Comparison of measurement methods for quantifying hand force

Hand force is a known risk factor for upper extremity disorders. The objective of the present study was to determine the characteristics of, and the relationships between, exposure assessment methods to quantify hand force. Five methods, used in the laboratory or the field, were used to quantify hand force at three force magnitudes: two direct (or technical) measurement methods, force transducers and electromyography; an observational method; and two self-report approaches, force matching and a visual analogue scale. Five tasks, simulating manual work activities, were performed by 20 participants. The coefficients of variation of measures within and between participants were moderate. All approaches clearly distinguished between the three force levels tested. The reliability of the methods ranged from poor (observation method without information) to good (force transducers method and observation method with information). The measurement methods correlated moderately over all five tasks. Predictions of grip force across all five tasks were poor and even for single tasks the predictions were not much better. The tasks in this study were still simplified; in the field tasks are even more complex and the measurement characteristics might be expected to be less good. A hand force exposure assessment method should therefore be calibrated and tested for each type of hand activity before use.     

Quantifying repetitive hand activityfor epidemiological research on musculoskeletal disorders – part I: individual exposure assessment

An exposure measurement approach is described for quantifying repetitive hand activity of individual workers in a prospective epidemiological study on work-related upper extremity musculoskeletal disorders. A total of 733 subjects were involved in this study at the baseline. Hand activities were quantified by force and repetition. Force levels were measured by workers' self-reports, ergonomists' estimates based on observation and measurements with instrumentation. Repetition levels were measured by detailed time–motion analyses using two repetitive hand activity definitions and ergonomists' estimates using scales for the American Conference of Governmental Industrial Hygienists hand activity level and the Strain Index. Results showed that the present exposure assessment approach seems to be able to quantify force level and repetitiveness of hand activities. Repetitive hand activity is quantified differently depending on whether forceful hand exertion or repetitive muscle activity is used as the definition. These hand activity definitions may quantify different physical exposure phenomena. Individual exposure assessment is important in epidemiological research of musculoskeletal disorders as there are interactions between the individual subjects and the measured parameters. These interactions may vary between exposure parameters.     

Quantifying repetitive hand activity for epidemiological research on musculoskeletal disorders--part I: individual exposure assessment

An exposure measurement approach is described for quantifying repetitive hand activity of individual workers in a prospective epidemiological study on work-related upper extremity musculoskeletal disorders. A total of 733 subjects were involved in this study at the baseline. Hand activities were quantified by force and repetition. Force levels were measured by workers' self-reports, ergonomists' estimates based on observation and measurements with instrumentation. Repetition levels were measured by detailed time-motion analyses using two repetitive hand activity definitions and ergonomists' estimates using scales for the American Conference of Governmental Industrial Hygienists hand activity level and the Strain Index. Results showed that the present exposure assessment approach seems to be able to quantify force level and repetitiveness of hand activities. Repetitive hand activity is quantified differently depending on whether forceful hand exertion or repetitive muscle activity is used as the definition. These hand activity definitions may quantify different physical exposure phenomena. Individual exposure assessment is important in epidemiological research of musculoskeletal disorders as there are interactions between the individual subjects and the measured parameters. These interactions may vary between exposure parameters.     

Comparison of self-report,video observation and direct measurement methods for upper extremity musculoskeletal disorder physical risk factors

The prevention of work-related musculoskeletal disorders has become a national priority in many countries. Increasingly, attempts are made to quantify those exposures that increase risk in order to set exposure limit values. This study used commonly employed field measurement methods and tools in order to perform an inter-method comparison between three primary methods of risk factor exposure assessment: self-report questionnaires, observational video analysis and direct measurement. Extreme posture duration, repetition, hand force (estimated from electromyography) and movement velocity were assessed for 18 subjects while performing each of three jobs processing tree seedlings. Results indicated that self-reports were the least precise assessment method, which consistently overestimated exposures for each of the measured risk factors. However, adjustment of the reports as psychophysical scales may increase agreement on a group level. Wrist flexion/extension duration and repetition were best measured by electrogoniometer. Electrogoniometric measures of wrist deviation duration and frequency were less precise than video analysis. Forearm rotation duration and repetition, grip force and velocity appeared to be best quantified by direct measurement as measured by electrogoniometer and electromyography (EMG) (as root-mean-square amplitude). The results highlight the fact that it is as important to consider and report estimated measurement error in order to reduce potential exposure misclassification in epidemiologic studies.     

Comparison of self-report, video observation and direct measurement methods for upper extremity musculoskeletal disorder physical risk factors

The prevention of work-related musculoskeletal disorders has become a national priority in many countries. Increasingly, attempts are made to quantify those exposures that increase risk in order to set exposure limit values. This study used commonly employed field measurement methods and tools in order to perform an inter-method comparison between three primary methods of risk factor exposure assessment: self-report questionnaires, observational video analysis and direct measurement. Extreme posture duration, repetition, hand force (estimated from electromyography) and movement velocity were assessed for 18 subjects while performing each of three jobs processing tree seedlings. Results indicated that self-reports were the least precise assessment method, which consistently overestimated exposures for each of the measured risk factors. However, adjustment of the reports as psychophysical scales may increase agreement on a group level. Wrist flexion/extension duration and repetition were best measured by electrogoniometer. Electrogoniometric measures of wrist deviation duration and frequency were less precise than video analysis. Forearm rotation duration and repetition, grip force and velocity appeared to be best quantified by direct measurement as measured by electrogoniometer and electromyography (EMG) (as root-mean-square amplitude). The results highlight the fact that it is as important to consider and report estimated measurement error in order to reduce potential exposure misclassification in epidemiologic studies.     

Quantifying repetitive hand activity for epidemiological research on musculoskeletal disorders--part II: comparison of different methods of measuring force level and repetitiveness

This paper focuses on comparisons between the different methods of assessing repetitive hand activities. Various methods were used to measure hand force and repetitiveness of hand activities on 733 subjects in the study described by Bao et al. (2006). Two definitions of repetitiveness were used in analysis of detailed time studies of repetitive hand activities and four parameters of the American Conference of Governmental Industrial Hygienists (ACGIH) hand activity level (HAL) and the Strain Index methods were estimated by ergonomists and used to quantify repetitiveness. Hand forces were measured or estimated using three different methods: 1) measured with a force gauge or mimicked on a force gauge (force matching); 2) estimated by ergonomists using rating scales; 3) self-reports by subjects. The jobs were also evaluated using the ACGIH HAL and Strain Index methods when different repetitiveness quantification methods were used. Results showed that different definitions of repetitive exertion might lead to measuring different physical exposure phenomena and produce very different results. There were poor correlations between the measures of repetitiveness estimated by the different methods. Correlations between force quantifications using different methods were also poor. This suggests that parameters measured by different methods might not be interchangeable. Both the ACGIH HAL and Strain Index methods identified more 'hazardous' jobs when repetitiveness was estimated by ergonomists than when it was calculated by detailed time studies of forceful hand exertions. The Strain Index method identified more 'hazardous' jobs than the ACGIH HAL method. Overall, the between-methods agreements were found to be moderate to substantial.     

Thumb force and muscle loads are influenced by the design of a mechanical pipette and by pipetting tasks

Work involving pipetting is associated with elevated rates of musculoskeletal disorders of the hand and wrist. The purpose of this study was to quantify thumb loading and muscle activity and determine if they varied among pipetting tasks. Fourteen experienced participants performed nine pipetting tasks while surface electromyography was measured for the extensor pollicis brevis, abductor pollicis longus, flexor pollicis longus, and abductor pollicis brevis muscles. For five tasks, participants used a pipette instrumented to measure the thumb force applied to the plunger. High-precision tasks significantly increased static muscle activity but reduced peak thumb force on average 5% as compared with low-precision tasks. Pipetting high-viscosity fluids increased peak thumb forces on average 11% as compared with pipetting low-viscosity fluids. Use of a latch pipette increased muscle activity of three muscles. We conclude that pipette design and pipetting tasks can influence applied thumb force and muscle activity. We recommend that pipettes be designed to limit applied peak forces and that pipette users be instructed in use patterns that will reduce applied forces. Actual or potential applications of this research include modifications to pipette designs and worker training in order to reduce hand pain associated with pipetting.     

Validity of fixed-interval observations for postural assessment in construction work

While observing six simulated construction tasks in the field, trained analysts recorded arm, trunk and leg postures categorically with two fixed-interval observational protocols. Observations were compared to measurements obtained with an electronic postural assessment system coupled with video analysis. The electronic postural assessment system consisted of electronic inclinometers to measure upper arm posture, knee flexion and trunk flexion, coveralls to house the inclinometer wiring, and an eletrogoniometric system to measure trunk lateral bending and twisting. Video analysis included frozen-frame analysis that corresponded to the moment of observation and simulated real-time analysis. Measurements were made on five male participants who each performed three tasks representative of construction laborers' work. Agreement among the observational and reference methods was generally high, although significant differences in measured frequency of exposure existed for knee flexion, trunk lateral bending and trunk twisting. The results suggest that, under appropriate conditions, discrete observations can be used to obtain reasonably accurate estimates of exposure frequency for broad categories of certain body postures.     

Work routinization and implications for ergonomic exposure assessment

Jobs in many modern settings, including manufacturing, service, agriculture and construction, are variable in their content and timing. This prompts the need for exposure assessment methods that do not assume regular work cycles. A scheme is presented for classifying levels of routinization to inform development of an appropriate exposure assessment strategy for a given occupational setting. Five levels of routinization have been defined based on the tasks of which the job is composed: 1) a single scheduled task with a regular work cycle; 2) multiple cyclical tasks; 3) a mix of cyclical and non-cyclical tasks; 4) one non-cyclical task; 5) multiple non-cyclical tasks. This classification, based primarily on job observation, is illustrated through data from a study of automobile manufacturing workers (n = 1200), from which self-assessed exposures to physical and psychosocial stressors were also obtained. In this cohort, decision latitude was greater with higher routinization level (p < 0.0001), and the least routinized jobs showed the lowest self-reported exposure to physical ergonomic stressors. The job analysis checklist developed for non-routinized jobs is presented, and limitations of the task analysis method utilized in the study are discussed. A work sampling approach to job analysis is recommended as the most efficient way to obtain a comparable unbiased exposure estimate across all routinization levels.     

Relationships between observational estimates and physical measurements of upper limb activity †

This study examined the internal validity of observational-based ergonomic job analysis methods for assessing upper limb force exertion and repetitive motion. Six manual tasks were performed by multiple ‘workers’ while direct measurements were made to quantify force exertion and kinematics of the upper limb. Observational-based analyses of force and upper limb motion/repetition were conducted by 29 professional ergonomists. These analysts overestimated the magnitude of individual force exertions – temporal aspects of force exertion (duty cycle) were estimated more accurately. Estimates of the relative severity of repetitive motions among the jobs were accurate. Absolute counts of repetitive motions were less accurate. Modest correlations (r² = 0.28 to r² = 0.50) were observed between ratings of hand activity level and measured joint velocities. Ergonomic job analyses relying on systematic observation should be applied and interpreted with consideration given to the capabilities and limitations of analysts in estimating the physical risk factors. These findings are relevant to a better understanding of the internal validity of ergonomic job analysis methods based on systematic observation.     

A study of the difference between nominal and actual hand forces in two-handed sagittal plane whole-body exertions

Given a task posture, changes in hand force magnitude and direction with regard to joint locations result in variations in joint loads. Previous work has quantified considerable vertical force components during push/pull exertions. The objective of this work was to quantify and statistically model actual hand forces in two-hand, standing exertions relative to the required nominal horizontal and vertical hand forces for a population of widely varying stature and strength. A total of 19 participants exerted force on a fixed handle while receiving visual feedback on the magnitude of force exerted in the required horizontal or vertical direction. A set of regression equations with adjusted R(2) values ranging from 0.20 to 0.66 were developed to define actual hand force vectors by predicting off-axis forces from the required hand force magnitude. Off-axis forces significantly increase the overall magnitude of force exerted in two-hand push/pull and up/down standing force exertions. STATEMENT OF RELEVANCE: This study quantifies and statistically models actual hand forces in two-hand, standing exertions. Inaccuracies in hand force estimates affect the ability to accurately assess task-oriented strength capability. Knowledge of the relationship between nominal and actual hand forces can be used to improve existing ergonomic analysis tools, including biomechanical simulations of manual tasks.     

Biomechanically determined hand force limits protecting the low back during occupational pushing and pulling tasks

Though biomechanically determined guidelines exist for lifting, existing recommendations for pushing and pulling were developed using a psychophysical approach. The current study aimed to establish objective hand force limits based on the results of a biomechanical assessment of the forces on the lumbar spine during occupational pushing and pulling activities. Sixty-two subjects performed pushing and pulling tasks in a laboratory setting. An electromyography-assisted biomechanical model estimated spinal loads, while hand force and turning torque were measured via hand transducers. Mixed modelling techniques correlated spinal load with hand force or torque throughout a wide range of exposures in order to develop biomechanically determined hand force and torque limits. Exertion type, exertion direction, handle height and their interactions significantly influenced dependent measures of spinal load, hand force and turning torque. The biomechanically determined guidelines presented herein are up to 30% lower than comparable psychophysically derived limits and particularly more protective for straight pushing.

Practitioner Summary: This study utilises a biomechanical model to develop objective biomechanically determined push/pull risk limits assessed via hand forces and turning torque. These limits can be up to 30% lower than existing psychophysically determined pushing and pulling recommendations. Practitioners should consider implementing these guidelines in both risk assessment and workplace design moving forward.     

An evaluation of predictive methods for estimating cumulative spinal loading

The focus of this study was to assess the amount of error present in several approaches that have been commonly used to estimate the cumulative spinal loading during manual materials handling tasks. Three male subjects performed three sagittal plane lifting tasks of varying loads and postural requirements. Video recordings of the tasks were digitized and a biomechanical model was used to calculate the spinal loading (compression, joint shear, reaction shear, and flexion/extension moment) at L4/L5 for each frame of data. The 'gold standard' for cumulative loading experienced by the subjects was obtained by integrating the resultant biomechanical model outputs for the entire lifting cycle. Five approaches that quantify cumulative spinal loading, four that use discrete measures and one that reduces the number of frames used (5 Hz), were used and compared with the gold standard. The four methods using discrete measures to quantify the cumulative demands of a task resulted in substantial errors (average error across task and subjects was 27-69%). Reducing the number of frames of data processed to 5 frames/s preserved the time varying information and was the only approach examined that did not induce significant error into the cumulative loading estimates. This study indicates that errors in cumulative spinal loading estimates can be large depending upon the approach used, which will hinder any progress in developing a dose-response link between cumulative exposure and an increased risk of low-back pain or injury.     

Spinal loads during individual and team lifting

The aim of this experiment was to compare lumbar spinal loads during individual and team lifting tasks. Ten healthy male subjects performed individual lifts with a box mass of 15, 20 and 25 kg and two-person team lifts with a box mass of 30, 40 and 50 kg from the floor to standing knuckle height. Boxes instrumented with force transducers were used to measure vertical and horizontal hand forces, whilst sagittal plane segmental kinematics were determined using a video based motion measurement system. Dynamic L4/L5 torques were calculated and used in a single equivalent extensor force model of the lumbar spine to estimate L4/L5 compression and shear forces. A significant reduction in L4/L5 torque and compression force of approximately 20% was found during team lifts compared to individual lifts. Two main reasons for the reduced spinal loads in team lifting compared to individual lifting were identified: (1) the horizontal hand force (i.e. pulling force) was greater in team lifting, (2) the horizontal position of the hands was closer to the lumbar spine during team lifts. The horizontal hand force and position of the hands had approximately equal contributions in reducing the spinal load during team lifting compared to individual lifting.     

Spinal loads during individual and team lifting

The aim of this experiment was to compare lumbar spinal loads during individual and team lifting tasks. Ten healthy male subjects performed individual lifts with a box mass of 15, 20 and 25 kg and two-person team lifts with a box mass of 30, 40 and 50 kg from the floor to standing knuckle height. Boxes instrumented with force transducers were used to measure vertical and horizontal hand forces, whilst sagittal plane segmental kinematics were determined using a video based motion measurement system. Dynamic L4/L5 torques were calculated and used in a single equivalent extensor force model of the lumbar spine to estimate L4/L5 compression and shear forces. A significant reduction in L4/L5 torque and compression force of approximately 20% was found during team lifts compared to individual lifts. Two main reasons for the reduced spinal loads in team lifting compared to individual lifting were identified: (1) the horizontal hand force (i.e. pulling force) was greater in team lifting, and (2) the horizontal position of the hands was closer to the lumbar spine during team lifts. The horizontal hand force and position of the hands had approximately equal contributions in reducing the spinal load during team lifting compared to individual lifting.     

An evaluation of predictive methods for estimating cumulative spinal loading

The focus of this study was to assess the amount of error present in several approaches that have been commonly used to estimate the cumulative spinal loading during manual materials handling tasks. Three male subjects performed three sagittal plane lifting tasks of varying loads and postural requirements. Video recordings of the tasks were digitized and a biomechanical model was used to calculate the spinal loading (compression, joint shear, reaction shear, and flexion/extension moment) at L4/L5 for each frame of data. The ‘gold standard’ for cumulative loading experienced by the subjects was obtained by integrating the resultant biomechanical model outputs for the entire lifting cycle. Five approaches that quantify cumulative spinal loading, four that use discrete measures and one that reduces the number of frames used (5 Hz), were used and compared with the gold standard. The four methods using discrete measures to quantify the cumulative demands of a task resulted in substantial errors (average error across task and subjects was 27–69%). Reducing the number of frames of data processed to 5 frames/s preserved the time varying information and was the only approach examined that did not induce significant error into the cumulative loading estimates. This study indicates that errors in cumulative spinal loading estimates can be large depending upon the approach used, which will hinder any progress in developing a dose-response link between cumulative exposure and an increased risk of low-back pain or injury.     

Insertion loads and forearm muscle activity during flexible hose insertion tasks

OBJECTIVE: To quantify the physical demands of hose insertion tasks in automotive assembly operations and how they are affected by method and the mechanical interference between the hose and the flange. BACKGROUND: Insertion tasks were identified by workers as physically demanding and can often lead to fatigue or losses in production attributable to pain or injury. METHODS: Six male and 6 female participants pushed a 25.4-mm flexible rubber hose onto a stationary flange during simulated insertions. Three insertion methods -- rock, straight, and twist -- were examined in the study. Muscle activity of the finger flexors was recorded to estimate grip effort during the simulated insertions. RESULTS: The twist method (114.8 N) resulted in a 26% reduction in axial force compared with the straight method (155.7 N). Average muscle activity ranged from a low of 14% maximum voluntary contraction (MVC; men, straight method) to a high of 67% MVC (women, twist method). Hose resultant forces ranged from a low of 52.2 N to a high of 461.1 N for all participants. Men exerted 6% higher resultant forces with 37% less muscle activity than women. CONCLUSION: There are situations when the 26% reduction in the axial force attributable to twisting may be helpful during an insertion, despite the fact that forearm muscle activity was highest for both male and female participants during twisting insertions. APPLICATION: The results of this study can be applied to the future design of tasks that involve the joining of two parts such as a hose and flange.     

Clinical-Based Engineering Assessment and Data Interpretation of Hand Strength for Task-Oriented Robotic Rehabilitation

A successful implementation of any effective rehabilitation process using robotics systems must consider both the clinical and engineering issues. Useful results obtained by clinical groups are not widely received in the robotics community. Many engineering works documented do not seem to focus on the crucial clinical problems. The lack of an effective means of communication between the clinical and engineering groups is a major hindrance to any effective rehabilitation process. The main objective of this study is to quantify hand strength in patients experiencing motor disability and loss of function in the hand (e.g., following stroke and spinal cord injury). In the present work, hand strength is measured by using surface electromyography and a force sensor during gripping/pinch tasks. With the development of a robotics system, which will obtain inputs from the clinician's assessment, together with the pre-measurement results at each progress interval and through an interactive control system, the robot will provide appropriate assistance to the patient to achieve specific tasks. This study presents clinical data from hand strength measurements that have been processed and streamlined to facilitate interpretation and characterization, by virtue of the design of experiments and multivariate data analysis data analysis methods. The proposed analysis is able to provide useful data (both statistical data and graphic information) for objective and quantitative assessment towards control applications on the hand rehabilitation device that is being developed.     

Task exposures in an office environment: a comparison of methods

Task-related factors such as frequency and duration are associated with musculoskeletal disorders in office settings. The primary objective was to compare various task recording methods as measures of exposure in an office workplace. A total of 41 workers from different jobs were recruited from a large urban newspaper (71% female, mean age 41 years SD 9.6). Questionnaire, task diaries, direct observation and video methods were used to record tasks. A common set of task codes was used across methods. Different estimates of task duration, number of tasks and task transitions arose from the different methods. Self-report methods did not consistently result in longer task duration estimates. Methodological issues could explain some of the differences in estimates seen between methods observed. It was concluded that different task recording methods result in different estimates of exposure likely due to different exposure constructs. This work addresses issues of exposure measurement in office environments. It is of relevance to ergonomists/researchers interested in how to best assess the risk of injury among office workers. The paper discusses the trade-offs between precision, accuracy and burden in the collection of computer task-based exposure measures and different underlying constructs captures in each method.     

Predicting the health risks related to whole-body vibration and shock: a comparison of alternative assessment methods for high-acceleration events in vehicles

In this paper, alternative assessment methods for whole-body vibration and shocks are compared by means of 70 vibration samples measured from 13 work vehicles, deliberately selected to represent periods containing shocks. Five methodologies (ISO 2631-1:1997, BS 6841:1987, ISO 2631-5:2004, DIN SPEC 45697:2012 and one specified by Gunston [2011], ‘G-method’) were applied to the vibration samples. In order to compare different evaluation metrics, limiting exposures were determined by calculating times to reach the upper limit thresholds given in the methods. Over 10-fold shorter times to exposure thresholds were obtained for the tri-axial VDV (BS 6841) than for the dominant r.m.s. (ISO 2631-1) when exposures were of high magnitude or contained substantial shocks. Under these exposure conditions, the sixth power approaches (ISO 2631-5, DIN SPEC, G-method) are more stringent than a fourth power VDV method. The r.m.s. method may lead to misleading outcomes especially if a lengthy measurement includes a small number of severe impacts. In conclusion, methodologies produce different evaluations of the vibration severity depending on the exposure characteristics, and the correct method must be selected.