Electromyography of the Upper Limbs During Computer Work: A Comparison of 2 Wrist Orthoses in Healthy Adults

Article Outline

• Abstract
• Methods
  • Participants
  • Equipment and Materials
    • Electromyograph
    • Orthoses
    • Scale and dynamometer
    • Computer and furniture
  • Procedures
    • Orthotic choice and manufacturing process
  • Electromyographic recordings
    • Electrode placement
    • Maximum voluntary contraction recordings
    • Preparation of the subjects for executing the tasks
    • Conditions for performing tasks
    • Typing
    • Use of mouse
  • Data Analysis
• Results
• Discussion
  • Study Limitations
• Conclusions
• References
• Copyright

Abstract 

Ferrigno IS, Cliquet Jr, A, Magna LA, Zoppi Filho A. Electromyography of the upper limbs during computer work: a comparison of 2 wrist orthoses in healthy adults.

Objective

To examine the effect of wrist orthoses on the electromyography activities of the extensor carpi ulnaris, flexor digitorum superficialis, and fibers of the upper trapezius muscles during computer work.

Design

A randomized, 3×2 factorial design: orthoses (no orthosis, wearing a custom-made orthosis, wearing a commercial orthosis) and tasks (typing, using the mouse).

Setting

Laboratory for biomechanics and rehabilitation.

Participants

Healthy university students (N=23), ranging from 18 to 26 years of age.

Intervention

Study volunteers performed standardized tasks such as typing and using the mouse while wearing 1 of 2 types of wrist orthoses or no orthosis.

Main Outcome Measures

We used surface electromyography and considered 100% maximum voluntary contraction to represent the amplitude of electromyographic activity.

Results

We observed a significant increase in the electromyographic activity of the trapezius (P<.05) with the use of orthoses. No significant difference was observed in the activities of the flexor digitorum superficialis or extensor carpi ulnaris in participants who typed with or without orthoses (P>.05). However, when the participants used the mouse, the extensor muscle presented an increase in activity with both orthoses, and the same pattern was observed in the flexor muscle when the volunteers used the custom-made orthosis.

Conclusions

Wrist orthoses affected the muscle activities in the upper limbs of healthy adults who were using a computer. Electromyographic activity increased in the trapezius when the subjects used either type of orthosis, and the same pattern was observed in the extensor carpi ulnaris when the subjects used the mouse. The flexor digitorum superficialis presented an increase in activity only when the subjects worked with the mouse and used a custom-made splint.

Key Words: Electromyography, Rehabilitation, Splints

List of Abbreviations: BMI, body mass index, ECU, extensor carpi ulnaris, EMG, electromyography, FDS, flexor digitorum superficialis, MVC, maximum voluntary contraction

THE COMPUTER HAS ACQUIRED an important role as a powerful means of work and communication around the world. With the advent of information technology, health problems related to the excessive use of the computers have begun to emerge, especially in the form of musculoskeletal pain in the cervical regions and in the upper limbs.¹, ², ³ Blatter and Bongers⁴ observed that 4 to 5 hours a day of computer work was associated with complaints and symptoms in the upper limbs. According to another study by Jensen et al,⁵ the prevalence of such computer work–related symptoms was approximately 19% in the hands and 53% in the shoulders. Changes in length, force, tension, and muscle contractions are related to patterns of movement repetition and prolonged fixation in 1 determined position.⁶ Furthermore, these factors could lead to microtraumas and macrotraumas, and could cause cervical myalgia, epicondylitis, carpal tunnel syndrome, Guyon canal syndrome, tendinitis, and de Quervain tenosynovitis.⁶, ⁷

The upper trapezius (trapezius), the ECU, and the FDS are the muscles most at risk for injury when specific repetitive motor tasks such as typing and using the mouse are performed. The upper trapezius is constantly engaged to ensure postural stability for long periods. This effort is greater when there is a demand for precision and ocular fixation.⁸ The flexor digitorum superficialis is involved in gripping objects and stabilizing abrupt movements while gripping the object. In situations in which strong finger compression is required, the muscle also exerts great activity in movements that require only 1 finger while the other fingers are flexed.⁹ The extensor carpi ulnaris is continuously involved in stabilizing the wrist to allow adequate gripping of the objects.¹⁰ Together with the other extensors, these muscles play an important role as antagonists of wrist flexion and are primarily responsible for the symptoms and clinical manifestations of repetitive stress disorders.⁹, ¹¹ The most common initial treatments for these disorders involve medication, kinesiotherapy, and temporary immobilization.⁷, ¹² When there are signs of muscle fatigue, which are generally more pronounced on the extensor side of the forearm, the recommended treatment is passive stabilization of the wrist to reduce strain on the extensor muscles.¹³ Medical professionals often agree that immobilization using plaster casts or orthoses promotes rest and prevents the appearance of muscular fatigue.¹⁴, ¹⁵ Orthoses that maintain the wrist in the extended position are the most frequently used.¹⁴, ¹⁵, ¹⁶ Although these immobilization treatments are widely used, there is still controversy regarding the actual efficacy of using the orthoses during forearm muscle rest.¹⁵, ¹⁷, ¹⁸, ¹⁹

EMG is an effective method for elucidating muscle behaviors during functional activities. This technique has been used for decades for ergonomic investigations.²⁰ Electromyographic evaluation allows the quantification of the overall muscle activity required for computer tasks, and is able to provide an assessment of the alterations that may result from splint support. Currently, there is still a lack of satisfactory understanding on the effects of using wrist orthosis. In fact, a review of the recent literature showed that very few studies provided any clear, objective, methodologic, or applicable information regarding the effects of this therapeutic tool on the upper limb.²¹ There are controversies over the different types of orthoses, their functionality, and their effect on muscle activation or deactivation.¹⁶ Thus, this study aims to understand muscle behavior under the influence of 2 different types of extensor wrist orthoses, 1 custom-made (orthosis A) and the other commercially produced (orthosis B), during typing and normal use of the computer mouse. Analysis of muscle behavior was achieved by using electromyography. The choice of the orthosis models was based not only on previous literature, which has confirmed the therapeutic values of these splints,¹⁶, ¹⁷, ²², ²³, ²⁴ but also on our clinical and teaching experiences. In Brazil, commercial orthoses are widely used among computer users. However, there have been no scientific studies done regarding the demand or functionality of different varieties of these commercial splints. We believe that studies involving healthy computer users may help elucidate a few of the effects brought on by the use of orthoses with respect to the pattern of normal hand movements. This may also provide comparison parameters for further studies on people with work-related musculoskeletal injuries and add new criteria for therapeutic indications of orthoses.

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Methods 

Participants 

Twenty-three volunteers (10 men, 13 women) participated in this study. The subjects ranged from 18 to 26 years in age. None of the subjects presented with any symptoms of discomfort, pathology, or sequelae related to the upper limbs. Participants were predominantly right-handed, were familiar with the use of the computer, and were not computer professionals or computer science students. On the day of the evaluation, the following exclusion criteria were used: BMI of 30kg/m² or more, use of myorelaxant medication 72 hours prior to evaluation, and history of extenuating physical exercise in the past 48 hours. These criteria were defined as those factors that might interfere significantly with the electomyographic signals.²⁵, ²⁶, ²⁷ This study was approved by the local ethics committee of the university, and all participants were asked to provide informed consent.

Equipment and Materials 

For this study we used the portable Miotool 400 system^a with a 1700-mA nickel metal hydride battery. The equipment has an analog-digital converter of 14 bits of resolution, a data acquisition board of 2000 samples a second, and 100dB of common-mode rejection, which produced a 1000 times increase in signal amplification. The surface sensors (SDS500)^a had an entrance impedance of 10¹⁰Ω//2pF (power factor). To minimize signal noise, we used a band-pass filter between 10Hz and 500Hz with a 60-Hz notch filter. Bipolar, adhesive, and disposable Ag/AgCl EMG surface electrodes were positioned with an interelectrode distance of 20mm. These were connected to the sensors by flexible clinches and secured with adhesive tape and an elastic band to ensure a stable fit on the study participants' forearms while wearing the orthoses.

Two wrist orthoses were selected for the tests. One was a custom-made, stiff, long, volar thermoplastic^b orthosis, with a wrist extension between 20° and 30° (orthosis A). The second was a short palmar orthosis^c made of canvas with a stiff plastic insert for support and approximately 20° of wrist extension (orthosis B, fig 1).

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• Fig 1. 

  Orthoses: (A) custom-made orthosis; (B) commercial orthosis.

We confirmed the anthropometric data with a digital scale, a vertical anthropometer fixed to the scale,^d and a Jamar dynamometer^e to measure the maximum isometric contraction of the flexor digitorum superficialis.

For the tasks, we used a keyboard and a standard mouse placed on a common office desk with an adjustable chair that had no forearm or wrist support. The use of an adjustable chair allowed for greater comfort when it was supporting the subjects' upper extremities, and the adequate distance from the screen as well as desk height minimized muscle demand.²⁸ The absence of an armrest was intended to recreate the subjects' daily working conditions and not promote rest of the proximal muscle groups in the upper limbs.²⁹

Procedures 

University students were asked to participate in this study and received a description of the study procedure and the techniques for measurement. The volunteers who fulfilled the inclusion criteria during a questionnaire and who fulfilled the requirements for height and weight were included in this study. The performed procedures are reported in the order in which they were carried out. Each experiment was carried out in 1 session to avoid possible errors in electrode replacement and differences in the data collection environment, and additional influences of these factors on the recording of electromyographic signals.²⁵

We chose the appropriate sizes for the commercial orthosis (small, medium, large) and modeled the custom-made orthosis with thermoplastic, then performed the necessary adjustments. The modeling of the thermoplastic splint followed the necessary functional and biomechanical specifications from our practice in this field and as recommendation of literature.¹⁶, ²⁴

Electromyographic recordings 

We prepared the participants for EMG data collection based on the recommendations of the group for surface electromyography for the noninvasive assessment of muscles.³⁰ We located the muscles by palpation and by a muscle function test, thereby defining the most appropriate regions for electrode placement. The electrodes were placed parallel to the direction of the muscle fibers and between the motor point area and respective tendons of the FDS, the ECU, and the trapezius as recommended by Hermens et al.³⁰ A reference electrode (ground wire) was attached to the prominent bone of the C7 spinous process. In order to attach the electrodes with minimal impedance, the skin was prepared by shaving the site, sponging any abrasions, and applying the area with 70° Gay-Lussec alcohol.³⁰

We carried out tests with manual resistance from an external rigid block for the trapezius and the ECU. To ensure isometric FDS contraction, we used a Jamar dynamometer in the standard position as recommended by the American Society of Hand Therapists.³¹ The volunteers received instructions before the tests and were verbally encouraged to grip with maximum strength for 4 seconds during the readings. Four readings were performed, with resting intervals of 10 seconds each³² (fig 2).

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• Fig 2. 

  Example of raw EMG signals obtained during the 4 consecutive contractions to obtain maximal voluntary contraction.

The computer tasks were demonstrated to the volunteer, who was comfortably and ergonomically seated in a straight-backed chair as close as possible to the screen and to the table. Volunteers were also informed that they should type, drag, and click the mouse using the wrist and finger movements that they were accustomed to using. From our point of view, these criteria reproduced conditions that a computer user was most likely to encounter. The participant was asked to carry out each task once or until the participant was confident in performing the tests. Participants were informed that they could commit errors, and tasks were to be completed at each person's pace to avoid stress from time and efficiency. This information was meant to minimize mental pressure and demand for precision, both of which could cause an increase in muscle activation.³³ Volunteers were not trained to use the computer with the orthoses in order to mimic the conditions similar to those encountered by most patients, in which the splint is used as soon as it is acquired, with generally no therapeutic supervision in the workplace.

Each task was performed under 3 different conditions: without orthosis, with orthosis A, and with orthosis B. Each condition was repeated 5 times, and a total of 30 electromyographic recordings were made for each participant. Each task lasted from 10 to 20 seconds, according to the pace and ability of each participant. There was a resting period of 60 seconds after each task to avoid muscle fatigue. The order of different experimental conditions was randomized. Each task was initiated from the resting position, with hands supported on the desk on each side of the keyboard for typing or with the right hand on the mouse. Recordings took place when each volunteer reached the maximum relaxation point and the simultaneous visualization of these electromyographic signals was registered. When this condition was reached, we requested the tasks using clear and precise commands.

The keyboard task involved typing a 40-character sentence, with the letters distributed equally between the left and right side of the keyboard and with no punctuation or accents. We dictated the sentence emphatically in order to mark the beginning of typing. The subject was instructed to look at the keyboard while typing in order to minimize head movements and to maintain normal hand movement while typing.

The task using the mouse involved coloring a geometrical figure from a computer game^f in an orderly and directed manner. The subjects were to click and drag the mouse for a total of 10 clicks (5 colors, 5 figures). We informed the volunteers that they could not correct errors, even when the mouse failed, in order not to affect the sequence and number of movements.

Data Analysis 

For the electromyographic analyses of each muscle studied, we selected the section corresponding to task execution and obtained the mean amplitude of myoelectric activity in root mean square using the Miograph 2.0 software.^a We calculated the arithmetic mean of the total 5 attempts from each participant and obtained a final value in microvolts. These results were stored in a database for subsequent normalization. Normalization is a process that permits the transformation of relative signal amplitude values (in microvolts) into absolute values (percentages) and enables the comparative analysis of intersubject and intrasubject amplitudes.⁹, ³⁴ The MVC value of each muscle studied was obtained using the dynamic mean method.³⁴ We first obtained the mean amplitude of each contraction, then calculated the arithmetic mean of 4 consecutive contractions in microvolts. This value referred to a 100% muscle contraction capacity. We adopted this absolute value as a reference to convert the amplitude values of the other electromyographic recordings from the typing and mouse use tasks. Data were statistically analyzed with statistics SPSS software (version 15.0).^g The Friedman and Wilcoxon tests were used to compare the mean of the correlated data with a significance level of P less than .05.

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Results 

We considered our sample to be homogenous with regard to age and BMI: the 13 women subjects had a mean age ± SD of 22.55±1.68 years and a mean BMI ± SD of 21.07±2.34. The mean age ± SD for men was 22.07±2.27, and the mean BMI ± SD was 21.78±2.16.

The results of this experiment showed that the mean amplitude values of electromyographic activity from the participants exhibited changes during computer use when the participants were submitted to different experimental conditions (table 1).

Table 1. Mean Electromyographic Activity of the Trapezius, FDS, and ECU During Computer Work

NOTE. Values are mean ± SD expressed as percentage MVC or as otherwise indicated.

The trapezius presented a significant increase in electromyographic activity with both orthosis A and orthosis B in the tasks of typing and using the mouse. The custom-made splint required a greater effort than that required for the commercial splint, though both required greater effort than the tasks performed with no orthosis (fig 3).

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• Fig 3. 

  Recording of EMG signals in root mean square of the trapezius obtained during the typing task.

FDS and ECU presented no significant differences (P>.05) in the typing task with or without the use of orthoses. However, the same did not occur with using the mouse: with orthosis A, there was an increase in the electromyography reading of the flexor and the extensor muscles (P<.05), and with orthosis B, there was no significant difference for FDS (P>.05) compared with the task performance without an orthosis. No decrease in the EMG activity of the ECU was observed when the subjects wore orthoses during the execution of the computer tasks.

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Discussion 

The results of the present study revealed an increase in the electromyographic activity of the upper limbs during computer use. Environmental factors, such as positions of the furniture and the peripherals used during the procedure (including desk height and arm support, keyboard, and mouse model) have previously been shown to be able to cause changes in muscle demand.²⁸, ²⁹, ³⁵, ³⁶, ³⁷ Based on these studies, we defined strict postural and experimental environmental orientations. We further sought to reproduce the typical conditions of movement and background environment for each volunteer. Thus, we believe that the inclusion of orthoses during the execution of the study's proposed activities demonstrated a significant interference in muscle activity.

The results related to the increase in electromyography activity of the trapezius (fig 4) were consistent with results from previous studies that showed a restriction of the wrist orthoses on wrist motion and the additional stress placed on the proximal muscle groups of the upper limb.¹⁷, ³⁸ However, clinical practice and treatment protocols usually do not indicate when wrist orthoses should be prescribed. This suggests that special attention should be paid to the functional and postural evaluations in addition to the monitoring of patients who use wrist splints while performing tasks on the computer. Adequate training for orthosis adaptation could minimize postural compensations and trapezium muscle fatigue in addition to helping evaluate a more functional orthosis model.

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• Fig 4. 

  Representation of the electric activity amplitude of the trapezius during typing and mouse use without orthosis, N; with orthosis A; and with orthosis B.

Of concern regarding the effect of the splints on forearm muscle behavior, the splints acted directly on the wrist and the palmar region, thus affecting the function of the hand. Therefore, the type of splint used is an important consideration for this study. The commercial orthosis is made of flexible material and is shorter than the custom-made orthosis. This allows greater dexterity and an improved functional position of the hand.²², ²³ In relation to the forearm flexor muscles, studies have reported that the use of this commercial orthosis did not significantly modify muscle demand during finger gripping and flexion. However, there was a significant increase in the readings of gripping strength when using a longer, more restrictive orthosis.¹⁰, ¹⁵ In our study, the same phenomenon occurred for FDS during typing and the use of mouse, when the commercial splint but not the custom-made orthosis was used. In fact, there was an increase in FDS activity while the subjects used the mouse with a custom-made orthosis (fig 5). The difficulty of maintaining the involvement of the entire hand in gripping objects should be considered with respect to rigidity of the thermoplastic and the absence of surface friction. These factors may obstruct the necessary level of neuromotor control for manipulation. We believe that the movement restrictions and the lack of adequate sensorial contact and friction, which are characteristics of the architecture of the cutaneous tissue of the hand volar region tissue, could hinder neurosensorial control and increase muscle demand. The increases in muscle activity of the forearm were related to the increases in the precision of hand movements.³⁹ These results suggest that significant attention should be paid to the type of splint prescribed when the main therapeutic aim is to relieve strain on the flexor digitorum superficialis muscles in computer users. The prescription of custom-made orthoses is a common practice regardless of the diagnosis and the tasks performed by the patient. However, in some situations, the main indication for the prescription must be clear because there are situations where immobilization and articular protection are of greater priority, and a stiff orthosis should therefore be prescribed. This may occur in carpal tunnel syndrome, in which the main aim is to decrease the attrition mechanisms on the tendon structures and minimize position changes, which alter the space between the carpal bones and the transverse ligament.¹²

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• Fig 5. 

  Representation of the electric activity amplitude of the flexor digitorum superficialis, during typing and mouse usage without orthosis, N; with orthosis A and with orthosis B.

Contrary to what was expected, ECU rest was not promoted by the use of orthoses. The aim of orthosis A, as previously suggested by Hägg and Milerad,¹³ was a passive wrist immobilization in order to reduce the load on extensor muscles, where fatigue signs are generally more pronounced. This orthosis is recommended by several authors for its length, which could provide greater support for two thirds of the forearm to balance hand weight and attain adequate stability and rest.²⁴, ⁴⁰, ⁴¹ The results of this study showed that there was no reduction in the extensor muscle electromyography activity (fig 6). In contrast, the use of orthoses promoted an increase of this activity when manipulating objects, a result that was consistent with other studies.¹⁵, ¹⁷ Similar results were found for 2 wrist orthoses by Jansen et al,¹⁸ who verified that, among the 3 different splint models, only the semicircular orthosis reduced the extensor load. Important methodologic differences must be considered in this study regarding the tasks and the orthosis models, because they generated different results. The tasks in the aforementioned study involved the suspension of large objects that did not require the same amount of precision necessary for executing fine and delicate movements such as those required for typing and mouse use. The circular orthosis provided greater support for both the volar and the dorsal regions, and could have provided more effective immobilization as well as contributed to a more efficient wrist stabilization.²¹ Contrary to our results, extensor muscle rest was also observed previously during a manual task test performed by 5 patients presenting with rheumatoid arthritis.¹⁹ In that study, in which the patients' mean disease period was 16.2 years, the authors did not mention how long the patients used the orthoses. This meant that at the time of the study, they could have already become functionally adapted to the orthoses, which was the main objective for the device and the tests. This adaptation could have been a relevant factor in the functional performance by the patients.⁸, ³⁸ Continued controversies regarding the effect of wrist orthoses on extensor muscles indicated the need for further studies on the effect of standardized interventions using different models of wrist orthoses.

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• Fig 6. 

  Representation of the electric activity amplitude of the extensor carpi ulnaris during typing and mouse usage without orthosis, N; with orthosis, A; and with orthosis, B.

Study Limitations 

Study limitations included the technical limitations and the nature of the studied population. The region where electromyography was carried out in the forearm contained many small muscles; thus, the risk of interference from other muscle signals (cross-talk) could not be excluded.⁹, ²⁷, ²⁵, ⁴² This phenomenon could cause alterations in the interpretation of electromyographic signals, and new measurement methodologies⁴³ should be developed by rehabilitation researchers. This study was carried out in subjects with no limitations or discomfort in their upper extremities, which did not allow us to make conclusions for populations who already have ailments related to computer work. However, this study should offer methodologic parameters for future research in this population.

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Conclusions 

According to our electromyography results on healthy adults, the use of wrist orthoses during computer use increased trapezius activity but did not reduce extensor carpi ulnaris muscle activity. The activity of the flexor digitorum superficialis increased during tasks involving use of the mouse while the subjects wore orthoses. With the stiff and long custom-made orthosis, we noted greater activation of the FDS (and trapezius) muscle than with the flexible commercial orthosis. The effect of an orthosis on FDS, ECU, and trapezius activity observed in this study suggests a need to monitor patients receiving a prescription for one. We believe that further research under controlled experimental conditions is necessary. Furthermore, subjects who present with musculoskeletal injuries related to computer use should be evaluated with proper criteria when selecting for the appropriate functional wrist orthosis.

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• a Miotec Equipamentos Equipamentos Biomédicos Ltda, Rua Washington Luiz, 675, sala 1102, Porto Alegre, RS—Brasil 90010-460.
• b Ezeform, Sammons Preston Rolyan, W68 N158 Evergreen Blvd, PO Box 886, Cedarburg, WI 53012.
• c Digitala-450; Salvapé Produtos Ortopédicos Ltda, Rua Cesário Ramalho, 90, São Paulo, SP, Brasil, cep 01521-000.
• d Filizola Balanças Industriais S.A., Rua João Ventura Batista, no. 450, São Paulo, SP, Brasil, cep 02054-100.
• e Jamar Hydraulic Hand Dynanometer; Patterson Medical/Sammons Preston, 1000 Remington Blvd, Ste 2210, Bolingbrook, IL 60440-5117.
• f MegaFile, Available at: http://www.megafile.com.br/tiatania.
• g SPSS Inc, 233 S Wacker Dr, 11th Fl, Chicago, IL 60606.