An examination of the handheld adapter approach for measuring hand-transmitted vibration exposure

The use of a handheld adapter equipped with a tri-axial accelerometer is the most convenient and efficient approach for measuring vibration exposure at the hand-tool interface, especially when the adapter is incorporated into a miniature handheld or wrist-strapped dosimeter. To help optimize the adapter approach, the specific aims of this study are to identify and understand the major sources and mechanisms of measurement errors and uncertainties associated with using these adapters, and to explore their improvements. Five representative adapter models were selected and used in the experiment. Five human subjects served as operators in the experiment on a hand-arm vibration test system. The results of this study confirm that many of the handheld adapters can produce substantial overestimations of vibration exposure, and measurement errors can significantly vary with tool, adapter model, mounting position, mounting orientation, and subject. Major problems with this approach include unavoidable influence of the hand dynamic motion on the adapter, unstable attachment, insufficient attachment contact force, and inappropriate adapter structure. However, the results of this study also suggest that measurement errors can be substantially reduced if the design and use of an adapter can be systematically optimized toward minimizing the combined effects of the identified factors. Some potential methods for improving the design and use of the adapters are also proposed and discussed.

Theoretical relationship between vibration transmissibility and driving-point response functions of the human body

The relationship between the vibration transmissibility and driving-point response functions (DPRFs) of the human body is important for understanding vibration exposures of the system and for developing valid models. This study identified their theoretical relationship and demonstrated that the sum of the DPRFs can be expressed as a linear combination of the transmissibility functions of the individual mass elements distributed throughout the system. The relationship is verified using several human vibration models. This study also clarified the requirements for reliably quantifying transmissibility values used as references for calibrating the system models. As an example application, this study used the developed theory to perform a preliminary analysis of the method for calibrating models using both vibration transmissibility and DPRFs. The results of the analysis show that the combined method can theoretically result in a unique and valid solution of the model parameters, at least for linear systems. However, the validation of the method itself does not guarantee the validation of the calibrated model, because the validation of the calibration also depends on the model structure and the reliability and appropriate representation of the reference functions. The basic theory developed in this study is also applicable to the vibration analyses of other structures.

Modeling of the biodynamic responses distributed at the fingers and palm of the hand in three orthogonal directions

The objectives of this study were to develop models of the hand-arm system in the three orthogonal directions (x_h, y_h , and z_h ) and to enhance the understanding of the hand vibration dynamics. A four-degrees-of-freedom (DOF) model and 5-DOF model were used in the simulation for each direction. The driving-point mechanical impedances distributed at the fingers and palm of the hand reported in a previous study were used to determine the parameters of the models. The 5-DOF models were generally superior to the 4-DOF models for the simulation. Hence, as examples of applications, the 5-DOF models were used to predict the transmissibility of a vibration-reducing glove and the vibration transmissibility on the major substructures of the hand-arm system. The model-predicted results were also compared with the experimental data reported in two other recent studies. Some reasonable agreements were observed in the comparisons, which provided some validation of the developed models. This study concluded that the 5-DOF models are acceptable for helping to design and analyze vibrating tools and anti-vibration devices. This study also confirmed that the 5-DOF model in the z_h direction is acceptable for a coarse estimation of the biodynamic responses distributed throughout the major substructures of the hand-arm system. Some interesting phenomena observed in the experimental study of the biodynamic responses in the three directions were also explained in this study.

Vibration-reducing gloves: transmissibility at the palm of the hand in three orthogonal directions

Vibration-reducing (VR) gloves are commonly used as a means to help control exposures to hand-transmitted vibrations generated by powered hand tools. The objective of this study was to characterise the vibration transmissibility spectra and frequency-weighted vibration transmissibility of VR gloves at the palm of the hand in three orthogonal directions. Seven adult males participated in the evaluation of seven glove models using a three-dimensional hand-arm vibration test system. Three levels of hand coupling force were applied in the experiment. This study found that, in general, VR gloves are most effective at reducing vibrations transmitted to the palm along the forearm direction. Gloves that are found to be superior at reducing vibrations in the forearm direction may not be more effective in the other directions when compared with other VR gloves. This casts doubts on the validity of the standardised glove screening test. Practitioner Summary: This study used human subjects to measure three-dimensional vibration transmissibility of vibration-reducing gloves at the palm and identified their vibration attenuation characteristics. This study found the gloves to be most effective at reducing vibrations along the forearm direction. These gloves did not effectively attenuate vibration along the handle axial direction.

Laboratory and field measurements and evaluations of vibration at the handles of riveting hammers

The use of riveting hammers can expose workers to harmful levels of hand-transmitted vibration (HTV). As a part of efforts to reduce HTV exposures through tool selection, the primary objective of this study was to evaluate the applicability of a standardized laboratory-based riveting hammer assessment protocol for screening riveting hammers. The second objective was to characterize the vibration emissions of reduced vibration riveting hammers and to make approximations of the HTV exposures of workers operating these tools in actual work tasks. Eight pneumatic riveting hammers were selected for the study. They were first assessed in a laboratory using the standardized method for measuring vibration emissions at the tool handle. The tools were then further assessed under actual working conditions during three aircraft sheet metal riveting tasks. Although the average vibration magnitudes of the riveting hammers measured in the laboratory test were considerably different from those measured in the field study, the rank orders of the tools determined via these tests were fairly consistent, especially for the lower vibration tools. This study identified four tools that consistently exhibited lower frequency-weighted and unweighted accelerations in both the laboratory and workplace evaluations. These observations suggest that the standardized riveting hammer test is acceptable for identifying tools that could be expected to exhibit lower vibrations in workplace environments. However, the large differences between the accelerations measured in the laboratory and field suggest that the standardized laboratory-based tool assessment is not suitable for estimating workplace riveting hammer HTV exposures. Based on the frequency-weighted accelerations measured at the tool handles during the three work tasks, the sheet metal mechanics assigned to these tasks at the studied workplace are unlikely to exceed the daily vibration exposure action value (2.5 m s(-2)) using any of the evaluated riveting hammers.

Assessment of hand-transmitted vibration exposure from motorized forks used for beach-cleaning operations

Motorized vibrating manure forks were used in beach-cleaning operations following the massive Deepwater Horizon oil spill in the Gulf of Mexico during the summer of 2010.

OBJECTIVES

The objectives of this study were to characterize the vibration emissions of these motorized forks and to provide a first approximation of hand-transmitted vibration exposures to workers using these forks for beach cleaning.

METHODS

Eight operators were recruited to operate the motorized forks during this laboratory study. Four fork configurations were used in the study; two motor speeds and two fork basket options were evaluated. Accelerations were measured near each hand as the operators completed the simulated beach-cleaning task.

RESULTS

The dominant vibration frequency for these tools was identified to be around 20 Hz. Because acceleration was found to increase with motor speed, workers should consider operating these tools with just enough speed to get the job done. These forks exhibited considerable acceleration magnitudes when unloaded.

CONCLUSIONS

The study results suggest that the motor should not be operated with the fork in the unloaded state. Anti-vibration gloves are not effective at attenuating the vibration frequencies produced by these forks, and they may even amplify the transmitted vibration and increase hand/arm fatigue. While regular work gloves are suitable, vibration-reducing gloves may not be appropriate for use with these tools. These considerations may also be generally applicable for the use of motorized forks in other workplace environments.

Frequency-dependent effects of vibration on physiological systems: experiments with animals and other human surrogates

Occupational exposure to vibration through the use of power- and pneumatic hand-tools results in cold-induced vasospasms, finger blanching, and alterations in sensorineural function. Collectively, these symptoms are referred to as hand-arm vibration syndrome (HAVS). Currently the International Standards Organization (ISO) standard ISO 5349-1 contains a frequency-weighting curve to help workers and employers predict the risk of developing HAVS with exposure to vibration of different frequencies. However, recent epidemiological and experimental evidence suggests that this curve under-represents the risk of injuries to the hands and fingers induced by exposure to vibration at higher frequencies (>100 Hz). To improve the curve, better exposure-response data need to be collected. The goal of this review is to summarize the results of animal and computational modeling studies that have examined the frequency-dependent effects of vibration, and discuss where additional research would be beneficial to fill these research gaps.

An analysis of contact stiffness between a finger and an object when wearing an air-cushioned glove: the effects of the air pressure

Air-cushioned gloves have the advantages of lighter weight, lower cost, and unique mechanical performance, compared to gloves made of conventional engineering materials. The goal of this study is to analyze the contact interaction between fingers and object when wearing an air-cushioned glove. The contact interactions between the the fingertip and air bubbles, which is considered as a cell of a typical air-cushioned glove, has been analyzed theoretically. Two-dimensional finite element models were developed for the analysis. The fingertip model was assumed to be composed of skin layers, subcutaneous tissue, bone, and nail. The air bubbles were modeled as air sealed in the container of nonelastic membrane. We simulated two common scenarios: a fingertip in contact with one single air bubble and with two air cushion bubbles simultaneously. Our simulation results indicated that the internal air pressure can modulate the fingertip-object contact characteristics. The contact stiffness reaches a minimum when the initial air pressure is equal to 1.3 and 1.05 times of the atmosphere pressure for the single air bubble and the double air bubble contact, respectively. Furthermore, the simulation results indicate that the double air bubble contact will result in smaller volumetric tissue strain than the single air bubble contact for the same force.

A practical biomechanical model of the index finger simulating the kinematics of the muscle/tendon excursions

Biomechanical models of the hand and fingers are useful tools for hand surgeons to improve surgical procedures and for biomedical researchers to explore the mechanical loading in the musculoskeletal system that cannot be easily measured in vivo. The purpose of the present study was to develop a realistic index finger model for solving practical problems. The model includes the meshes of four bony sections (distal, middle, proximal and metacarpal bones) obtained via micro-CT scans. The tendon attachment sites are adopted from the normative finger model. A total of seven tendon/muscles are included in the model. The predicted tendon excursions and moment arms were compared with published experimental data. One of the advantages of the current approach over previous studies is that the current model has been developed on a platform of a commercial software package, such that researchers can apply it as a universal tool for practical problems.

An evaluation of the methods for deriving representative frequency response functions of the human whole-body system

The biodynamic response functions of the human whole-body system measured with subjects participating in an experiment are commonly arithmetically averaged and used to represent their mean response functions in many studies. The reported means were further averaged to form the reference means for standardization and various applications. The objectives of this study are to clarify whether this response-based averaging process could significantly misrepresent the characteristics of the original functions, and to explore appropriate methods for deriving representative functions. A set of reported mechanical-equivalent models for 12 subjects was used to derive the vertical and fore-and-aft cross-axis response functions expressed in apparent mass. The response-based average was directly compared with the response derived from a property-based derivation method. This study found that the response-based average could differ from the property-based mean response by more than 30%, especially in the fore-and-aft cross-axis response functions. This study also theoretically demonstrated that the discrepancies result from the non-linear relationship between the apparent mass and the properties of a dynamic system. Therefore, the discrepancies depend on the variability of the subjects' dynamic properties. Practically, the discrepancies in the vertical response could be reduced to an acceptable level (e.g., <10%) if a sufficient number of subjects with similar body weights are selected or grouped in the measurement. However, it is very difficult to reduce the discrepancies in the fore-and-aft cross-axis to such a level. While more demanding than the response-based method, the property-based method is theoretically more reliable for deriving the representative response functions for each axis.

Comparing three methods for evaluating impact wrench vibration emissions

To provide a means for comparing impact wrenches and similar tools, the international standard ISO 8662-7 prescribes a method for measuring the vibrations at the handles of tools during their operations against a cotton-phenolic braking device. To improve the standard, alternative loading mechanisms have been proposed; one device comprises aluminum blocks with friction brake linings, while another features plate-mounted bolts to provide the tool load. The objective of this study was to evaluate these three loading methods so that tool evaluators can select appropriate loading devices in order to obtain results that can be applied to their specific workplace operations. Six experienced tool operators used five tool models to evaluate the loading mechanisms. The results of this study indicate that different loads can yield different tool comparison results. However, any of the three devices appears to be adequate for initial tool screenings. On the other hand, vibration emissions measured in the laboratory are unlikely to be fully representative of those in the workplace. Therefore, for final tool selections and for reliably assessing workplace vibration exposures, vibration measurements should be collected under actual working conditions. Evaluators need to use appropriate numbers of tools and tool operators in their assessments; recommendations are provided.

Modeling of the muscle/tendon excursions and moment arms in the thumb using the commercial software anybody

A biomechanical model of a thumb would be useful for exploring the mechanical loadings in the musculoskeletal system, which cannot be measured in vivo. The purpose of the current study is to develop a practical kinematic thumb model using the commercial software Anybody (Anybody Technology, Aalborg, Denmark), which includes real CT-scans of the bony sections and realistic tendon/muscle attachments on the bones. The thumb model consists of a trapezium, a metacarpal bone, a proximal and a distal phalanx. These four bony sections are linked via three joints, i.e., IP (interphalangeal), MP (metacarpophalangeal) and CMC (carpometacarpal) joints. Nine muscles were included in the proposed model. The theoretically calculated moment arms of the tendons are compared with the corresponding experimental data by Smutz et al. [1998. Mechanical advantage of the thumb muscles. J. Biomech. 31(6), 565-570]. The predicted muscle moment arms of the majority of the muscle/tendon units agree well with the experimental data in the entire range of motion. Close to the end of the motion range, the predicted moment arms of several muscles (i.e., ADPt and ADPo (transverse and oblique heads of the adductor pollicis, respectively) muscles for CMC abduction/adduction and ADPt and FPB (flexor pollicis brevis) muscle for MP extension/flexion) deviate from the experimental data. The predicted moment potentials for all muscles are consistent with the experimental data. The findings thus suggest that, in a biomechanical model of the thumb, the mechanical functions of muscle-tendon units with small physiological cross-sectional areas (PCSAs) can be well represented using single strings, while those with large PCSAs (flat-wide attachments, e.g., ADPt and ADPo) can be represented by the averaged excursions of two strings. Our results show that the tendons with large PCSAs can be well represented biomechanically using the proposed approach in the major range of motion.

An investigation on the biodynamic foundation of a rat tail vibration model

The objectives of this study are to examine the fundamental characteristics of the biodynamic responses of a rat tail to vibration and to compare them with those of human fingers. Vibration transmission through tails exposed to three vibration magnitudes (1 g, 5 g, and 10 g r.m.s.) at six frequencies (32 Hz, 63 Hz, 125 Hz, 160 Hz, 250 Hz, and 500 Hz) was measured using a laser vibrometer. A mechanical-equivalent model of the tail was established on the basis of the transmissibility data, which was used to estimate the biodynamic deformation and vibration power absorption at several representative locations on the tail. They were compared with those derived from a mechanical-equivalent model of human fingers reported in the literature. This study found that, similar to human fingers, the biodynamic responses of the rat tail depends on the vibration magnitude, frequency, and measurement location. With the restraint method used in this study, the natural frequency of the rat tail is in the range 161-368 Hz, which is mostly within the general range of human finger resonant frequencies (100-350 Hz). However, the damping ratios of the rat tail at the unconstrained locations are from 0.094 to 0.394, which are lower than those of human fingers (0.708-0.725). Whereas the biodynamic responses of human fingers at frequencies lower than 100 Hz could be significantly influenced by the biodynamics of the entire hand-arm system, the rat tail biodynamic responses can be considered independent of the rat body in the frequency range used in this study. Based on these findings it is concluded that, although there are some differences between the frequency dependences of the biodynamic responses of the rat tail and human fingers, the rat tail model can provide a practical and reasonable approach to examine the relationships between the biodynamic and biological responses at midrange to high frequencies, and to understand the mechanisms underlying vibration-induced finger disorders.

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.

Three-Dimensional Finite Element Simulations of the Dynamic Response of a Fingertip to Vibration

Although excessive dynamic deformation of the soft tissues in the fingertip under vibration loading is thought to induce hand-arm vibration syndrome, the in vivo distributions of the dynamic stress/strain of the tissues in the fingertip under vibration conditions have not been studied because they cannot be measured experimentally. In the present study, we analyzed the dynamic responses of a fingertip to vibrations by extending our previously proposed three-dimensional finite element (FE) model. The FE model of the fingertip contains the essential anatomical structures of a finger, such as skin layers (dermis and epidermis), subcutaneous tissue, bone, and nail. Our analysis indicated that the fingertip has a major local resonance around 100Hz and that the vibration displacement in the soft tissues under the nail bed is less than 10% of those in the finger pad for all precompression levels and vibration range. The resonant frequency of the fingertip was found to increase from 88Hzto125Hz with the static precompression increasing from 0.5mmto2.0mm. These results suggest that structural and functional changes in vascular function will likely initiate from the fingerpad, the location that undergoes the greatest deformation during vibration exposure. The current predictions are qualitatively consistent with the physiological data collected from workers with vibration white finger.

An evaluation of impact wrench vibration emissions and test methods

In the interest of providing more effective evaluations of impact wrench vibration exposures and the development of improved methods for measuring vibration emissions produced by these tools, this study focused on three variables: acceleration measured at the tool surface, vibration exposure duration per test trial, and the amount of torque required to unseat the nuts following a test trial. For this evaluation, six experienced male impact wrench operators used three samples each of five impact wrench models (four pneumatic models and one battery-powered model) in a simulated work task. The test setup and procedures were based on those provided by an International Organization for Standardization (ISO) Technical Committee overseeing the revision of ISO 8662-7. The work task involved the seating of 10 nuts onto 10 bolts mounted on steel plates. The results indicate that acceleration magnitudes vary not only by tool type but also by individual tools within a type. Thus, evaluators are cautioned against drawing conclusions based on small numbers of tools and/or tool operators. Appropriate sample sizes are suggested. It was further noted that evaluators could draw different conclusions if tool assessments are based on ISO-weighted acceleration as opposed to unweighted acceleration. As expected, vibration exposure durations varied by tool type and by test subject; duration means varied more for study participants than they did for tool types. For the 12 pneumatic tools evaluated in this study, torque varied directly with tool handle acceleration. Therefore, in order to reduce vibration exposure, tools should be selected and adjusted so that they produce no more than the needed torque for the task at hand.

A new approach to characterize grip force applied to a cylindrical handle

The grip force applied to a cylindrical handle is a function of the measurement reference axis. So far, however, no attempt has been made to fully describe the exact form of this function. The objectives of this study were to examine some fundamental characteristics of grip forces and to explore the basic pattern of the grip force function. Twenty subjects (10 males and 10 females) participated in the experiment. The subjects alternately used their left and right hands to apply maximum grip forces and medium grip forces (about 40% of maximum) to a 30 mm handle. A flexible pressure sensor mat was used to measure the grip pressure. The pressure was integrated with respect to different measurement axes; this resulted in the grip force function. This study found that every gripping action produces maximum and minimum force axes; these axes are separated by about 90 degrees . The maximum force is correlated with the minimum force, but the former is generally about 1.42 times the latter. The principal grip direction is about 78 degrees from the z(h)-axis of the hand biodynamic coordinate system defined in ISO 8727 [ISO 8727. Mechanical vibration and shock - human exposure - biodynamic coordinate systems. Geneva, Switzerland: International Organization for Standardization; 1997]. More interestingly, each of the 160 sets of experimental data reasonably fit this study's proposed elliptical model. The implications of the findings are discussed.

A method to quantify hand-transmitted vibration exposure based on the biodynamic stress concept

This study generally hypothesized that the vibration-induced biodynamic stress and number of its cycles in a substructure of the hand-arm system play an important role in the development of vibration-induced disorders in the substructure. As the first step to test this hypothesis, the specific aims of this study were to develop a practical method to quantify the biodynamic stress-cycle measure, to compare it with ISO-weighted and unweighted accelerations, and to assess its potential for applications. A mechanical-equivalent model of the system was established using reported experimental data. The model was used to estimate the average stresses in the fingers and palm. The frequency weightings of the stresses in these substructures were derived using the proposed stress-cycle measure. This study found the frequency dependence of the average stress distributed in the fingers is different from that in the palm. Therefore, this study predicted that the frequency dependencies of finger disorders could also be different from those of the disorders in the palm, wrist, and arms. If vibration-induced white finger (VWF) is correlated better with unweighted acceleration than with ISO-weighted acceleration, the biodynamic stress distributed in the fingers is likely to play a more important role in the development of VWF than is th e biodynamic stressdistributed in the other substructures of the hand-arm system. The results of this study also suggest that the ISO weighting underestimates the high-frequency effect on the finger disorder development but it may provide a reasonable risk assessment of the disorders in the wrist and arm.

Analysis of musculoskeletal loading in an index finger during tapping

Since musculoskeletal disorders of the upper extremities are believed to be associated with repetitive excessive muscle force production in the hands, understanding the time-dependent muscle forces during key tapping is essential for exploring the mechanisms of disease initiation and development. In the current study, we have simulated the time-dependent dynamic loading in the muscle/tendons in an index finger during tapping. The index finger model is developed using a commercial software package AnyBody, and it contains seven muscle/tendons that connect the three phalangeal finger sections. Our simulations indicate that the ratios of the maximal forces in flexor digitorum superficialis (FS) and flexor digitorum profundus (FP) tendons to the maximal force at the fingertip are 0.95 and 2.9, respectively, which agree well with recently published experimental data. The time sequence of the finger muscle activation predicted in the current study is consistent with the EMG data in the literature. The proposed model will be useful for bioengineers and ergonomic designers to improve keyboard design minimizing musculoskeletal loadings in the fingers.

Finite element analysis of the penetrations of shear and normal vibrations into the soft tissues in a fingertip

It is well accepted that the effects of mechanical vibration on the finger-hand-arm system are strongly frequency-dependent: low frequency vibration can transmit from hand to arm, while high frequency vibration is absorbed in the local tissue of fingers. This assertion has not been validated directly. The purpose of the present study is to analyze the frequency- and deformation-dependent dynamic strains in the soft tissues in a fingertip that is subjected to vibration normal or tangential to the contact surface. The dynamic responses of the fingertip were analyzed using a multi-layered two-dimensional finite element model. The major anatomical substructures, i.e., skin, subcutaneous tissue, bone, and nail, are included in the model. The fingertip was found to have a major resonance around 100-125 Hz and a second resonance around 250 Hz. The resonances of the fingertip are found to be independent of the direction of exposure (in normal or shear direction). The simulations further indicated that the dynamic strains induced by the vibration at low frequencies will penetrate deeper into the tissue (> 3 mm) while that at high frequencies will be concentrated in the superficial skin layer (< 0.8 mm). The model predictions are consistent with the published experimental observations.