Point Mechanical Impedance Research Articles

Simulation of soft tissue stimulation–Indication of a skull bone vibration mechanism in bone conduction hearing

Soft tissue conduction has been proposed as an alternative to bone conduction (BC) for hearing vibrations applied at soft tissue positions at the human head. Arguments for soft tissue conduction originate primarily from experimental studies with stimulation applied to different positions such as the neck, the eye, and directly to the dura. To investigate the mechanism for hearing when stimulations are at soft tissue positions, experimental studies were replicated using the finite element model for BC research, the LiUHead. The vibrations at the cochlear promontory and the sound pressure in the cerebrospinal fluid (CSF) close to the inner ear were extracted from simulations in the LiUHead. The LiUHead simulations were able to replicate data in the literature of cochlear promontory vibration levels and CSF sound pressures with stimulation applied at the soft tissue positions and at the skin covered mastoid. It was shown that the mechanical point impedance of the soft tissue positions affected the output of the BC transducer at frequencies below 1 kHz. The LiUHead simulated cochlear promontory velocities predicted the soft tissue position's hearing thresholds reported in the literature within the inter-study range. This indicates that the hearing mechanism for stimulation at soft tissue positions equals the hearing mechanism for conventional BC hearing, and that soft tissue conduction is not an alternative hearing mechanism. Moreover, the simulations indicated that the CSF sound pressure is not an important pathway for BC hearing and that the CSF pressure is generated by the local skull bone vibrations.

Development and analysis of a novel biomechanical model for seated occupants

Seated occupants may experience discomfort or meet with injuries if the vertical vibration frequency lies in the range of 4–20 Hz. To examine the effect of vibration on seated occupants, it is necessary to develop a biomechanical model that can precisely predict the biodynamic responses (seat to head transmissibility, driving point mechanical impedance, and apparent mass) of a real human. In the past, none of the studies segregated the human body mass into bones and muscles mass that have distinct mechanical properties. In this article, the work is carried out in four steps: first, development of a novel 8- dofs biomechanical model by considering both bone and muscle masses; second, development of all the feasible configurations of 8- dofs model followed by identification and optimization of model parameters with firefly algorithm (FA); third, calibration of all the feasible models as per the standards of biodynamics and selection of best four models; and fourth, comparison of biodynamic responses of best four models with experimental responses and past developed models. The goodness of fit of a seat to head transmissibility, driving point mechanical impedance, and apparent mass for the best model is obtained 0.986, 0.976, and 0.956, respectively.

An Improved Biomechanical Model to Optimize Biodynamic Responses Under Vibrating Medium

The vibration is a mechanical phenomenon created by man or machine. Persons involved in on/off-road vehicle driving expose to intense vibrations and shocks owing to uneven and asymmetrical terrains. The prolonged vibration exposure may lead to musculoskeletal disorders followed by injuries. To examine human body movements and biodynamic responses precisely, it is obligatory to establish a reliable biomechanical model. To do so, the present paper proposes a ten degrees-of-freedom (dofs) biomechanical model of the seated human. The novelty of the proposed model is that it hypothetically divides the real human structure into segments as a head, thorax, abdomen, pelvis, left upper arm, left forearm, left hand, right upper arm, right forearm, and right hand, respectively, which are mainly missing in the human model developed in past. The mechanical parameters are utilized to define the biomechanical model and optimized through the firefly algorithm. After optimization, the biodynamic responses: seat to head transmissibility, apparent mass and driving point mechanical impedance are calculated for the proposed model and compared with Allen2-dofs model, Wan and Schimmels4-dofs model, Bai et al.4-dofs model, Darling et al.7-dofs model and Boileau et al.experimental. The overall goodness of fit is compared for the proposed model with five other models and found that the proposed model gives maximum goodness of fit (93.47). Also, the sensitivity analysis (± 10% variation in mass, stiffness and damping) is performed to validate the reliability of the developed model. And, it is observed that the mass, stiffness, and damping of the pelvis region have a significant role in peak modulus of biodynamic responses.

The Mechanical Impedance of the Human Skull via Direct Bone Conduction Implants.

PurposeThe mechanical skull impedance is used in the design of direct bone drive hearing systems. This impedance is also important for the design of skull simulators used in manufacturing, service, and fitting procedures of such devices.Patients and MethodsThe skull impedance was measured in 45 patients (25 female and 20 male) who were using percutaneous bone conduction implants (Ponto system or Baha system). Patients were recruited as a consecutive prospective case series and having an average age of 55.4 years (range 18–80 years). Seven patients were treated in Gothenburg, Sweden, and 38 patients in Edmonton, Canada. An impedance head (B&K 8001), driven by an excitation transducer with emphasized low-frequency response, was used to measure the mechanical point impedance with a swept sine from 100 to 10k Hz.Results and DiscussionThe skull impedance was found to have an anti-resonance of approximately 150 Hz, with a median maximum magnitude of 4500 mechanical ohms. Below this anti-resonance, the mechanical impedance was mainly mass-controlled corresponding to an effective skull mass of 2.5 kg at 100 Hz with substantial damping from neck and shoulder. Above the anti-resonance and up to 4 kHz, the impedance was stiffness-controlled, with a total compliance of approximately 450n m/N with a small amount of damping. At frequencies above 4 kHz, the skull impedance becomes gradually mass-controlled originating from the mass of the osseointegrated implant and adjacent bone. No significant differences related to gender or skull abnormalities were seen, just a slight dependence on age and major ear surgeries. The variability of the mechanical impedance among patients was not found to have any clinical importance.ConclusionThe mechanical skull impedance of percutaneous implants was found to confirm previous studies and can be used for optimizing the design and test procedures of direct bone drive hearing implants.

Characteristics of Bone-Conduction Devices Simulated in a Finite-Element Model of a Whole Human Head.

Nowadays, many different kinds of bone-conduction devices (BCDs) are available for hearing rehabilitation. Most studies of these devices fail to compare the different types of BCDs under the same conditions. Moreover, most results are between two BCDs in the same subject, or two BCDs in different subjects failing to provide an overview of the results between several of the BCDs. Another issue is that some BCDs require surgical procedures that prevent comparison of the BCDs in the same persons. In this study, four types of skin-drive BCDs, three direct-drive BCDs, and one oral device were evaluated in a finite-element model of the human head that was able to simulate all BCDs under the same conditions. The evaluation was conducted using both a dynamic force as input and an electric voltage to a model of a BCD vibrator unit. The results showed that the direct-drive BCDs and the oral device gave vibration responses within 10 dB at the cochlea. The skin-drive BCDs had similar or even better cochlear vibration responses than the direct-drive BCDs at low frequencies, but the direct-drive BCDs gave up to 30 dB higher cochlear vibration responses at high frequencies. The study also investigated the mechanical point impedance at the interface between the BCD and the head, providing information that explains some of the differences seen in the results. For example, when the skin-drive BCD attachment area becomes too small, the transducer cannot provide an output force similar to the devices with larger attachment surfaces.

Biomechanical Modeling of a Seated Human Body Exposed to Vertical and Horizontal Vibrations Using Genetic Algorithms

Investigation of vibrations transmitted from automotive to the human body, for evaluating comfort parameters, always has been a concern for car manufacturers. Hence, the study of vertical vibration is more important in comparison with other directions. To fined and study vertical vibrations in a sitting position in two states, some error criteria and the backrest support effect on the apparent mass responses, as well as biomechanical responses consisting of apparent mass, driving point mechanical impedance and seat-to-head transmissibility. In this paper, two new biomechanical models are presented in order to study vertical vibrations in the seated posture in both with and without backrest support states. The first model is an optimized five-degrees-of-freedom (5-DoF) lumped-parameter model with a unique structure to display vertical vibrations in one direction. The other one is a new type of model called the matrix model, on which the stiffness and damping matrixes are employed instead of the spring and damper scalar parameters to present vertical vibrations in two directions: vertical and horizontal. The optimal parameters for 5-DoF and matrix models have been determined using genetic algorithms. Some criteria such as sum of square error, root mean square error, mean absolute percentage error and coefficient of determination (R2) were used to evaluate models’ accuracy. The backrest support showed different effects on the module apparent mass responses, and the module apparent mass responses values without backrest support were higher than those of values obtained in the presence of backrest support. As the extracted scientific results from this research we could said that matrix model is a new method which has many advantages over previous methods, such as simplicity, fewer degrees of freedom and high accuracy.

Investigation and Application on the Vertical Vibration Models of the Seated Human Body

Investigation of the vertical vibration characteristics of the seated human body is beneficial for the design and development of vehicle ride comfort. In this study, we first established models of the seated human body with two, three and four degrees of freedom (DOF). Then, the vibration characteristics of 30 volunteers were tested under standard conditions with a vibration test rig to obtain data for the apparent mass, driving point mechanical impedance, and seat-to-head transfer function. Based on the experimental data, the parameters of these models are identified and the results show that the four-DOF model can simulate the vertical vibration characteristics of the seated human body more comprehensively. Then, different seated human body models were applied to optimize the damping of shock absorber. The results show that the optimized damping with the four-DOF Chinese seated human body model is 27% more than that with rigid mass and 7% less than that with ISO 5982:2001 seated human body model.

The soil plate oscillator: Measuring nonlinear mesoscopic elasticity of granular medium in flexural vibration

The soil plate oscillator apparatus (SPO) consists of two circular flanges (5 inch ID, 8 inch OD, 2.5 inch thick) sandwiching and clamping a thin (1/8 inch) circular elastic acrylic plate. Granular media (soil, masonry sand, glass beads, or even edible granular material) is supported at the bottom by the plate and side walls of the upper flange. A magnetic disk (1 cm) fastened below the plate (on center) is driven by a swept sinusoidal current in a coil placed below the magnet. This electrodynamic SPO, placed in a Wheatstone bridge, was used to measure the on-axis motional electrical impedance Zmot with and without granular loading. Nonlinear tuning curves near resonance exhibit softening with increasing drive amplitude. Measurements of Real{Zmot} and Imag{Zmot} were used to predict the nonlinear mechanical point impedance (Zmech = constant / Zmot) behavior of the SPO system. A lumped element model (granular media and elastic plate system) help determine the mesoscopic nonlinear properties of the granular media alone. Nonlinear tuning curves of the magnet’s acceleration were also measured. A bilinear hysteresis model fits the tuning curve results. See I. T. Perez-Miravete et al., JASA 125, 1302 (2009) for a nonlinear mesoscopic analysis of a flexural bar.

The development of a whole-head human finite-element model for simulation of the transmission of bone-conducted sound.

A whole head finite element model for simulation of bone conducted (BC) sound transmission was developed. The geometry and structures were identified from cryosectional images of a female human head and eight different components were included in the model: cerebrospinal fluid, brain, three layers of bone, soft tissue, eye, and cartilage. The skull bone was modeled as a sandwich structure with an inner and outer layer of cortical bone and soft spongy bone (diploë) in between. The behavior of the finite element model was validated against experimental data of mechanical point impedance, vibration of the cochlear promontories, and transcranial BC sound transmission. The experimental data were obtained in both cadaver heads and live humans. The simulations showed multiple low-frequency resonances where the first was caused by rotation of the head and the second was close in frequency to average resonances obtained in cadaver heads. At higher frequencies, the simulation results of the impedance were within one standard deviation of the average experimental data. The acceleration response at the cochlear promontory was overall lower for the simulations compared with experiments but the overall tendencies were similar. Even if the current model cannot predict results in a specific individual, it can be used for understanding the characteristic of BC sound transmission in general.

Feasibility analysis of low-cost flexible resistive sensors for measurements of driving point mechanical impedance of the hand-arm system

This study explored the feasibility of the flexible resistive (FlexiForce) force sensors for measurement of the hand-arm biodynamic response. Two FlexiForce sensors were installed on an instrumented handle to measure the palm-handle and finger–handle interface dynamic forces. The measurements were performed with six subjects grasping a 38 mm diameter instrumented handle with nine different combinations of grip (10, 30 and 50 N) and push (25, 50 and 75 N) forces and two levels of broad-band random vibration (1.5 and 3.0 m/s2 weighted rms acceleration) in the 4–1000 Hz frequency range. The data acquired from the instrumented handle was analyzed to determine the palm and finger impedance responses, which served as the reference values to evaluate feasibility of the FlexiForce sensors. The comparisons revealed very similar trends, while the impedance magnitude responses obtained from the FlexiForce sensors were substantially lower in the entire frequency range than the reference values, except at very low frequencies. A correction function was subsequently developed and applied to the FlexiForce measured data, which resulted in similar hand-arm impedance response trends compared to the reference values. It was concluded that the low-cost FlexiForce sensors could be applied for measurements of biodynamic responses of the hand-arm system in real tool handles in the field. Due to the physiological risks associated with prolonged exposure to tool vibration the applicability of a low-cost biodynamic response measurement system can be used as a preventative measure for such risks. Relevance to industryThe measurement of hand–handle interface forces is vital for assessing the hand-transmitted vibration exposure and the biodynamic responses of the hand-arm system to vibration. The low cost and flexible sensors, proposed in the study, could be conveniently applied to the curved surfaces of real power tool handles in the field. The most significant benefit of the sensors lies with its negligible mass and thereby the instrumented handle inertia-induced errors in the biodynamic responses can be eliminated.

Evaluation of Bio-Dynamic Responses of Human Body Subjected to Impacts Due to Speed Humps

Bio-dynamics of seated human subjects exposed to whole body vibration has been a topic of interest over the years. Most of the motorcycle riders are exposed to health risks, due to repeated exposure of mechanical shock when frequently riding over traffic speed humps. The present study describes a novel method to assess the acceleration transmissibility through the body and the apparent mass of the subject while riding a motorised two-wheeler over a speed hump. Sensitivity of human subjects to whole body vibration under low-frequency excitation in seated posture is also addressed by driving point mechanical impedance. Experiment methodology involves use of body mounted accelerometer as motion sensors to track these shocks for four healthy subjects. A four-degree of freedom lumped parameter model is simulated to quantify the instantaneous effects of mechanical shocks. This understanding is likely to aid the driving community, in general, and the motorcycle manufactures to incorporate better suspension and appropriate ergonomics considerations.

A three-dimensional finite-element model of a human dry skull for bone-conduction hearing.

A three-dimensional finite-element (FE) model of a human dry skull was devised for simulation of human bone-conduction (BC) hearing. Although a dry skull is a simplification of the real complex human skull, such model is valuable for understanding basic BC hearing processes. For validation of the model, the mechanical point impedance of the skull as well as the acceleration of the ipsilateral and contralateral cochlear bone was computed and compared to experimental results. Simulation results showed reasonable consistency between the mechanical point impedance and the experimental measurements when Young's modulus for skull and polyurethane was set to be 7.3 GPa and 1 MPa with 0.01 and 0.1 loss factors at 1 kHz, respectively. Moreover, the acceleration in the medial-lateral direction showed the best correspondence with the published experimental data, whereas the acceleration in the inferior-superior direction showed the largest discrepancy. However, the results were reasonable considering that different geometries were used for the 3D FE skull and the skull used in the published experimental study. The dry skull model is a first step for understanding BC hearing mechanism in a human head and simulation results can be used to predict vibration pattern of the bone surrounding the middle and inner ear during BC stimulation.

Evaluation of bio-dynamic responses of human body subjected to impacts due to speed humps

Bio-dynamics of seated human subjects exposed to whole body vibration has been a topic of interest over the years. Most of the motorcycle riders are exposed to health risks, due to repeated exposure of mechanical shock when frequently riding over traffic speed humps. The present study describes a novel method to assess the acceleration transmissibility through the body and the apparent mass of the subject while riding a motorised two-wheeler over a speed hump. Sensitivity of human subjects to whole body vibration under low-frequency excitation in seated posture is also addressed by driving point mechanical impedance. Experiment methodology involves use of body mounted accelerometer as motion sensors to track these shocks for four healthy subjects. A four-degree of freedom lumped parameter model is simulated to quantify the instantaneous effects of mechanical shocks. This understanding is likely to aid the driving community, in general, and the motorcycle manufactures to incorporate better suspension and appropriate ergonomics considerations.

A Vibration Investigation of a Flat Surface Contact to Skull Bone for Direct Bone Conduction Transmission in Sheep Skulls In Vivo

Bone conduction implant (BCI) attached with a flat surface contact will offer efficient and linear vibration transmission over time. Despite that percutaneous bone conduction devices (PBCD) are successful in treating patients with conductive hearing loss, there are some drawbacks related to the need of a permanent skin penetration. The BCI system is designed as an alternative to the PBCD because it leaves the skin intact. BCI dummy implants were installed in 3 sheep skulls in vivo to study the vibration transmission characteristics over time. Mechanical point impedances and vibration transfer response functions of the BCI implants were measured at the time of surgery and after a healing period of 8 months. In 1 sheep both implants healed without complications. In the other 2 sheep, the implants were either partially loose or lost to follow up. In the sheep with stable implants, it was found by the resonance frequency shift of the mechanical point impedance that a firmer integration between the implant and bone tissue as seen in osseointegrated surfaces developed over time. It was also shown that the transcranial vibration transmission remains stable and linear. Providing bone chips in the contact between the implant and the bone did not enhance vibration transmission. The surgical procedure for installing the BCI dummy implants was uneventful. The mechanical point impedances and vibration transfer response functions indicate that the BCI implants integrate and that transmission conditions remain stable over time.

Hand-arm mechanical impedance in presence of unknown vibration direction

This work describes the results of an experimental campaign whose aim was the identification of the hand-arm driving point mechanical impedance (DPMI) in presence of a stimulus having an unknown direction. This situation is relevant in case of cylindrical handles, where the tool can be gripped in different ways independently from the vibration direction. In this case, xh and zh ISO 10068 curves can be conveniently substituted by a unique parameter, here referred to as “radial impedance”. Radial impedance values have been determined with an experimental campaign based on the classical setup in which the vibration is generated by an electrodynamic shaker and the force component parallel to the vibration is measured by an instrumented handle. The vibration direction was always vertical, while the different postures entailed a different stimulus orientation with respect to the hand, forearm and shoulder. The hand-arm posture was monitored using a purposely designed measurement system. The push forces transmitted by the operator to the handle were measured using load cells, while the grip force was measured before the tests using a pressure matrix. A statistical analysis on the radial DPMI data was performed using the analysis of variance (ANOVA) techniques. Results showed that the uncertainty deriving from the adoption of the radial impedance in presence of cylindrical handles does not always lead to a larger uncertainty with respect to the ISO 10068 data. Coherently with the existing literature studies, also the radial DPMI depends on the forces transmitted to the handle and on the wrist, elbow and shoulder postures. The opportunity of adopting (in specific cases) the radial impedance instead of the xh and zh is discussed. Relevance to industryHand-transmitted vibration may cause a variety of vascular and musculoskeletal disorders that often result in loss of work capacity and compromised quality of life. The concept of radial impedance described in this paper can be used in the design of devices meant to reduce the vibration transmitted to the workers, thus reducing the above mentioned risks.

Analysis of the point mechanical impedance of fingerpad in vibration

Biodynamic responses of the finger–hand–arm system, such as apparent mass and mechanical impedance, characterize the relationship between the motion of the finger–hand–arm system and the dynamic force acting on the driving point, and they are useful for vibration exposure assessment. In the present study, a two-dimensional finite element (FE) model has been proposed to simulate the biodynamic responses of the fingerpad in vibration tests. The fingernail was supported by the rigid ground while the fingerpad was activated by a vibration probe. The fingertip model is composed of skin, subcutaneous tissue, bone, and nail. The soft tissues (i.e., skin and subcutaneous tissues) were assumed to be non-linearly elastic and linearly viscoelastic. The FE model was applied to predict the effects of pre-indentation of the vibration probe onto the fingerpad, the damping of the soft tissues, and probe mass on the magnitude and phase angle of the mechanical impedance and the apparent mass, as measured in the vibration tests. The model predictions showed that the probe mass has non-negligible effects on the measured biodynamic responses in the vibration tests. In order to determine “true” biodynamic responses of the finger–hand–arm system, the mass effects have to be cancelled using an appropriate approach. The present analysis provided a theoretical explanation, from a biomechanical point-of-view, for the inconsistencies in the published experimental data for the biodynamic responses of fingerpad.

Influence of hand–arm posture on biodynamic response of the human hand–arm exposed to zh-axis vibration

The influence of hand–arm posture on the biodynamic responses under z h-axis vibration is investigated in terms of driving point mechanical impedance and absorbed power under various combinations of hand grip and push forces, handle sizes and excitation levels. Laboratory measurements of the biodynamic responses were performed on seven healthy male subjects exposed to two levels of broadband random vibration in the 8–1000 Hz frequency range using three instrumented cylindrical handles of different diameter (30, 40 and 50 mm), and different grip (10, 30 and 50 N) and push (25, 50 and 75 N) forces. The experiments involved grasping the handle with two different postures, consisting in flexed forearm with elbow angle of 90° and extended forearm with elbow angle of 180°, with wrist being in the neutral position for both postures. The results revealed remarkable effects of the hand–arm posture on both the mechanical impedance and the absorbed power characteristics. The low-frequency apparent mass magnitude of the hand–arm with the extended forearm posture was observed to be approximately three times than that with the flexed forearm posture. Furthermore, the effects of handle size, and push and grip forces on the biodynamic responses of the human hand–arm exposed to vibration were observed to be more significant for the extended forearm posture. This posture revealed considerably higher coupling with the vibrating handle and a damper-like behavior of the hand–arm system in the very low-frequency range. This posture also resulted in considerably higher power absorption than the flexed forearm posture for the majority of the combinations of hand forces considered. Depending on the handle size, excitation level, and hand forces, the total power absorbed by the hand–arm with an extended hand–arm could be up to 96% higher than that with the flexed elbow. From the results, it is evident that the hand–arm posture strongly affects the biodynamic response, when exposed to z h-axis vibration. Relevance to industry Operators assume considerable variations in the hand–arm posture while operating hand-held power tools. Such variations coupled with variations in the hand forces imparted on the tool handle could cause considerably different biodynamic responses of the hand–arm system. The current International Standard describes the range of idealized biodynamic response in terms of the driving-point mechanical impedance under a fixed posture involving 90° elbow angle (ISO-10068, 1998). The standard on the assessment of anti-vibration gloves also requires the same posture (ISO-10819, 1996). This study presents the effect of hand–arm posture on the biodynamic responses in terms of both driving-point mechanical impedance and power absorption by the hand–arm system. The influences of hand forces and handle sizes on both measures are also presented. The results of the study clearly suggest high significance of the hand–arm posture in view of the biodynamic responses. The results of the study attained on the driving-point mechanical impedance are expected to provide the vital knowledge and data for enhancing the current standard and its applications. The results attained on the influence of posture on the absorbed power characteristics of the hand–arm system are further expected to enhance our knowledge on the assessment of vibration exposure.

Transmission properties of bone conducted sound: Measurements in cadaver heads

In the past, only a few investigations have measured vibration at the cochlea with bone conduction stimulation: dry skulls were used in those investigations. In this paper, the transmission properties of bone conducted sound in human head are presented, measured as the three-dimensional vibration at the cochlear promontory in six intact cadaver heads. The stimulation was provided at 27 positions on the skull surface and two close to the cochlea; mechanical point impedance was measured at all positions. Cochlear promontory vibration levels in the three perpendicular directions were normally within 5 dB. With the stimulation applied on the ipsilateral side, the response decreased, and the accumulated phase increased, with distance between the cochlea and the excitation position. No significant changes were obtained when the excitations were on the contralateral side. In terms of vibration level, the best stimulation position is on the mastoid close to the cochlea; the worst is at the midline of the skull. The transcranial transmission was close to 0 dB for frequencies up to 700 Hz; above it decreased at 12 dB/decade. Wave transmission at the skull-base was found to be nondispersive at frequencies above 2 kHz whereas it altered with frequency at the cranial vault.

Mechanical Impedance and Absorbed Power of Hand-Arm under xh-Axis Vibration and Role of Hand Forces and Posture

The biodynamic responses of the hand-arm system under x(h)-axis vibration are investigated in terms of the driving point mechanical impedance (DPMI) and absorbed power in a laboratory study. For this purpose, seven healthy male subjects are exposed to two levels of random vibration in the 8-1,000 Hz frequency range, using three instrumented cylindrical handles of different diameters (30, 40 and 50 mm), and different combinations of grip (10, 30 and 50 N) and push (0, 25 and 50 N) forces. The experiments involve grasping the handle while adopting two different postures, involving elbow flexion of 90 degrees and 180 degrees, with wrist in the neutral position for both postures. The analyses of the results revealed peak DPMI magnitude and absorbed power responses near 25 Hz and 150 Hz, for majority of the test conditions considered. The frequency corresponding to the peak response increased with increasing hand forces. Unlike the absorbed power, the DPMI response was mostly observed to be insensitive to variations in the excitation magnitude. The handle diameter revealed obvious effects on the DPMI magnitude, specifically at frequencies above 250 Hz, which was not evident in the absorbed power due to relatively low velocity at higher frequencies. The influence of hand forces was also evident on the DPMI magnitude response particularly at frequencies. above 100 Hz, while the effect of hand-arm posture on the DPMI magnitude was nearly negligible. The magnitude of power absorbed within the hand and arm was observed to be strongly dependent upon the excitation level over the entire frequency range, while the influence of hand-arm posture on the total absorbed power was observed to be important. The effect of variations in the hand forces on the absorbed power was relatively small for the bent elbow posture, while an increase in either the grip or the push force coupled with the extended arm posture resulted in considerably higher energy absorption. The results suggested that the handle size, hand-arm posture and hand forces, produce coupled effect on the biodynamic response of the hand-arm system.

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.