Upholstery Hand Tool
Risk Factors
Upon review of the results of our biomechanical analysis of the upholstery operation it became clear that the principle risk factors for hand/wrist stress were the repetitive, forceful and static pinch grip exertions performed by the operators’ non-dominant hand. Specifically the operator would pull the fabric tight with their non-dominant hand using a pinch grip between their thumb and index finger and would secure the fabric using the staple gun in their dominant hand. The operator would then release the grip on the fabric and move their hand down ~2” and repeat the process. Each one of these grips required a forceful pinch grip and were repeated every .5 seconds. The operators complained of pain in the fingers, wrists and forearm.
Engineering Design of Intervention Prototype
The first level prototype for the upholstery handtool began with a simple welding clamp. On this clamp was added a 6 inch wide nose-plate that was covered with an abrasive surface to increase gripping power. The benefits of this design are that it changes the pinch grip to a power grip, eliminates the prolonged static exertion by utilizing a clamping mechanism, decreases repetition by covering more linear distance per gripping exertion, and decreases the force required by the use of the mechanical advantage of the tool. Subsequent modifications to this tool included replacing the abrasive surface with a series of pins that pierce the fabric, thus increasing the gripping power of the device.
Based on some filed observations that showed some limitations of the first level prototype, a second prototype was developed. The specific limitations cited by the furniture workers were that the wide noseplate made it difficult to get into many positions in the piece of furniture. They noted that the tool was excellent for working on the outside of the piece, particularly the long stretches that would be found on sofas, but when trying to use the tool in the more confined spaces on the inside of the piece the tool was too cumbersome. In the development of the second level prototype our goal was to maintain the ergonomic benefits of the first prototype (elimination of the sustained pinch grips) while making the tool more nimble/versatile. Field-testing of this prototype is currently underway. Preliminary results have shown this handtool to be more usable.
Laboratory Evaluation of Prototype
Subjects
Six subjects (four men and two women) were recruited from the university population and signed an Informed Consent form before participation. All subjects were in good health and had no serious musculoskeletal problems. Some pertinent anthropometric data are shown below.
Experimental Design
A mock upholstery workstation was created to simulate the conditions of upholstering in the furniture manufacturing industry. Fabric was draped over the workstation and a wooden frame. Marks were drawn across the fabric to indicate where the subject was to pull the fabric. The subject pulled the fabric tight over the frame with their right hand using standard pinch grip or the upholstering hand tool. A staple gun was held in their left hand and at each pull, the subjects simulated stapling the fabric to the frame.
Three different weights (20N, 40N, 50N) were suspended in the fabric to simulate the different force requirements of different fabrics and spring constants experienced by upholsterers. The subject completed one pass across the workstation (~36 inches), rested, and then repeated it. Each weight trial was completed using the pinch grip and again with the hand tool.
These pictures show the testing set-up. In the right-hand picture, the subject is using the hand tool to pull the fabric.
Independent Variables
There were two independent variables in this study: type of grip and level of resistance. The type of the grip had 2 levels: the standard pinch grip and the upholstery hand tool. The level of resistance had 3 weight levels: 20N, 40N, and 50N.
Dependent Variables
There were four dependent variables in this study: wrist posture, muscle activity, productivity, and subjective preferences.
Two electrogoniometers were placed on the subject’s right wrist. One monitor was located on the top of the wrist with the hinge aligned with the wrist’s center of rotation. This monitor measured radial and ulnar deviation. The second monitor was located on the side of the wrist with the hinge aligned with the wrist’s center of rotation. This monitor measured flexion and extension.
Normalized integrated electromyography (NIEMG) was used to measure muscle activity in the right arm. The muscles sampled included the flexor digitorum, extensor digitorum, generalized thenar group, and the first dorsal interosseous. The processed EMG data was collected at 100 Hz for 30 seconds during each trial.
Productivity was measured by timing the length of time it took the subject to complete one full pass on the mock upholstering workstation.
At the end of the experiment, the subject completed a survey reviewing the pinch grip and the hand tool. The questions allowed the user to determine their subjective preference of upholstering method.
Procedure
The subjects completed the experiment in one day. The subject was informed of the experiment procedures and was asked to sign an Informed Consent Form. Anthropometric measurements were taken. The subject’s skin was then prepared for electrode placement over the four muscle groups to be sampled.
Four maximal voluntary exertions (MVE) were performed to get a maximum contraction from each sampled muscle group. The subject was instructed on how to perform the MVEs and then practiced each one. They were to exert a maximum exertion for several seconds and then rested before performing the next MVE. The MVE for the flexor digitorum was achieved when the subject made a fist with their right hand, turned their arm to the supinated position, and rolled their wrist upward against resistance. The MVE for the extensor digitorum was achieved when the subject made a fist with their right hand, turned their arm to a pronated position, and rolled their wrist downward against resistance. The MVE for the generalized thenar group was achieved by having the subject pull their thumb across their palm against resistance. The MVE for the first dorsal interosseous was achieved by pushing their finger laterally against resistance. After sufficient rest period, resting values were recorded.
Two electrogoniometers were attached to the subject’s wrist using hypoallergenic, flexible surgical tape. These monitors measured wrist posture throughout the trials. The wrist was first placed in a neutral posture and these values were recorded.
Subjects were standing at the upholstering workstation to complete the tasks. Breaks were provided after each trial. Then, after all of the trials were completed, the subject completed a short questionnaire regarding the ease of use of the different methods as well as their preference of method.
Results
The results of this analysis revealed a considerable reduction in the intrinsic muscles of the hand and a slight increase in the
extrinsic muscles in the forearm. These results, which demonstrate a shift to the use of the forearm muscles, show the benefit of
changing from a pinch to a power grip. The power grip is preferred because the forearm muscles are larger and can exert more
force than the intrinsic hand muscles. When productivity was considered, the results showed a slight improvement in
productivity during tasks requiring the higher pull forces and a slight reduction in productivity during the tasks requiring less pull
force.
FIELD TESTING
This tool was developed to be used by workers who upholster large areas of a furniture frame (i.e. outside or inside
sections). However, the furniture manufacturing plant that we visited with this tool had the upholstery tasks divided into small
parts. When an upholstery only works on a small part (such as an arm), the task can be performed much more quickly with a
pinch grip. We discovered that a pulling tool is not useful for workers in these types of plants. At this particular plant, however,
workers pulled large sections of fabric to cover partitions (used in office settings). One of the workers used the tool for this
task. During the testing, two of the needles on the tool broke when the tool was pulled against the fabric. Otherwise, the tool
pulled the fabric effectively. In this task, however, few pulls were needed because the fabric did not need to be pulled tightly to
cover the partition. Therefore, the tool is not practical for this task.
From our industry testing with the second prototype, we found that we needed to have stronger needles in the tool. Titanium
needles were purchased and used in the redesigned prototypes. Also, an attachment system was developed that allows the user
to quickly replace a needle if it does break during use.
For more information about ergonomic interventions for the furniture manufacturing industry, please contact:
Dr. Gary Mirka, Professor of Industrial Engineering at North Carolina State University