INTRODUCTION

The human hand-arm system is most frequently and repetitively used part of the body in occu-pational repetitive tasks, e.g. assembly work, wire cutting, packaging, hammering, screwing, etc.Sufficient grip strength is necessary to successfully complete daily and industrial activities such asremoving a lid or using hand tools. Literature review showed that a higher grip force is one of thecritical factors in causing the carpal tunnel syndrome (CTS) or hand-wrist cumulative trauma disor-ders, especially when combined with awkward postures (Armstrong and Chaffin, 1979; Silverstein etal., 1987). To get knowledge about the reduced risk of injury, hand strength data is necessary for theanalysis and design of equipment concerning the grip held or manipulated by the hands (Seo et al.,2005). This data may be used for the design of tools or tasks with a low level of discomfort in therepetitive tasks. Over-exertion injuries of the hand/wrist and forearm have been reported to occur inseveral manufacturing industries at an increased rate worldwide. According to the Swedish statisticson occupational injuries, 32% of all reported accidents involve the fingers, hands or wrists. On theaverage, these accidents lead to 20 days of sick leave per occurrence and 15% of them lead to perma-nent disability. In the United States, hand and finger injuries constitute 35% of the total injuries(Mital and Kilbom, 1992). In a survey conducted by the Institute for Occupational Safety and Health(IOSH, 1998), 531 construction workers in Northern Taiwan were interviewed concerning their expe-rience with musculoskeletal symptoms. Aches in the upper extremity were found to be associatedwith the extensive use of hand tools and/or awkward working postures. Seventy four percent of the

J. Human Ergol., 38: 1-9, 2009

Received 13 August 2007; accepted 16 February 2009.

EFFECTS OF ULNAR DEVIATION OF THE WRIST COMBINED WITH FLEXION/EXTENSION ON THE MAXIMUM VOLUNTARY

CONTRACTION OF GRIP

SERAJUL HAQUE1 AND ABID ALI KHAN2*

1 Department of Mechanical Engineering, Z H College of Engineering & Technology, Aligarh Muslim University, Aligarh-202002, UP, India

2 Ergonomics Laboratory, Department of Mechanical Engineering, Z H College of Engineering & Technology, Aligarh Muslim University, Aligarh, UP, India

*Email: abida.khan@amu.ac.in

Work-related musculoskeletal disorders (WMSDs) is related with the frequency of exertionfor repetitive tasks requiring heavy load. Different researchers have reported that a poorposture is very much responsible for WMSD if combined with increased load and/or fre-quency. In the assembly tasks in different industries involve the gripping very commonly.Therefore in the present study it was tried to find the effect of a wrist posture on gripstrength. For design of experiment, subjects, flexion/extension and ulnar deviation of thewrist were taken as independent variables and the dependent variable was maximum volun-tary contraction (MVC) of grip. The results showed that the effect of flexion/extensionangle, ulnar angle and the subject on MVC grip were highly significant (i.e., p<0.001,0.001 and 0.002 respectively). The two-way interaction effect of flexion/extension angle ofthe wrist and the subject on MVC grip was also found significant at p<0.001. The otherinteraction effects were not found significant. MVC grip was found maximum at the neu-tral wrist posture. Key words: flexion/extension of wrist, ulnar deviation of wrist, MVC grip

interviewees reported that they used hand tools to accomplish their tasks. Among them, 57% reportedthat they experienced aches in the upper extremity in the past 12 months.

There are several studies that indicate the connection between the discomfort felt by the opera-tors and the applied force and the repetition rate of hand movements in its different positions (Cirielloet al., 2001; Carey & Gallwey, 2005; Khan et al., 2009). Poor design, excessive use of hand tools,working conditions and postures are associated with an increased incidence of acute trauma and sub-acute chronic disorders of the hand, wrist and forearm (Armstrong, 1983; Agahazadeh and Mital,1987). Sub acute/chronic disorders are so-called cumulative trauma disorder (CTD), also namedrepetitive strain injury (RSI), or work-related musculoskeletal disorders (WMSDs). The excessiveuse of hand tools (especially ergonomically poor design) coupled with awkward working postureslead to the exertion of large grip forces, extreme wrist and finger deviations, high local pressures inthe hand, unsuitable shoulder/neck postures, repetitive movements and productivity losses (Tichauerand Gage, 1977), and hand tools are the main contributors of upper extremity musculoskeletal disor-ders.

Numerous studies indicated a strong correlation between upper extremity WMSD and forcefulexertions. The effects are more severe, if forceful exertions are accompanied by the high frequency ofexertions and awkward postures (Silverstein et al., 1986; Armstrong et al., 1986; Putz-Anderson,1988; Kroemer, 1989). Carey and Gallwey (2002) investigated the effects of exertions on discomfort.They found that extreme flexion caused higher discomfort than the other simple types of deviations,and the combination of flexion and ulnar deviation resulted in higher discomfort than the other typesof combined deviation.

From the literature and by interviewing the frequent users of the hand tools, it was found thatpoor design might cause decreases in productivity with increased fatigue and more errors. This ispossibly because of the increases in injuries to the wrist, forearm and shoulders. Injuries mightenhance the compensation costs. Many of the absenteeism cases owing to illness and motivated bymusculoskeletal disorders are linked to the wrist, i.e., by repetitive movements and hand-appliedforces (Bernard, 1997). Reduction in grip strength may increase the likelihood that the user will losethe control of the tool or drop it, leading to an injury or poor quality work. The awkward posture ofthe hand during the use of hand tools is a major cause of work-related injuries which are previouslystudied by many researchers.

A number of different ergonomics design changes intended to reduce the incidence of upperlimb injuries have been reported in the literature. In the main, changes have focused on reducing therequirement for deviated wrist positions and reducing the loads on the hand and wrist. Kattel et al.(1996) investigated the effects of flexion and ulnar deviation of wrist combined with elbow flexionand shoulder abduction angles on grip strength. They found that peak grip strength occurred at acombined posture of shoulder abducted 00, elbow flexed 1350, and the wrist in the neutral posture.Decrement of up to 42% in grip strength was reported as elbow and wrist angles deviated. Shih(2005) investigated the effect of wrist flexion/extension combined with shoulder rotation on MVCgrip and found that upper limb postures had significant effect on MVC and there was highly signifi-cant effect of gender on MVC. However no study was found indicating the effect of the combinedwrist radial/ulnar deviation with flexion/extension on grip strength (MVC). Therefore the aim of thepresent study is to find out best posture of the hand and wrist with a high level of MVC grip withoutpain/discomfort. The results of the present study will be helpful to design the tools providing maxi-mum efficiency/effectiveness in tasks with a lower level of error and with the reduced risk of discom-fort, stress, fatigue, etc.

METHODS

SubjectFifteen male university student of the age group of 20 to 24 years volunteered in the study.

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Subjects who informed any detectable physical abnormality of the hand and forearm, or who had apast history of significant injury or disease involving the hand were not allowed to participate. Beforestarting the experiment, a consent form was given each subject following briefing about the experi-ment. It was dully signed by the subjects.

Experimental DesignFor the experiment, the neutral condition of the wrist was assumed to be approximately 10°

ulnar and ≤5° extension angle with respect to the midline of the forearm and 0° ulnar and 0° exten-sion/flexion angle of the wrist as (Figure 1). Three levels of ulnar deviation of the wrist (0°, 10° and20°) combined with five levels of flexion/extension of the wrist (0° and 20° flexion, mid position and100 and 200 extension) were investigated for the maximum voluntary contraction (MVC) of gripstrength. The 3 (three levels of ulnar deviation of wrist) x 5 (five levels of flexion/extension of thewrist) x 15 (subjects) full design of experiment was used for the study. On fifteen combinations ofexperimental treatment, MVC grip was also recorded for fifteen subjects. The experiment wasordered based on 15 (experimental treatments) x 15 (subjects) full Latin squares to randomize theexperiment so that it would comply with the assumptions of analysis of variance (ANOVA). Latinsquare was used to conduct the experiment in random order. That was supposed to be verified by atest of normality of the data. The subjects were chosen randomly as their availability was consideredto be a random factor instead of a fixed factor.

Experimental setupThe schematic diagram of the experimental setup is shown in Figure 2. Figure 3 is the picture of

the experimental rig. For minimizing the variation of upper arm abduction and elbow flexion andextension, a fixture is fabricated. An adjustable chair and a fixed table was used so as to adjust thechair according to anthropometric dimensions of volunteers. An elbow guide commonly used incricket and other games was used for comfortable support of the elbow during observations. This fix-ture also supported the hand while observing the maximum grip force. The grip dynamometer(Model Anand Agencies, India, with least count 1 kg and range 1-100 kg) was used to record theMVC grip for each of the experimental treatments. Two protectors were attached in the experimentalrig as shown in Figure 3 to measure horizontal and vertical movement of the wrist.

ProcedureAfter giving the consent, each subject was asked to sit in a fixed posture with self-adjustability

on the comfortable chair. The back was vertical in coronal plane, with the upper arm vertical, and theforearm was fixed with bands over a comfortable support attached with the rig. In this posture, thesubject held the grip meter for each treatment of the experiment. As per order selected by LatinSquare design, each subject was asked to squeeze the grip dynamometer up to the maximum possiblestrength for that subject with an interval of two minutes or till the subject felt fully comfortable. Out

WRIST POSTURE AND MAXIMUM CONTRACTION OF GRIP 3

Fig. 1. Postural deviations of the wrist relevant to the present study.

of two attempts, the maximum value of MVC grip was recorded for each treatment of the experi-ment. Than rest of the treatments were completed in the same manner for each subject with an inter-val of five minutes.

RESULTS

The summary of the data is presented in Table 1. All statistical analysis was performed usingSPSS 13.0. Initially data was tested for normality, i.e. the assumption of using ANOVA. The his-togram and normal score plot of the data of MVC grip showed normality of the data. The skew nesson the histogram was almost the same on both sides. Also the normal scores plot showed a closenessof straight line indicating signs of normality of the data. Thus it was clear that data was normally dis-tributed and it satisfied the assumption of using ANOVA. The results of ANOVA for dependentvariable MVC grip are presented in Table 2. The results showed that effects of flexion/extensionangle, ulnar angle and the subject on MVC grip were highly significant (i.e., p<0.001, 0.001 and0.002 respectively). The two-way interaction effect of flexion/extension angle of wrist and subject onMVC grip was also found significant at p<0.001. The other interaction effects were not found signifi-cant.

The graphical analysis of the main effects and interaction effects are shown in Figures 4 and 5.The figures also showed that there was no significant two-way interaction of flexion/extension withulnar deviation. In Figure 4, very close lines are seen for different levels of ulnar deviation with an

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Fig. 2. Schematic diagram of the experimental setup.

Fig. 3. Experimental rig and grip dynamometer used for the study.

approximate difference of 49.05N to a maximum of 29.43N. For the deviation from the neutral wristposition to flexion and extension, flexion had more decreasing effect on MVC grip compared toextension of the wrist. This difference was approximately 29.43N for 100 in flexion/extension and49.05N for 200 in flexion/extension.

In Figure 5, a decreasing trend of MVC grip for increasing ulnar deviation was observed for allangular deviation of wrist flexion/extension except for 200 flexion of the wrist. Also an increase inMVC grip was noticed for 200 ulnar deviation compared to 100 ulnar deviation for 200 extension ofthe wrist.

The pairwise t-test was performed between certain positions of the wrist posture, and these posi-tions were not identified as significantly different from each other by ANOVA (as shown in Figure 4and 5). The results showed that MVC grip for different angles of ulnar deviation were not significant-ly different at 200 extension of the wrist (p>0.10). For 200 flexion of the wrist, the MVC grip was notsignificantly different for 100 and 200 ulnar deviation of the wrist (p=0.438), while 100 and 200 ulnardeviation had a significantly different effect on MVC grip compared to the neutral wrist position atp=0.003 and 0.049 respectively. Also at 100 extension, the neutral wrist position was found to have asignificantly different effect on MVC grip compared to 100 and 200 ulnar deviation at p=0.038 and0.002, respectively. Other differences of ulnar deviation at different values of wrist extension andflexion were not found significantly different at all the levels of extension/flexion of the wrist consid-ered in the present investigation.

WRIST POSTURE AND MAXIMUM CONTRACTION OF GRIP 5

Table 1. Summary of the data of MVC grip (Newtons) with standard deviation.

Table 2. The result of ANOVA using MVC grip (Newtons) as dependent variable.

DISCUSSION

Mogk and Keir (2003) found that a flexed wrist reduced the maximum grip force by 40-50%and their results supported the present findings. In line with the study of Mogk and Keir (2003), theresults of the present study indicated that approximately 10% decrease for 100 flexion and 20%decrease for 200 flexion of the wrist. However, the grip MVC was found to be reduced more for thesame angular extension of the wrist, i.e. about 20% reduction for 100 extensions and 25% reductionfor 200 extension of the wrist. Kee & Karwowski (2001) reported the ranges of motion of wrist flex-ion and extension as 720 and 650 respectively. It means that the range of the maximum possible exten-sion (degrees) of the wrist was less than the maximum possible flexion of the wrist. The reason canbe the ability to extend the wrist was lesser than that for flexion. It means that reduction in grip MVCcould be greater for extension than for flexion in terms of the angular deviation of the wrist. Terrelland Purswell (1976) reported that grip strength is reduced if the wrist deviates from the neutral pos-ture. In their study, it was concluded that the supine palm had the maximum grip strength withrespect to the neutral wrist position. The effect of the change in grip strength with respect to wristdeviation is thus in line with the findings of the present study. It is also found that the grip strengthreduced with the ulnar wrist deviation and extension/flexion of the wrist.

Dempsey et al. (2004) found that the relationship between productivity and deviated wrist pos-

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Fig. 4. The profile of MVC grip for different levels of extension/flexion at differentlevels of ulnar deviation of the wrist.

Fig. 5. The profile of MVC grip for different levels of ulnar deviation of the wrist atdifferent levels of extension/flexion.

tures was inversely related. That is why it can be said that the deviated wrist, even if it is due to thebad design of the tool or task, can cause reduced productivity as well as WMSDs. These findings ofinverse effects of the bad posture of the wrist can reduce human performance in accordance with thepossibility of injury or reduced strength due to discomfort/pain. The present results also showed thatthe MVC grip reduced with the increase in ulnar deviation of the wrist as well as with the increase inextension/flexion of the wrist. That could lead to more stress and less productivity with wrist devia-tion.

The findings in terms of the reduction in MVC grip relative to wrist extension/flexion were inline with the findings of Keir et al. (2007). Keir et al. (2007) reported that with the angular deviationin extension of about 200, carpal tunnel pressure (CTP in mmHg) increased compared to 10 mmHg atthe neutral wrist position. Although, the increase in CTP was very less for 200 flexion of the wrist,that was increased beyond 200 flexion of the wrist. Therefore it was somewhat explaining the reasonsfor the reduction of MVC more in extension than in flexion of the wrist. Keir et al. (2007) alsoreported the increase in CTP for ulnar deviation of 200 (about 25 compared to 12 mmHg at the neutralwrist position) that may be the one of the causes of reduction in MVC grip strength.

CTP was supposed to be a strong factor to explain the reasons for change in wrist grip strength.However, it was not the complete answer for the reasons of the change in MVC grip strength.Therefore the muscle architecture (MA) in relation to wrist deviations was investigated through theliterature and few studies were found to help in this regard (Pigeon, et al., 1996 and Gonzalez et al.,1997). Muscle architecture mainly concerns the physiological cross Sectional area (PCSA), momentarm (MA), muscle length, and motor unit recruitment. PCSA is determined by dividing muscle vol-ume by true fibre length that represents the muscle strength (An et al., 1981). The function of thewrist joint is determined primarily by the contour of its contact surfaces and the constraints of its sur-rounding soft tissue (An and Chao, 1991). Biomechanical potential (BP) for force exertion has beendefined as the product of the PCSA and MA of muscles (Schoenmarklin and Marras, 1993). Itappears from the above that, if the BP of a particular muscle is greater, greater force can be generatedwith the least effort. From the findings of Pigeon et al. (1996) and Gonzalez et al. (1997), Table 3was constructed for five muscles of the forearm. The five muscles chosen were the flexor carpiulnaris (FCU), extensor carpi radialis brevis (ECRB), flexor carpi radialis (FCR), extensor carpi radi-alis longus (ECRL) and extensor carpi ulnaris (ECU), as these were expected to be the most activesurface muscles for the wrist grip. Khan et al. (2009) also studied four muscles (FCR, FCU, ECRBand ECU) for the evaluation of the muscle efforts during flexion force exerted by the wrist. Table 4shows that wrist radial/ulnar deviation leads to drastic changes in BP and MA. In ulnar deviation, BPis reduced compared to the neutral condition. That might be the cause of a reduced MVC grip inulnar deviation because a lower BP means that more effort is required to produce the same level offorce. It can be seen in Table 3 that wrist flexion/extension led to change in BP and MA. That couldbe the result of reduced MVC grip in extension/flexion of the wrist compared to the neutral wristposition.

The present results showed that the MVC of grip strength was at the neutral wrist position.These findings contradict with the findings of O'Driscoll et al. (1992). They stated that the self-select-ed position for recording grip strength was 350 of extension and 70 of ulnar deviation. Grip strengthwas significantly less in any position of deviation from this self-selected position. The reason for thiscould be because the subject might be using his hand only in the position selected for the experiment.And the deviation at which reduced grip strength was recorded was only 150 of extension or in neu-tral radial/ulnar deviation.

However, some anomalies were noticed in Figure 4. MVC grip was higher for higher ulnar devi-ation compared to lower ulnar deviation, this may be due to some subjects with lesser ability of ulnardeviation of the wrist from the neutral position. These differences were very little as per the differ-ence in MVC grip of the order of 1.962N, as these were tested using the pairwise t-test and found notsignificantly different.

In Figure 5, a decreasing trend of MVC grip for increasing ulnar deviation was observed for all

WRIST POSTURE AND MAXIMUM CONTRACTION OF GRIP 7

the angular deviation of wrist flexion/extension except for 200 flexion of the wrist. Also an increasein MVC grip was noticed for 200 ulnar deviation compared to 100 ulnar deviation for 200 extension ofthe wrist. This may be due to the reason that at higher ulnar deviation and high rotation of the wrist,the degree of compression on the median nerve increases. Another reason may be that the ability ofhigh wrist deviation may vary from subject to subject.

CONCLUSIONS

The information of the MVC grip at different angles will be useful for the design of hand toolsin accordance with the grip strength. The results of the present study suggest that the posture close tothe neutral position will help working more efficiently and comfortably while gripping.

MVC grip was the maximum at the neutral condition of the wrist, i.e., approximately 100 ulnardeviation of the wrist, with no rotation of the forearm and no angular deviation in the flexion/exten-sion of the wrist.

ACKNOWLEDGEMENT

Authors are thankful to the Department of Mechanical Engineering, Aligarh Muslim University,India for providing necessary support and facilities to carry out this research.

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