پديد آورندگان :
پويا كيان، مصطفي دانشگاه علوم پزشكي شهيد بهشتي - دانشكده بهداشت و ايمني - گروه مهندسي بهداشت حرفه اي و ايمني كار، تهران، ايران , رنجبريان، محمد دانشگاه علوم پزشكي شهيد بهشتي - دانشكده بهداشت و ايمني - گروه مهندسي بهداشت حرفه اي و ايمني كار، تهران، ايران , خادمي كلانتري، خسرو دانشگاه علوم پزشكي شهيد بهشتي - دانشكده علوم توانبخشي - گروه فيزيوتراپي، تهران، ايران , خداكريم، سهيلا دانشگاه علوم پزشكي شهيد بهشتي - دانشكده بهداشت و ايمني - گروه اپيدميولوژي، تهران، ايران , احمدلو، مهناز دانشگاه علوم پزشكي شهيد بهشتي - دانشكده بهداشت و ايمني - گروه اپيدميولوژي، تهران، ايران
چكيده لاتين :
International Labor Organization (ILO) 2015 report shows that about 40% of the all total compensation for work-related diseases and accidents in the world is due to musculoskeletal disorders. According to UK Health and Safety Executive (2019) 41% of all registered Work related Musculoskeletal Disorders (WMSDs) are due to upper limbs. Barr et al. (2004) reported that most absence from work in the United States was due to musculoskeletal injuries to the hand and wrist. Hand, arm, and forearm are most important limbs of human body to work. Hand tools, especially non-powered hand tools, are responsible for a significant portion of damages. Many different types of hand tools are used in industrial, services, and domestic activities including wrenches, pliers, screwdrivers, clamps, snips, saws, etc. The use of hand tools is accompanied by repetitive movements and previous studies have shown that inappropriate design of hand tool can damage human joints, tendons and muscles. Repetitive movements are one of main causes of musculoskeletal injuries. In order to achieve high levels of comfort, safety, and efficiency of hand tools, physical abilities and limitations of users must be taken into account in design phase. Also, women and left-handed people must be considered in design of hand tools due to their different body dimensions and abilities. These factors are directly influences the trading of hand tools. Handle design is an important factor in the safe, convenient and ease of use of non-powered hand tools. Socket wrench is a single-wrench that is widely used to open and close the bolts and nuts that require high force and high speed operation. Due to wide usage of socket wrench this study aimed to investigate the relationship between physical parameters of length and diameter of handle, and weight of different models of socket wrench with electrical activity of two selected forearm muscles. We examined the correlation between three physical parameters of six trade model of socket wrenches.
Methods: This descriptive-analytical study performed among 58 male workers of the installation and maintenance unit staff of a university. Their ages ranged from 25 to 45 years. All participants were right-handed, healthy, and without history of upper limb injury or musculoskeletal disorders. They had at least one year of job experience in maintenance unit of university, and their body mass index was between 20-30. Six different types of socket wrench were selected and named with alphabetic codes A, B, C, D, E, F. Selected wrenches were differed in length and diameter of handle, and weight (Table 1).
Table 1. Characteristics of the Socket wrenches
Type of
socket wrench Handle length
(mm)Handle diameter
(mm)Weight
(gr)
A225.5225.76439.77
B158.3018.14240.21
C235.1637.82570.58
D228.7818.36592.08
E186.6829/20348.90
F185.7033.56235.21
A simulated task were designed according to usual usage of wrench users. This task was consist of opening the bolts using 6 types of wrenches. For this purpose, a wooden plate was fitted on a table with 12 fitted bolts and nuts of size 10. All bolts and nuts were fastened on the plate using a torque meter with 8 Nm. The height of task surface was in 95 percentile of elbow height of males (104 cm). However, it was possible to adjust the height of the table with the elbow height of each participant. Participants were asked to stand behind the desk and perform the activity of opening the bolts with each of six wrenches. The order of use of wrenches for each participant was random. The steps of each trial were as follows: steps 1) select the right socket; step 2) attach the socket to the handle; step 3) check turning direction; step 4) open the bolts.
Surface Electromyography (SEMG) was applied to record the electrical activity of forearm muscles. The flexing muscle of the fingers and wrist extensor muscles play important role in gripping the wrench handle and using it to perform the simulated task. Therefore, the Flexor Digitorum Superficialis (FDS) and Extensor Carpi Radialis (ECR) muscles were considered for the recording the surface EMG signals. The steps of recording EMG signals were as follows: step 1) finding the flexor digitorum superficialis and extensor carpi radialis muscles on participant’s forearm; step 2) preparing the skin to conncet the electrodes; and step 3) connecting the electrodes. A surface EMG apparatus made by Biometrics Ltd. was used in this experiment. Since the independent (length and diameter of handle and weight of wrench) and dependent variables (EMG signals) were both quantitative, linear regression model was used to examine the relationship between the two quantitative variables and the data were analyzed by SPSS® 21 software.
Result: The mean of participants’ age, height and weight were 39.95 year, 175.7 cm, and 72.9 kg, respectively. Mean and standard deviation of normalized electrical activity of FDS and ECR muscles are shown in Table 2.
Table 2. Mean (SD) of normalized electrical activity of flexor digitorum superficialis and extensor carpi radialismuscles
Type of socket wrenchMean (SD) of electrical activity of muscles (mv/s)
FDS muscleECR muscle
A0.45(0.21)0.47(0.22)
B0.61(0.25)0.48(0.23)
C0.43(0.23)0.46(0.24)
D0.55(0.24)0.45(0.21)
E0.48(0.23)0.44(0.21)
F0.50(0.22)0.43(0.22)
The results showed that the lowest level of electrical activity of FDS muscle was belonged to type C wrench, which had the longest handle length and largest handle diameter. On the opposite side, the highest level of electrical activity of FDS muscle was belonged to type B wrench with shortest handle length and smallest handle diameter. Lowest and highest levels of electrical activity of ECR muscle were belonged to type F and B wrenches, respectively.
A linear regression test was used to examine the relationship between the electrical activity of Flexor Digitorum Superficialis muscle and the physical parameters of the socket wrenches. The results showed that there is a significant relationship between the electrical activity of FDS muscle and the wrenches’ handle length and diameter (p-value<0.001). Based on regression models, for one millimeter increase in handle length and diameter, the level of electrical activity of the flexor muscles decreased by 0.001 and 0.007 mV, respectively. Simple and multiple regression models showed no significant relationship between the electrical activity of ECR muscle and physical parameters of wrenches (p-value>0.001).
Discussion and conclusion: In the present study, relationship between three physical parameters (handle length, handle diameter and weight of wrench) of six type of socket wrenches with electrical activity of two important muscles involved in using this hand tool was investigated. The results showed that increasing or decreasing socket wrench length, handle diameter, and weight were associated with changes in the electrical activity level of FDS muscle. However, no relationship found for ECR muscle. The findings showed that lowest and highest muscle force for opening the bolts were applied in using wrench type C (maximum length) and wrench type B (lowest length), respectively.
Li (2002) showed that new designs of handle for powered and non-powered hand tools can decrease the unnatural postures, including flexion, extension, radial deviation, and ulnar deviation. He showed that flexor digitorum superficialis and the flexor carpi ulnaris of the right forearm may be significantly reduced when using the new designs of pliers. Dong et al (2006) and Ozawa (2001) reported similar finding. They showed that physical characteristics of hand tools can strongly effect the muscular effort required to work with hand tools used by dentists.
The present study showed that there is no significant relationship between the weight of the wrench and the electrical activity level of the Flexor Digitorum Superficialis muscle. This indicates that muscles other than FDS are involved in controlling the weight of hand tool.
In power grip tasks (such as working with a wrenches), the handle diameter and length of the tool are important parameters. Therefore, wrenches with a longer handle length and a larger handle diameter provide the lowest level of electrical activity of the FDS muscle to open the bolts. This results to lower levels of muscle fatigue. Therefore, the force required to open the bolts can be reduced by choosing a longer handle and a larger handle diameter (near to optimum diameter). The findings of this study can be used by hand tools manufacturers to improve tool design. Also, differences in the ergonomic characteristics of different types of a hand tool can be well documented using electromyography. The generalization of the findings of this study should be considered to its limitations. In this study, the performance of Flexor Digitorum Superficialis muscle and Extensor Carpi Radialis muscle were recorded for a short time task. Therefore, the level of muscle fatigue that occurs in participants may vary in actual and long-term conditions. Also, the effect of roughness and softness of the handle on the grip power and electrical activity of the muscles were not examined in this study. Thus, it is suggested that the effect of this variables along with other physical factors be investigated in future studies.
