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The authors ask you:
Unlike the physical risk factors associated with working posture and repetitive motion, the magnitude of exerted grip force is not readily observable in a real-time or video-based assessment of risk factors for work related musculoskeletal disorders (WMSD). Thin profile pressure/force sensor technologies have been employed to characterize grip force in laboratory-based studies and, to a lesser degree, industry settings. There may be several reasons for the limited use of these technologies in field settings. The existing technologies have limitations in terms of cost, durability, localization of sensor placement, adequacy of coverage of the contact surface on the palm, and the “wearability” of the sensor(s) and necessary cabling to the computer interface. Some researchers have developed glove-based and skin attachment interfaces of these sensors to the palmar surface of the hand, but the more widespread use of these technologies by practitioners may be hindered by limitations in the sensor designs. With improvements in these sensor technologies a “wearable” system for assessing exposure to hand grip force exertion may be attainable for field use by health/safety practitioners.
Repetitive high grip force exertion has been identified as a risk factor for work related musculoskeletal disorders (WMSDs) of the upper limbs, particularly when combined with awkward posture and repetitive motions. Unlike posture and repetitive motion, which are associated with observable changes in the position and displacement of joints and limb segments, the magnitude of hand force exertion is difficult to assess visually. Thus, it is believed that a system to reliably quantify hand force will greatly benefit efforts to assess exposure to risk factors for WMSDs.
Thin profile force/pressure sensors have been used to quantify hand and finger contact force in hand grips [1,2,3], mostly in the grip of hand tools. Improvements in the design of thin profile force/pressure sensors since the early 1990’s may make current technologies better suited to applications involving hand grip force measurement. This poster describes the development of a measurement system using thin profile force sensors attached to a glove for the measurement of hand contact force and outlines the possibility for improvements in related technologies for the purpose of hand grip force exposure assessment. It is believed that thin profile pressure/force sensors have potential as a technology for quantifying hand force in the assessment of WMSD risk. However, some limitations need to be overcome before these technologies are likely to receive widespread use among health/safety practitioners for assessing worker exposure to hand grip force exertions.
Our approach [4] incorporates 20 thin profile conductive polymer resistance-based force sensors attached to a thin leather athletic grip glove over the regions shown in Figure 1. The active area of each sensor is circular with a diameter of 9.53 mm. The thickness of each sensor is 0.127 mm. The sensors have leads that are wrapped behind the glove where they terminate at a pin connector block held in place with a Velcro strap on the dorsum of the wrist. In industrial applications these thin sensors are prone to mechanical damage due to abrasion, tear, or chemical contamination. As the cost of these sensors is not high, individual sensor replacement is feasible. We have used golf grip gloves that are available in incremental sizes for both hands so that a form-fitting glove can be donned on any worker’s hand. Raw sensor data are sampled and digitized through a USB data acquisition device with control from a LabVIEW (National Instruments) virtual instrument (VI). Each sensor is individually calibrated by placing it between the investigator’s thumb and a flat metal platform mounted over a button style load cell. The investigator manually applies a thumb force, ramping from zero force to approximately 7 kg and back to zero force. A least squares linear regression is fit to the force vs. sensor voltage output. The high linearity of response typically results in an r2 > 0.98. The raw voltage data from the sensors are converted to calibrated force units and can be displayed in real time with a color intensity display of individual force levels. The system also includes synchronized video capture of the work process through a standard USB camcorder (5).
One basic approach we have employed to characterize the hand force exposure of a job is to calculate a relative frequency histogram (RFH) for total hand force. Total hand force is calculated by simply summing over all sensors at each sampling interval of the recording duration. Following the recording period, the system can instantly compile a RFH for total hand force for the job. Figure 2 shows the RFH for two jobs assessed at a musical instruments assembly facility - a "low" force job on the left (saxophone key assembly) and a "high" force job on the right (trombone buffing). The terms low force and high force are used only with respect to one another - there are no current absolute criteria for defining a valid threshold for acceptable levels of hand force exertion from the RFH. However, the densities in the RFHs along the x-axes clearly reflect differences between the two jobs. A limitation of this approach is that interpretation of the physical demand of the jobs based on total hand force levels should also be informed by the nature of the individual grip exertions (the area and palmar regions over which the contact force is distributed) in addition to the strength capability of the worker. Otherwise, the relative exertion level of a particular grip exertion can be grossly misrepresented. For example the force levels in Figure 3 both represent approximately 50% of the subject's maximum force capability - for the one-point pinch grip (left) and hook grip (right). The sum of all individual force levels are very different between the two situations, however, the relative exertion levels with respect to the individual strength capability for each grip are similar.
Attaching thin profile pressure/force sensors to a thin snug fitting glove has the advantage of facilitating measurement of hand grip force on any object grasped in the work process, without the need to embed transducers in, or wrap sensor media around, the object itself. We believe that a wearable glove-based system designed with “pockets” for sensor insertion will serve to protect the membrane layer of the sensors and standardize their spatial placement with respect to the palm. It is suggested that sensor designs be ruggedized for higher durability in harsh environments and sensor cost be reduced so that replacement of damaged sensors is more economically tolerable. Concerns about adequate sensor coverage of relevant palmar regions, accurate reconstruction of localized contact force in these regions, and “bulkiness” of the sensor leads and cabling may be addressed through the physical and electrical design of the sensor. A disadvantage of this measurement approach that can not be surmounted is that the properties of the glove and/or sensor material may alter the frictional conditions between the hand and the gripped object and may degrade tactile feedback of the shear forces at this interface. If this latter limitation is acceptable thin profile force sensor technologies may have potential for quantifying hand grip forces in industrial settings. Our belief is that there are opportunities to improve upon existing sensor technologies and methods for thin profile pressure/force sensing to improve current methods for assessment of exposure to hand grip force.
With advances in thin profile force sensing technology it is conceivable that a “wearable” direct reading measurement instrument, or even a dosimeter, could be developed for hand grip force exposure assessment. NIOSH and Sungkyunkwan University are in the process of developing a prototype of a wireless system that moves toward fulfilling this objective.
The authors wish to acknowledge the support of Mitsui Sumitomo Insurance Group in facilitating industrial-based pilot tests of the measurement system.
The findings and conclusions in this poster are those of the author(s) and do not necessarily represent the views of the National Institute for Occupational Safety and Health. Citations to Web sites external to NIOSH do not constitute NIOSH endorsement of the sponsoring organizations or their programs or products. Furthermore, NIOSH is not responsible for the content of these Web sites.
