This study evaluated the validity of force matching in a field study using hand force estimation procedures commonly used by ergonomists (Bao et al., 2006
). The subjects selected for this study were unfamiliar with the force match procedure and received only a brief introduction and opportunity for practice prior to data collection which is the typical scenario in many field studies. The results showed that subjects had inconsistent responses and large coefficients of variation among repeated estimation trials as well as among multiple trials of direct measurement of the actual fastener installation. The force estimation method produced reasonably valid results for three of the six tools as evidenced by the low level of estimation errors and modest correlations. The three tools with high estimation errors may be due to a poor interface of the tool and pressure mat (tool 5) or caused by the nature of the tool operation (tools 4 and 6). Tool 5 showed significant differences in area of coverage of the mat on the handles of the tools and the dynamometer and this affected the pressure readings. Tool 4 was a light weight tool that required a low hand force to operate the tool during the installation; tool 6 was a heavier tool (4.3 kg) and used a pull force with a “break” at the end of the operation that may have been difficult to replicate. These two issues were unique to these two tools so it is unknown if this would be repeated with different tools that have the same characteristics.
In order to relate physical exposures to adverse health outcomes, it is important to have valid exposure estimates. Many studies have reported large within subject and/or between subject variability of physical exposures (Burdorf, 1993
; Kromhout et al., 1993
). The variability may be due to the nature of the work tasks, the environment, or the method for measuring the physical exposures. Force matching gives a subjective estimation of the perceived effort and even though the estimate does not represent the actual force, the values may be adequate for use in epidemiological studies to show a relationship to poor health outcomes. Using cut-points to categorize the force estimates as high or low exposure can result in misclassification of cases soresearchers should be cautious in applying defined cut-points to estimated values.
The current study was intended to mimic the way data collection occurs in ergonomic field studies since most companies cannot afford to have workers taken off the assembly line for extended periods of time. The worker volunteers were given specific instructions about the force match procedures and they had the opportunity to practice the protocol before the start of data collection. The procedure for testing initially had each subject install four fasteners with a single tool to help them adjust to the test set-up before performing the first force match trial. Despite these efforts, there was large within subject variability, and this occurred within the estimation and the directly measured trials. Factors contributing to this variability may have included the short time (2–4 seconds) required for installation of each fastener, the low force required to hold and operate the tool during the installation, and subtle differences between arm and fastener position in successive trials. Some researchers have found a tendency for more frequent misclassification and exposure underestimation in tasks with low force levels (Kumar et al., 1997
; Lowe, 1995
; McGorry et al., 2004
) and this may account for the large estimation errors found in tool 4 of the current study. McGorry and colleagues (2004)
reported a high mean error of estimation (194.7%) in more complex tasks such as meat cutting even when performed by experienced meat cutters. The high variability of grip force measured among individual experienced workers while performing the same task with the same tool demonstrates the inherent variability of hand force exposures in workplace settings, a fact that poses a significant challenge for all methods of exposure assessment.
We found high estimation errors within some tools and these variable responses occurred more frequently in four of the eight subjects. This indicates that some subjects were less able to perceive and reproduce efforts, particularly for some tools. This is not surprising as there is a wide range in human abilities (memory, physical strength, cognition) so it is likely that some subjects are more aware of physical performance and therefore able to replicate forces during estimation (Casey et al., 2002
). The large errors among some subjects may indicate the lack of understanding of the force match process. Training was not a part of this protocol; some studies that have emphasized training and used a reference for force estimation report improved precision of estimates (Marshall et al., 2004
; Spielholz, 2006
) but others found no difference (McGorry et al., 2004
). Ideally, several workers would provide estimated values on several trials of a particular task to reduce the effects introduced by worker variability.
This study had several limitations. Using the pressure mat may have altered the normal tool-hand interface and the tactile feedback from the fastening operation and therefore contributed to some of the variability in the measurement, In addition, the mat sensor was designed to detect perpendicular forces, so shear forces that may have been present with some operations would have been missed. Most tools used a rotational mechanism to install the ¼-inch diameter threaded fasteners; the small fastener size, light weight tools, and relatively low hand forces would have produced minimal shear forces during the installations. A comparison of the mat on the dynamometer to the dynamometer readings showed a 23% lower force value indicative of some loss in detected pressures, although the area of coverage of the mat on the tool handles were similar to the dynamometer handle mat area for most tools. Lower measured pressures from non-perpendicular forces may lead to lower correlations and larger estimation errors. These problems likely contributed to the lack of agreement shown in some of the tools. In addition, the limited number of workers tested may have contributed to some of the variability in results. The installation tools used in this fastening operation required a generally low hand force and a short duty cycle, both of which may have affected workers’ ability to reproduce estimated forces.
The strengths of the study are the measurements taken from experienced workers, using the tools, materials, and work procedures that exist in an actual work setting. We tested the validity of force matching, a method commonly used by researchers and consultants to obtain hand force estimates from work tasks in manufacturing settings.
Our results suggest that force matching provides a reasonable estimate of hand grip forces for some tools under field research settings. Further investigation is needed to determine the validity of using the force match method to collect force estimates on tools requiring low forces to operate or with impulsive mechanisms. Use of direct measurement techniques will provide more accurate estimates of exposures, but in many settings may result in fewer workers or work tasks being evaluated. Common to epidemiological study designs is the trade-off between obtaining high quality exposure data on few subjects versus lower quality data on many subjects. Force matching gives acceptably accurate estimates for some tools and tasks, and will allow more workers or work tasks to be studied.