantial effect of the back support on the vertical STHT responses for the male as well as the female subjects. In the fore-aft STHT responses, the effect of the back support was evident for the male subjects, however the responses for both the sitting support conditions were comparable for the female subjects. The data were further analyzed to identify the mean and standard deviations of the peak vertical and fore-aft s of the male and female subjects together with the corresponding frequencies, which are summarized in Table 3. Important gender effect on the STHT responses is evident from the results, which appears to be coupled with the sitting condition and the excitation magnitude. The peak vertical is reduced by about 16% and 8% for the male and female subjects, respectively, when sitting condition was changed from the NB to WB support. Furthermore, the change in sitting condition from NB to WB support resulted in lower primary resonance frequency, irrespective of the excitation level. For the three excitations considered, the mean change in the primary vertical STHT frequency was 0.26 and 0.38 Hz for the male and female subjects, respectively, when the sitting condition was altered from NB to WB. The reduction in the peak fore-aft magnitude was 11% for the male subjects, while the peak magnitude increased 6% for the female subjects, when the sitting condition was changed from the NB to WB support. Mean increase in the primary resonance frequency of the fore-aft STHT response was 0.48 and 0.20 Hz, respectively for the male and female subjects. Figure 4 compares the mean vertical and fore-aft magnitude responses obtained under three excitation magnitudes for the male and female subjects when sitting without a back support. The results show softening tendency of the human body, i.e. Table 3 Mean (standard deviation) of the peak sand the corresponding frequencies of the 31 male and 27 female subjects for the two sitting conditions and three levels of excitation. Excitation (m/s 2 ) Male Female NB WB NB WB Peak vertical STHT 0.25 2.25(0.37) 1.86(0.17) 7(0.28) 1.93(0.21) 0 2.13(0.23) 1.82(0.17) 7(0.27) 1.90(0.23) 0.75 2.18(0.32) 1.85(0.18) 2.13(0.30) 1.92(0.20) Frequency corresponding to peak vertical STHT 0.25 5.69(0.61) 5.35(1) 5.11(6) 4.70(0.47) 0 5.14(0.40) 5.00(5) 4.82(0.47) 4.40(3) 0.75 4.97(0.44) 4.66(0) 4.65(0.44) 4.35(0.44) Peak fore-aft STHT 0.25 2.78(6) 8(0.38) 2.34(8) 3(0.43) 0 8(0.61) 2.35(0.35) 2.13(0.66) 2.20(0.37) 0.75 2.43(7) 2(0.33) 1.98(7) 2.12(0.42) Frequency corresponding to peak fore-aft STHT 0.25 5.40(0.81) 4.83(0.71) 4.95(0.48) 4.79(9) 0 5.05(0.78) 4.65(0.74) 4.75(8) 4.49(6) 0.75 4.76(0.60) 4.39(0.66) 4.64(0.47) 4.36(0.60) NB: without back support; WB: with back support Vol. 32 No. 1+2 2013 19

and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects Without back support With back support Fore-a Figure 3. Fore-a Comparisons of mean s of 31 male and 27 female subjects seated with and without a back support under 0 m/s 2 excitations: male subjects; and female subjects. 0.25 m/s² 0 m/s² 0.75 m/s² Fore-a Figure 4. Influence of excitation magnitude on the mean responses of 31 male and 27 female subjects sitting without a back support: male; and female. a decrease in the primary resonance frequency with increase in the magnitude of excitation. Softening tendency was also evident with the back support condition. The softening tendency was more pronounced with increase in excitation from 0.25 to 0 m/s 2, while this effect was smaller with increase in excitation from 0 to 0.75 m/s 2. The softening tendencies, however, differed between the male and female subjects. The effect of the excitation magnitude on the peak s was relatively small, irrespective of the back support. The softening tendency among the male and female subjects is further studied by considering changes in the primary resonance frequency and the corresponding with increase in excitation magnitude from 0.25 to 0.75 m/s 2 (Table 3). Important gender effect on the STHT responses is evident from the results, which appears to be coupled with the sitting condition and the excitation magnitude. The results in Table 3 also show relatively higher softening effect of increasing excitation on the male subjects response compared to the female subjects. For the NB support, increasing vibration level from 0.25 to 0.75 m/s 2 resulted in reductions in the primary resonance Fore-a 20 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL

Krishna N. Dewangan, Arman Shahmir, Subhash Rakheja and Pierre Marcotte frequencies of the vertical STHT of 0.72 and 0.46 Hz for the male and female subjects, respectively,. The corresponding changes for the WB support were 0.69 and 0.35 Hz. Similar softening trends were also evident in the fore-aft STHT response. The frequency corresponding to the peak fore-aft magnitude was 0.64 and 0.31 Hz lower for the male and female subjects seated with the NB support, respectively. However, with the WB support, the corresponding reductions in the frequency were 0.44 and 0.43 Hz. 3.2 Gender and Mass Related Anthropometry The vertical STHT responses of the male and female subjects with body mass within three mass ranges of each gender (Table 1) are presented in Figures 5 and 5 for the two back support conditions, respectively, and 0 m/s 2 excitation. The figures show the mean response of the subjects within each mass range. The vertical STHT responses of subjects within the three mass groups show that the primary resonance frequency for the lighter subjects group is higher than that of the heavier subjects group for both the genders, irrespective of the sitting condition. The differences in the primary resonance frequencies between the light and heavy subjects groups are more important for the NB support as compared to the WB support, and the differences are more pronounced for the male subjects than the female subjects, particularly for the WB condition. Furthermore, the heavier subjects revealed higher vertical in the vicinity of the primary resonance frequency, when seated without a back support. The light-weight subjects, however, resulted in substantially higher around the secondary resonance for both back support conditions. The differences in the STHT magnitude with the body mass, however, were substantially lower in the presence of the vertical back support for both the gender groups. The fore-aft STHT responses of the male and female subjects with body mass within the three mass ranges for each gender (Table 1) are presented in Figure 6 for the two sitting conditions and the 0 m/s 2 excitation. Like vertical STHT responses, the fore-aft STHT responses of the three mass groups show that the primary resonance frequency of the lighter subjects group is generally higher than that of the heavier subjects group for both genders, irrespective of the back support condition. However, the effect on the primary resonance frequency was more pronounced for the male subjects as compared to the female subjects. Furthermore, the heavier female subjects revealed slightly higher fore-aft in the vicinity of the primary resonance Male (60.1 kg) Male (81.6 kg) Male (96.7 kg) Female (50.4 kg) Female (6 kg) Female (69.1 kg) Figure 5. Mean vertical responses of the male and female subjects within three different mass groups corresponding to different back support condition and 0 m/s 2 excitation: without back support; and with back support. Vol. 32 No. 1+2 2013 21

and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects Fore-a Fore-a Figure 6. Male (60.1 kg) Male (81.6 kg) Male (96.7 kg) Fore-a Fore-a Female (50.4 kg) Female (6 kg) Female (69.1 kg) Mean fore-aft responses of the male and female subjects within three different mass groups corresponding to different back support condition and 0 m/s 2 excitation: without back support; and with back support. Male (21.6 kg/m²) Male (25.6 kg/m²) Male (3 kg/m²) Female (19.4 kg/m²) Female (22.6 kg/m²) Female (25.3 kg/m²) Male (1 kg) Male (16.6 kg) Male (26.2 kg) Female (13.5 kg) Female (19.1 kg) Female (23.7 kg) Male (50.1 kg) Male (6 kg) Male (68.8 kg) Female (36.0 kg) Female (41.4 kg) Female (47.3 kg) Figure 7. (c) Effects of mass-related parameters on the mean vertical responses of the male and female subjects: BMI; body fat mass; and (c) lean body mass (NB posture, 0 m/s 2 excitation). 22 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL

Krishna N. Dewangan, Arman Shahmir, Subhash Rakheja and Pierre Marcotte frequency for the both back support conditions, while the three body mass ranges of the male subjects revealed comparable peak magnitudes. Similar trends in both the STHT responses were observed under the other excitations. Figure 7 shows the effects of mass-related parameters (BMI, body fat and lean body mass) on the mean vertical responses. The results are obtained considering the male and female subjects datasets within three ranges of mass-related factors, listed in Table 1. The results are presented only for the NB support and 0 m/s 2 excitation, although similar trends were also observed under other excitations and WB sitting condition. Results show that the primary resonance frequency was higher for the subjects with lower values of all the mass related parameters for both the gender groups, compared to that observed for the subjects with higher BMI, body fat and lean body mass. The peak magnitude corresponding to the primary resonance frequencies were generally comparable for the three ranges of the mass related parameters considered. Figure 8 shows the effects of massrelated parameters on the mean fore-aft responses for the male and female subjects for the NB support and 0 m/s 2 excitation. The results trends shown are to be similar to those observed in the vertical responses. The effects of the mass related parameters, however, is relatively more pronounced for the male subjects compared to the female subjects. The above results suggest that the gender effect on the STHT responses may be coupled with the body mass. Considering substantially higher body mass of the male subjects compared to the female subjects employed in the study, the gender effect is further investigated by comparing the responses of the male and female subjects of comparable body mass and related parameters (BMI, body fat mass and lean body mass), as summarized in Table 2. The vertical and fore-aft STHT responses of the male and female subjects of two similar mass groups (G60 and G70) are compared Fore-a Fore-a Fore-a Figure 8. Male (21.6 kg/m²) Male (25.6 kg/m²) Male (3 kg/m²) Male (1 kg) Male (16.6 kg) Male (26.2 kg) Male (50.9 kg) Male (6 kg) Male (68.8 kg) Fore-a Fore-a Fore-a (c) Female (19.4 kg/m²) Female (22.6 kg/m²) Female (25.3 kg/m²) Female (13.5 kg) Female (19.1 kg) female (23.7 kg) Female (36.0 kg) Female (41.4 kg) Female (47.3 kg) Effects of mass-related parameters on mean fore-aft responses of the male and female subjects: BMI; body fat mass; and (c) lean body mass (NB posture, 0 m/s 2 excitation). Vol. 32 No. 1+2 2013 23

and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects in Figure 9 for the two back support conditions (NB and WB) and 0 m/s 2 excitation. The results show that the peak vertical and fore-aft s of the male and female subjects within both the mass groups are comparable. The primary resonance frequency of the female subjects, however, is lower than that of the male subjects of comparable body mass for both the back support conditions. While the peak magnitudes of both gender groups are quite comparable, the female subjects yield lower vertical STHT at frequencies above 10 Hz for the NB condition. The high frequency s of both genders are also comparable for the WB condition. These trends are considerably different from those observed from the mean responses of all the male and female subjects shown in Figure 2. This difference is likely caused by large difference in the mean masses of the two gender groups and suggests strong coupling of the gender effect with the body mass. Male-G60 Female-G60 Male-G70 Female-G70 Fore-and-a Figure 9. Mean responses of the male and female subjects within two mass groups (G60 and G70) corresponding to different sitting conditions and 0 m/s 2 excitation: without back support; and with back support. Fore-and-a The results attained through ANOVA of the datasets corresponding to the G60 and G70 groups, considering three main factors (G gender, BS back support and E excitation magnitude), suggest that the resonance frequency, observed from both the vertical and fore-aft STHT responses, of the female subjects was significantly (p<05) different from that of the male subjects within both the body mass groups (Table 4). The difference in the peak magnitudes of the two gender groups, however, is insignificant. The results also show significant effect of the back support condition on the peak fore-aft and the corresponding frequency (p<5) for both the body mass groups. The effect on the peak vertical STHT magnitude was significantly different (p<01) while the frequency corresponding to peak magnitude was not significant. Furthermore, the primary resonance frequency of both the STHT responses was significantly (p<5) affected by the magnitude of excitation, while the effect on peak peak vertical was statistically insignificant for the male and female subjects within the two mass groups (G 60 and G70). The excitation magnitude, however, revealed significant effect on the fore-aft STHT peak magnitude (p<5). The interaction effect of the gender and other two factors (back support condition and excitation magnitude) was not significantly different. The results obtained from the paired t-test further showed that the vertical vibration transmissibility of the two genders of comparable mass are significantly different in most of the 3 to 7 Hz range, irrespective of the back support and excitation level (Table 5), while the fore-aft is significant in the 5.5 to 7.5 Hz range (Table 6). 24 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL

Krishna N. Dewangan, Arman Shahmir, Subhash Rakheja and Pierre Marcotte Table 4 p-values obtained from a three-factor (G, BS and E) analysis of variance (ANOVA) of the primary resonance frequency and peak s for the two gender groups of comparable body mass (60 and 70 kg). Body mass Measure G BS E G60 resonance frequency 00 0.104 36 peak magnitude 0.716 00 0.766 G70 resonance frequency 02 55 14 peak magnitude 0.369 01 0.748 G60 Fore-and-aft resonance frequency 00 13 20 Fore-and-aft peak magnitude 93 14 12 G70 Fore-and-aft resonance frequency 00 01 45 Fore-and-aft peak magnitude 0.211 00 01 G - gender (male and female), BS - back support (Without back support and with back support), E - excitation magnitude (0.25, 0 and 0.75 m/s 2 ) Table 5 p-values obtained from paired t-test of vertical s of male and female subjects within two comparable body mass groups (60 and 70 kg) for two back support conditions and three levels of excitation. Frequency Without back support With back support (Hz) 0.25 m/s 2 0 m/s 2 0.75 m/s 2 0.25 m/s 2 0 m/s 2 0.75 m/s 2 G60 G70 G60 G70 G60 G70 G60 G70 G60 G70 G60 G70 2 0.104 0.243 0.161 0.218 0.888 0.149 0.172 0.200 0.184 57 55 56 3 0.123 0.338 08 81 41 09 03 00 05 12 08 49 4 17 26 01 46 0.196 28 11 0.116 48 0.256 0.287 0.147 4.5 26 0.288 89 0.454 0.919 0.291 60 0.169 0.473 30 0.834 0.801 5 0.242 0.802 0.673 11 0.202 0.454 0.119 0.756 0.117 0.326 86 16 5.5 0.755 0.197 46 07 27 14 37 13 11 11 01 00 6 41 13 06 07 05 14 03 04 03 20 02 02 6.5 01 04 01 27 02 15 00 27 01 46 20 0.132 7 01 08 00 0.108 0.210 0.294 02 46 66 0.421 02 0.682 7.5 01 44 05 0.605 0.287 0.936 92 0.439 31 0.374 0.191 0.185 8 18 0.496 0.805 95 0.233 0.608 54 0.240 40 0.225 0.114 0.332 9 0.610 73 93 0.154 0.296 0.472 0.137 0.164 0.326 0.250 0.241 0.362 10 30 0.226 0.357 0.347 0.287 0.154 57 0.322 0.335 0.228 59 0.436 12 0.850 0.368 0.187 0.254 0.644 0.724 0.327 0.835 0.723 0.283 0.943 0.692 15 0.407 0.826 0.140 92 0.819 11 0.473 0.158 0.324 0.894 35 0.761 The mean vertical and fore-aft STHT responses of the male and female subjects of comparable BMI (23.3-27.5 kg/m 2 for male and 24.4-26.3 kg/m 2 for female), body fat mass (19.0-29.0 kg for male and 2-25.3 kg for female) and lean body mass (43.3-54.5 kg for male and 45.4-49.5 kg for female) are also compared in Figure 10. The comparisons are presented only for the NB condition and 0 m/s 2 excitation. The results show substantial gender effect on the STHT responses, when the data for male and female subjects with comparable BMI and lean body mass are considered. The effects are very pronounced on the peak vertical as well as on the fore-aft s and the corresponding frequencies. The gender effect, Vol. 32 No. 1+2 2013 25

and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects Table 6 p-values obtained from paired t-test of fore-and-aft s of the male and female subjects within two comparable body mass groups (60 and 70 kg) for two back support conditions and three levels of excitation. Frequency No back support back support (Hz) 0.25 m/s 2 0 m/s 2 0.75 m/s 2 0.25 m/s 2 0 m/s 2 0.75 m/s 2 G60 G70 G60 G70 G60 G70 G60 G70 G60 G70 G60 G70 2 0.625 0.251 71 0.755 97 0.292 17 0.298 11 0.769 0.287 92 3 0.689 0.411 0.674 0.976 0.195 0.309 0.807 0.214 64 0.218 0.901 0.249 4 91 0.167 0.309 0.157 00 19 0.480 0.165 0.227 0.151 0.684 03 4.5 0.164 0.366 0.158 0.379 0.960 47 14 0.153 0.714 35 0.983 0.213 5 19 0.988 0.381 0.239 0.212 0.637 0.405 0.605 0.788 0.420 24 40 5.5 0.163 0.111 0.186 14 21 0.102 35 77 21 03 07 13 6 63 03 11 04 06 49 18 34 11 14 08 42 6.5 13 00 03 00 01 31 11 29 47 05 25 0.191 7 04 00 01 11 01 22 47 20 76 06 55 32 7.5 02 01 02 32 05 54 73 04 77 04 0.160 0.901 8 05 02 17 98 0.195 0.219 74 0.126 68 81 0.109 0.813 8.5 05 43 77 0.168 99 0.243 0.147 0.196 0.133 0.197 0.249 0.332 9 32 0.146 0.902 0.653 0.901 89 0.266 0.294 0.252 0.474 0.252 0.348 10 0.237 0.729 0.658 0.432 46 69 0.228 0.782 0.144 0.771 0.700 54 Figure 10. Male (25.6 kg/m²) Female (25.3 kg/m²) Male (23.3 kg) Female (23.7 kg) Male (48.6 kg) Female (47.3 kg) Fore-a Fore-a Fore-a (c) Effects of gender on mean considering comparable mass-related parameters: BMI; fat body mass; and (c) lean body mass (NB posture, 0 m/s 2 excitation). 26 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL

Krishna N. Dewangan, Arman Shahmir, Subhash Rakheja and Pierre Marcotte Table 7 p-values obtained from paired t-test of vertical peak and the primary resonance frequency between the male and female subjects of comparable anthropometric dimensions. Anthropometric parameters Primary resonance frequency Peak magnitude without back support with back support Without back support With back support 0.25 m/s 2 0 m/s 2 0.75 m/s 2 0.25 m/s 2 0 m/s 2 0.75 m/s 2 0.25 m/s 2 0 m/s 2 0.75 m/s 2 0.25 m/s 2 0 m/s 2 0.75 m/s 2 Mass-related BMI 04 11 00 12 46 44 0.697 0.883 0.488 0.875 0.680 0.455 Body fat mass 0.357 0.401 56 91 0.106 0.671 0.246 0.356 0.757 65 0.435 0.309 Percent body fat 0.833 0.969 0.741 0.254 80 0.946 0.238 0.273 0.696 0.362 0.463 0.212 Lean body mass 01 20 20 21 10 47 46 11 34 45 34 40 Build-related Hip circumference 45 26 02 01 14 06 47 50 40 38 14 28 Seat-pan contact area 27 49 45 21 32 49 0.485 87 0.467 0.911 24 0.743 Mean contact pressure 0.808 0.341 09 0.244 0.149 0.243 0.622 0.946 0.835 0.135 0.469 0.971 Stature-related Stature 26 41 12 05 44 49 48 44 20 47 50 54 Sitting height 0.231 0.374 80 0.432 53 0.720 0.303 0.764 0.421 0.259 0.858 80 C7 height 0.334 0.271 0.369 36 55 0.641 0.632 0.840 0 0.121 94 0.187 Vol. 32 No. 1+2 2013 27

and Fore-Aft Seat-to-Head Transmissibility Response to Whole Body Vibration: Gender and Anthropometric Effects however, is small when the vertical and fore-aft STHT data for the male and female subjects of comparable body fat mass are considered, particularly up to 10 Hz. For comparable BMI and lean body mass, the peak vertical s as well as primary resonance frequencies of the female subjects are considerably lower compared to those of the male subjects, while the mean female subjects response shows more prominent secondary peak. The fore-aft peak of the female subjects is also lower compared to male subjects of comparable BMI, while the resonance frequencies are comparable. For comparable lean body mass, the foreaft of the female subjects is higher, while the corresponding frequency is considerable lower compared to those of the male subjects. This trend is opposite to that observed for comparable BMI. Paired t-tests of the selected datasets show significant differences (p<5) in the vertical peak STHT magnitudes of the male and female subjects of comparable lean body mass (Table 7). The gender effect is also significant (p<5) in view of the primary resonant frequency, when comparable BMI and lean body mass are considered. 3.3 Gender and Build-Related Anthropometry The effect of build related anthropometric parameters (hip circumference, seat-pan contact area and mean contact pressure) on the vertical STHT responses of the male and female subjects are presented in Figure 11. The results are obtained considering the male and female subjects datasets within three ranges of build-related factors, listed in Table 1, and the NB support and 0 m/s 2 excitation. The results show that the primary resonance frequencies are higher for the subjects with lower hip circumference and lower seat-pan contact area. An opposite trend, however, is evident with the mean contact pressure for both genders, since a lower contact area would yield a higher mean contact pressure. The peak s of the male Male (95.5 cm) Male (102.8 cm) Male (110.7 cm) Female (92.6 cm) Female (100.4 cm) Female (105.2 cm) Male (362 cm²) Male (556 cm²) Male (666 cm²) Female (350 cm²) Female (510 cm²) Female (682 cm²) Male (9.1 N/cm²) Male (13.2 N/cm²) Male (17.1 N/cm²) Female (7.2 N/cm²) Female (9.3 N/cm²) Female (12.3 N/cm²) Figure 11. (c) Effects of build-related parameters on mean vertical responses of the male and female subjects: hip circumference; seat-pan contact area; and (c) mean contact pressure (NB posture, 0 m/s 2 excitation). 28 JOURNAL OF LOW FREQUENCY NOISE, VIBRATION AND ACTIVE CONTROL

Krishna N. Dewangan, Arman Shahmir, Subhash Rakheja and Pierre Marcotte and female subjects, however, are quite comparable for the ranges of build-related factors considered. Comparisons of the fore-aft responses for the 3 groups of build related factors of the male and female subjects also revealed similar trends in the primary resonance frequency (Figure 12). The responses of the male and female subjects of comparable build-related parameters are compared in Figure 13, for the NB condition and 0 m/s 2 excitation. These include the datasets of male and female subjects with comparable hip circumference (98.3-106.4 cm for male and 97.0-103.0 cm for female), seat-pan contact area (615-695 cm 2 for male and 600-760 cm 2 for female) and mean contact pressure (8.1-10.4 kpa for male and 8.7-10.2 kpa for female), as summarized in Table 2. The results show that the peak vertical and fore-aft s and the corresponding frequencies of the male subjects are slightly higher compared to the female subjects of comparable hip circumference and mean contact area. The frequencies corresponding to the peak vertical and fore-aft responses of the male and female subjects of comparable mean contact pressure, however, are quite similar, while the peak magnitudes are higher for the male subjects. The results obtained from the paired t-tests of the selected datasets also show that the vertical peak of the male and female subjects are significantly different (p<5), as seen in Table 7 for the hip circumference. The results also show significant differences (p<5) in the primary resonance frequencies of the male and female subjects responses for comparable hip circumference and seat-pan contact area. Fore-a Fore-a Fore-a Figure 12. Male (95.5 cm) Male (102.8 cm) Male (110.7 cm) Male (362 cm²) Male (556 cm²) Male (666 cm²) Male (9.1 N/cm²) Male (13.2 N/cm²) Male (17.1 N/cm²) Fore-a Fore-a Fore-a (c) Female (92.6 cm) Female (100.4 cm) Female (105.2 cm) Female (350 cm²) Female (510 cm²) Female (682 cm²) Female (7.2 N/cm²) Female (9.3 N/cm²) Female (12.3 N/cm²) Effect of build-related parameters on mean fore-aft responses of the male and female subjects: hip circumference; seat-pan contact area; and (c) mean contact pressure (NB posture, 0 m/s 2 excitation). Vol. 32 No. 1+2 2013 29