Mechanical impedance of the human body in vertical direction

Dynamic interaction between the human body and the seat during vertical vibration: effect of inclination of the seat pan and the backrest on seat transmissibilities

Seat inclinations at the seat pan and backrest may affect the sitting comfort. This study was designed to quantify the effect of inclination of a seat pan (0°, 10°, and 20°) and backrest (0°, 15°, and 30°), either foamed or rigid, on the transmissibilities measured at the seat pan and backrest. Seat transmissibilities were measured with fifteen subjects exposed to vertical random vibration between 1 and 15 Hz at 0.5 ms^-2 r.m.s. It was found the resonance frequencies in transmissibilities measured at the seat pan and backrest increased with increasing the backrest inclination but were not affected by the seat pan angle. Increasing the foamed backrest inclination increased the peak transmissibilities. Inclination of the rigid seat pan or the rigid backrest reduced the transmissibilities measured at the backrest or the seat pan, respectively. Transmissibilities were more significantly affected by the backrest inclination than the seat pan inclination. Practitioner summary: Seat inclinations may alter the human-seat dynamic interaction and hence the riding discomfort. This study was designed to quantify the effect of inclined seats, either foamed or rigid, on the transmissibilities. It was found the backrest angle affected the transmissibilities more strongly than the seat pan angle.

Assessment of passenger long-term vibration discomfort: a field study in high-speed train environments

This study revealed the mechanism of long-term passenger vibration discomfort in high-speed trains and proposed a novel evaluation model to assess it, while the most popular international standard ISO 2631-1 is unsuitable. Here, a field test was conducted to investigate the long-term passenger vibration comfort in high-speed trains under different operation environments by the measurement of the whole-body vibration (WBV) and the subjective ratings of passenger comfort. During the whole sitting period of high-speed train passengers, the phenomena 'compensatory degradation' and 'discomfort accumulation' were found, which meant that the brief termination of vibration cannot fundamentally alleviate passenger vibration comfort. And the vibration comfort can be evaluated by the product of exposure time and the novel vibration acceleration index we proposed. Meanwhile, high-speed trains with higher velocities or running in tunnel environments have higher frequency-weighted WBV amplitude than open-air and lower velocities, which caused more vibration discomfort of passengers. Practitioner Summary: This field study provided data support for ensuring the occupational health of train drivers whose work routes involve a large number of tunnels and improving passenger vibration comfort. Meanwhile, a novel idea was provided for evaluating the vibration comfort of passengers who prolonged exposure to low-amplitude environments.

Relationships between Height, Mass, Body Mass Index, and Trunk Muscle Activation during Seated Whole-Body Vibration Exposure

Operators of heavy equipment are often exposed to high levels of whole-body vibration (WBV), which has been associated with a variety of adverse health outcomes. Although anthropometric factors are known to impact vibration dose and risk of low back pain, studies have yet to investigate the influence of anthropometric factors on muscle activation during WBV exposure. This study quantified the relationships between muscle activation, vibration frequency, body mass, body mass index (BMI), and height both pre- and post-fatigue. Muscle activation of the external oblique (EO), internal oblique (IO), lumbar erector spinae (LE) and thoracic erector spinae (TE) were quantified using surface electromyography. Results indicate increased activation with increased mass, BMI, and frequency for the LE, TE, and IO, which may be a result of increased activation to stabilize the spine. Decreased muscle activation with increased height was seen in the TE, IO, and pre-fatigue EO, which could indicate higher risk for low back injury since height is associated with increased forces on the spine. This may contribute to the association between increased low back pain incidence and increased height. Results suggest that ISO 2631-1 health guidance should incorporate anthropometric factors, as these may influence muscle activation and back injury risk.

Apparent mass of the seated human body during vertical vibration in the frequency range 2-100 Hz

We studied the apparent mass during vertical whole-body vibration in the frequency range 2-100 Hz at four magnitudes (sinusoidal sweep signals, 1.0, 1.5, 2.0 and 2.5 ms^-2 r.m.s.) in 12 males and 12 females with upright and relaxed sitting postures. The first two peaks of apparent mass decreased with increasing vibration magnitude with both postures. The non-linearity characteristics became obscured at the two largest magnitudes and were less transparent with relaxed sitting posture. The peak frequencies and the normalised apparent masses were similar between males and females with both postures. The standardised three degrees-of-freedom parametric model with modified parameters was proposed to predict well the apparent mass of seated human body during vertical vibration in the frequency range 2-100 Hz and in the magnitude range 1.0-2.5 ms^-2 r.m.s. Practitioner summary: This study shows the frequency-dependence and magnitude-dependence of biodynamic responses in the frequency range of 2-100 Hz. The magnitude of apparent mass at frequencies above 20 Hz may not be negligible. The proposed 3 DOF model with modified parameters would help with understanding and developing the human-seat system. Abbreviations: WBV: whole-body vibration; DOF: degrees-of-freedom; CMIFs: complex mode indicator functions.

Analysis of non-linear response of the human body to vertical whole-body vibration

The human response to vibration is typically studied using linear estimators of the frequency response function, although different literature works evidenced the presence of non-linear effects in whole-body vibration response. This paper analyses the apparent mass of standing subjects using the conditioned response techniques in order to understand the causes of the non-linear behaviour. The conditioned apparent masses were derived considering models of increasing complexity. The multiple coherence function was used as a figure of merit for the comparison between the linear and the non-linear models. The apparent mass of eight male subjects was studied in six configurations (combinations of three vibration magnitudes and two postures). The contribution of the non-linear terms was negligible and was endorsed to the change of modal parameters during the test. Since the effect of the inter-subject variability was larger than that due to the increase in vibration magnitude, the biodynamic response should be more meaningfully modelled using a linear estimator with uncertainty rather than looking for a non-linear modelling.

Response of the seated human body to whole-body vertical vibration: biodynamic responses to sinusoidal and random vibration

The dependence of biodynamic responses of the seated human body on the frequency, magnitude and waveform of vertical vibration has been studied in 20 males and 20 females. With sinusoidal vibration (13 frequencies from 1 to 16 Hz) at five magnitudes (0.1-1.6 ms(-2) r.m.s.) and with random vibration (1-16 Hz) at the same magnitudes, the apparent mass of the body was similar with random and sinusoidal vibration of the same overall magnitude. With increasing magnitude of vibration, the stiffness and damping of a model fitted to the apparent mass reduced and the resonance frequency decreased (from 6.5 to 4.5 Hz). Male and female subjects had similar apparent mass (after adjusting for subject weight) and a similar principal resonance frequency with both random and sinusoidal vibration. The change in biodynamic response with increasing vibration magnitude depends on the frequency of the vibration excitation, but is similar with sinusoidal and random excitation.

Predictive discomfort of non-neutral head-neck postures in fore-aft whole-body vibration

It seems obvious that human head-neck posture in whole-body vibration (WBV) contributes to discomfort and injury risk. While current mechanical measures such as transmissibility have shown good correlation with the subjective-reported discomfort, they showed difficulties in predicting discomfort for non-neutral postures. A new biomechanically based methodology is introduced in this work to predict discomfort due to non-neutral head-neck postures. Altogether, 10 seated subjects with four head-neck postures--neutral, head-up, head-down and head-to-side--were subjected to WBV in the fore-aft direction using discrete sinusoidal frequencies of 2, 3, 4, 5, 6, 7 and 8 Hz and their subjective responses were recorded using the Borg CR-10 scale. All vibrations were run at constant acceleration of 0.8 m/s² and 1.15 m/s². The results have shown that the subjective-reported discomfort increases with head-down and decreases with head-up and head-to-side postures. The proposed predictive discomfort has closely followed the reported discomfort measures for all postures and rides under investigation. STATEMENT OF RELEVANCE: Many occupational studies have shown strong relevance between non-neutral postures, discomfort and injury risk in WBV. With advances in computer human modelling, the proposed predictive discomfort may provide efficient ways for developing reliable biodynamic models. It may also be used to assess discomfort and modify designs inside moving vehicles.

Energy absorption of seated occupants exposed to horizontal vibration and role of back support condition

Absorbed power characteristics of seated human subjects under fore-aft (x-axis) and lateral (y-axis) vibration are investigated through measurements of dynamic interactions at the two driving-points formed by the body and the seat pan, and upper body and the backrest. The experiments involved: (i) three back support conditions (no back support, and back supported against a vertical and an inclined backrest); (ii) three seat pan heights (425, 390 and 350 mm); and three magnitudes (0.25, 0.5 and 1.0 m/s2 rms acceleration) of band limited random excitations in 0.5-10 Hz frequency range, applied independently along the x- and y- axes. The force responses, measured at the seat pan and the backrest are applied to characterize total energy transfer reflected on the seat pan and the backrest. The mean responses suggest strong contributions due to back support, and direction and magnitude of vibration. In the absence of a back support, the seat pan responses dominated in lower frequency bands centered at 0.63 and 1.25 Hz under both directions of motion. Most significant interactions of the upper body against the back support was observed under fore-aft vibration. The addition of back support caused the seat pan response to converge to a single primary peak near a higher frequency of 4 Hz under x- axis, with only little effect on the y-axis responses. The back support serves as an additional source of vibration to the occupant and an important constraint to limit the fore-aft movement of the upper body and thus relatively higher energy transfer under. The mean responses were further explored to examine the Wd frequency-weighting used for assessing exposure to horizontal vibration. The results show that the current weighting is suited for assessing the vibration exposure of human subjects seated only without a back support.

The apparent mass of the seated human exposed to single-axis and multi-axis whole-body vibration

Most workplaces where workers are exposed to whole-body vibration involves simultaneous motion in the fore-and-aft (x-), lateral (y-) and vertical (z-) directions. Previous studies reporting the biomechanical response of people exposed to vibration have almost always used single-axis vibration stimuli. This paper reports a study where apparent masses of 15 subjects were measured whilst exposed to single-axis and tri-axial whole-body vibration. Each subject was exposed to 28 vibration conditions comprising every combination of single-axis and tri-axial vibration with magnitudes of 0.4 and 0.8 ms(-2) r.m.s. in each direction, once with backrest contact and once without backrest contact. Results show that increasing the magnitude of vibration in directions orthogonal to that being measured affects the apparent mass, causing a reduction in the resonance frequency as the total magnitude of vibration increases. It is demonstrated that the apparent mass resonance frequency is a function of the total vibration magnitude in all axes rather than a function of the vibration magnitude in the direction being measured. It is also shown that, for individuals, the frequency of the peak in the apparent mass in one direction is not related to the frequency of the peak in another direction. It is concluded that more complex biomechanical models are required in order to simulate human response to multi-axis vibration.

Effect of vibration magnitude, vibration spectrum and muscle tension on apparent mass and cross axis transfer functions during whole-body vibration exposure

Twelve seated male subjects were exposed to 15 vibration conditions to investigate the nature and mechanisms of the non-linearity in biomechanical response. Subjects were exposed to three groups of stimuli: Group A comprised three repeats of random vertical vibration at 0.5, 1.0 and 1.5 ms(-2) r.m.s. with subjects sitting in a relaxed upright posture. Group B used the same vibration stimuli as Group A, but with subjects sitting in a 'tense' posture. Group C used vibration where the vibration spectrum was dominated by either low-frequency motion (2-7 Hz), high-frequency motion (7-20 Hz) or a 1.0 ms(-2) r.m.s. sinusoid at the frequency of the second peak in apparent mass (about 10-14 Hz) added to 0.5 ms(-2) r.m.s. random vibration. In the relaxed posture, frequencies of the primary peak in apparent mass decreased with increased vibration magnitude. In the tense posture, the extent of the non-linearity was reduced. For the low-frequency dominated stimulus, the primary peak frequency was lower than that for the high-frequency dominated stimulus indicating that the frequency of the primary peak in the apparent mass is dominated by the magnitude of the vibration encompassing the peak. Cross-axis transfer functions showed peaks of about 15-20% and 5% of the magnitudes of the peaks in the apparent mass for x- and y-direction transfer functions, respectively, in the relaxed posture. In the tense posture, cross-axis transfer functions reduced in magnitude with increased vibration, likely indicating a reduced fore-aft pitching of the body with increased tension, supporting the hypothesis that pitching contributes to the non-linearity in apparent mass.

Comparison of the apparent mass during exposure to whole-body vertical vibration between Japanese subjects and ISO 5982 standard

OBJECTIVE

The purpose of this study was to investigate the apparent mass of the sitting human body and to compare it with current experimental data and the ISO 5982 standard impedance model.

METHOD

The apparent mass of the seated human body in the vertical direction was measured. Twelve male subjects were exposed to random whole-body vibration of frequency range (1-20 Hz), with a vibration excitation level of 1.0 m/s2 r.m.s. The body posture was upright with no backrest contact.

RESULTS

The obtained apparent masses were compared to the International Standard, (ISO 5982). The biodynamic response of the seated Japanese subjects peaked in the 4-6.5 Hz frequency range, which is little bit higher than the reported range of fundamental frequencies (4.5-5 Hz) in most other studies which used different experimental conditions. The outcomes show a clear difference between apparent mass of Japanese subjects and ISO 5982 data.

CONCLUSION

It is not sufficient to apply the ISO 5982 standard to Japanese vehicle design or dummy design.

Impedance methods (apparent mass, driving point mechanical impedance and absorbed power) for assessment of the biomechanical response of the seated person to whole-body vibration

Exposure to whole-body vibration is a risk factor for the development of low back pain. In order to develop a fuller understanding of the response of the seated person to vibration, experiments have been conducted in the laboratory investigating the biomechanics of the seated person. Some of these methods are based on the driving force and acceleration at the seat and are reported in the literature as apparent mass, driving point mechanical impedance or absorbed power. This paper introduces the background behind such impedance methods, the theory and application of the methods. It presents example data showing typical responses of the seated human to whole-body vibration in the vertical, fore-and-aft and lateral directions. It also highlights problems that researchers might encounter in performing, analysing and interpreting human impedance data.

Influence of back support conditions on the apparent mass of seated occupants under horizontal vibration

The response characteristics of seated human subjects exposed to fore-aft (x-axis) and lateral (y-axis) vibration are investigated through measurements of dynamic interactions between the seated body and the seat pan, and the upper body and the seat backrest. The experiments involved: (i) three different back support conditions (no back support, and upper body supported against a vertical and an inclined backrest); (ii) three different seat pan heights (425, 390 and 350 mm); and three different magnitudes (0.25, 0.5 and 1.0 m/s2 rms acceleration) of band limited random excitations in the 0.5-10 Hz frequency range, applied independently along the fore-aft and lateral directions in an uncoupled manner. The body force responses, measured at the seat pan and the backrest along the direction of motion, are applied to characterize the total body apparent mass (APMS) reflected on the seat pan, and those of the upper body reflected on the backrest. Unlike the widely reported responses of seated occupants under vertical vibration, the responses to horizontal vibration show strong effect of excitation magnitude. The large displacements at lower frequencies cause considerable rotations of the upper body, and the knees and ankles, particularly when seated without a back support, which encouraged the occupants to continually shift larger portion of the body weight towards their feet. This together with the strong dependence on the excitation magnitude resulted in considerable inter-subject variability of the data. The addition of a back support causes stiffening of the body to limit the low frequency rocking motion of the upper body under x-axis motion, while considerable dynamic interactions with the backrest occur. The mean apparent mass (APMS) responses measured at the seat pan and the backrest suggest strong contributions due to the back support condition, and the direction and magnitude of horizontal vibration, while the role of seat height is important only in the vicinity of the resonant frequencies. In the absence of a back support, the seat pan responses predominate at a lower frequency (near 0.7 Hz) under both directions of motion, while two secondary peaks in the magnitude also occur at relatively higher frequencies. The addition of back support causes the seat pan response to converge mostly to a single primary peak, resulting in a single-degree-of-freedom like behavior, with peak occurring in the 2.7-5.4 Hz range under x-axis, and 0.9-2.1 Hz range under y-axis motions, depending upon the excitation magnitude and the back support condition. This can be attributed to the stiffening of the body in the presence of the constraints imposed by the backrest. A relaxed posture with an inclined backrest, however, causes a softening effect, when compared to an erect posture with a vertical backrest. The backrest, however, serves as another source of vibration to the seated occupant, which tends to cause considerably higher magnitude responses. The considerable magnitudes of the apparent mass response measured at the seat back under fore-aft motions suggest strong interactions with the backrest. Such interactions along the side-to-side motions, however, are relatively small. The results suggest that the biodynamic characterization of seated occupants exposed to horizontal vibration requires appropriate considerations of the interactions with the backrest.

Comparison of the apparent mass of the seated human measured using random and sinusoidal vibration

Exposure to whole-body vibration is generally accepted as being a risk factor for low back pain and therefore exposure to vibration should be minimised. The results of previous laboratory based research investigating the biomechanical response of the seated human to vibration has been used to develop models that can be used within tools that are capable of predicting the response of seats. Several studies in the literature have reported apparent masses of seated human subjects whilst exposed to either random or sinusoidal vibration. Although these studies have shown similar trends, there have been no systematic comparisons of apparent mass for the same subjects exposed to random and sinusoidal vibration. This paper reports a study where twelve male subjects were exposed to random whole-body vibration at 1.0 m/s2 r.m.s. and to sinusoidal vibration at 1, 2, 4, 8, 16 and 32 Hz. The modulus of the apparent masses were nominally identical when measured using random or sinusoidal vibration. The phase of the apparent masses were similar at 1, 2 and 4 Hz, when measured using random or sinusoidal vibration, but showed consistent differences at 8, 16 and 32 Hz. As the results between experiments using different waveforms are similar, models derived from experimental work based on one type of stimulus could be applied in scenarios involving the other type of stimulus.

Apparent mass of small children: experimental measurements

A test facility and protocol were developed for measuring the seated, vertical, whole-body vibration response of small children of less than 18 kg in mass over the frequency range from 1 to 45 Hz. The facility and protocol adhered to the human vibration testing guidelines of BS7085 and to current codes of ethics for research involving children. Additional procedures were also developed which are not currently defined in the guidelines, including the integral involvement of the parents and steps taken to maximize child happiness. Eight children were tested at amplitudes of 0.8 and 1.2 m/s(2) using band-limited, Gaussian, white noise acceleration signals defined over the frequency interval from 1 to 50 Hz. Driving point apparent mass modulus and phase curves were determined for all eight children at both test amplitudes. All results presented a single, principal, anti-resonance, and were similar to data reported for primates and for adult humans seated in an automotive posture which provided backrest support. The mean frequency of the apparent mass peak was 6.25 Hz for the small children, as compared to values between 6.5 - 8.5 Hz for small primates and values between 6.5 - 8.6 Hz for adults seated with backrest support. The peak value of the mean, normalized, apparent mass was 1.54 for the children, which compares to values from 1.19 to 1.45 reported in the literature for small primates and 1.28 for adults seated with backrest support. ISO standard 5982, which specifies a mean, normalized, apparent mass modulus peak of 1.50 at a frequency of 4.0 Hz for adults seated without backrest support, provides significant differences.

Influence of tyre inflation pressure on whole-body vibrations transmitted to the operator in a cut-to-length timber harvester

The influence of tyre inflation pressure on whole-body vibrations transmitted to the operator during the movement of a cut-to-length timber harvester was evaluated. Vibration measurements were taken in three orthogonal (x, y, z) axes at tyre pressure settings of 138, 345 and 414 kPa. Vibration was predominant in the vertical (z) direction with the peak rms acceleration value for the operator seat (0.281 ms(-2)) occurring at approximately 3.2 Hz. The corresponding peak value for the operator cabin chassis was 0.425 m s(-2) at 4 Hz. At 414 kPa, there was potential health risk on the operator for exposures above 8h duration. The vibration total values recorded for the operator seat at the maximum tyre inflation pressure setting were classed as "fairly uncomfortable" (ISO standard 2631-1), and vertical seat vibration transmissibility was highest between 4 and 8 Hz at the 345 kPa tyre pressure setting. The recorded values of WBV were significantly reduced by a reduction in tyre inflation pressure which may therefore be used to moderate the magnitude of WBV on wheeled timber harvesters.

Whole-body vibration during manual wheelchair propulsion with selected seat cushions and back supports

Although the exposure to whole-body vibrations (WBV) has been shown to be detrimental to seated humans, the effects of wheelchairs and seating systems on the transmission of vibration to an individual have not been thoroughly examined. The purpose of this study was to determine if the selected wheelchair seat cushions and back supports minimize the transmission of vibrations. Thirty-two wheelchair users traversed an activities of daily living course three times using 16 randomly selected seating systems as well as their own. Vibrations were measured using triaxial accelerometers at the seat and participant's head. The weighted fore-to-aft (Tx), vertical (Tz), and resultant (Tr) transmissibility based on the vibrational-dose-value (VDV) were used to determine if differences existed among the four seat cushions and back supports. The obstacles that seem to have the largest effect on the transmission of WBV are the single event shocks and the repeated event shocks. Comparisons between the individuals own seating system and the tested seating systems suggest that the individuals are not using the most appropriate seating system in terms of the reduction of vibration transmission.