Thermal behavior of the skin on the wrist and finger extensor muscles during a typing task

| Background: Occupational diseases are the second leading cause of sick leave in Brazil, among which musculoskeletal disorders are very common especially among workers whose job includes typing tasks. Thermography analyzes the temperature distribution on the skin surface and is used for diagnosis and prevention of musculoskeletal disorders. Objective: To investigate the thermal behavior of the skin on the wrist and finger extensor muscle area before, during and after a typing task. Methods: Twenty-four workers whose job involves typing were allocated to two groups—with or without elbow, forearm or injury—and performed a 10-minute typing task. Four thermography images were captured from the forearms and fingers at baseline, 0–2, 3–5 and 8–10 minutes and the minimum, maximum and mean temperature was calculated. The data were subjected to factorial ANOVA with software SPSS v 20.0. The significance level was set to 5%. Results: Minimum (mean difference–d=1.7), maximum (d=0.8) and mean (d=0.39) temperature was lowest on the elbow of participants with forearm injury; maximum temperature was lower on the right compared to the left side (d=0.39). Temperature did not vary as a function of time. Conclusion: There was difference in skin temperature between individuals with or without forearm injury and between the right and left sides, but not as a function of time. In future studies tasks should be longer and/or have set typing speed and goals.


INTRODUCTION
As a result of the ongoing technological development, thermography is increasingly used for diagnosis of musculoskeletal disorders (MSD) 1 . This method detects variations in the skin temperature, and the acquired images may serve to ground inferences on the onset of an inflammatory process or still asymptomatic disease or presenting with symptoms other than the typical (pain, swelling and paresthesia). Thus, thermography has preventive application 2-5 and enables early 6 diagnosis through a painless 2 , noninvasive 7 , non-radioactive, rapid, low-cost and safe technique with a broad scope of applications and no restrictions 3 .
Chronic conditions may lead to disuse of the involved musculoskeletal structures, with consequent decrease of the blood flow (vasoconstriction) and local temperature 4 . In turn, acute injury and inflammation increase the skin temperature (vasodilation) 3,7 . In the state of rest, the skin temperature is about 33ºC, with some variability; the deeper a tissue, the higher its temperature, for instance, the intramuscular temperature is about 35ºC [8][9][10] . When combined to other diagnostic methods, such as imaging, physical and functional tests and clinical examination, thermography contributes to efficient diagnosis by detecting changes in tissues such as synovial membranes, tendons and bones, eventually also malignant tumors 11 .
Occupational diseases are the second leading cause of sick leave in Brazil 12 . Work-related MSD are frequently reported by typists, assembly line workers, telemarketers, bank employees, secretaries and journalists. Among these populations of workers, wrist, hand and finger joints are the most frequently involved 13 . Symptoms include fatigue, pain, paresthesia, feeling of heaviness, weakness, tremors, numbness, tingling and cold hands 14 . Most affected workers are aged 30 to 40, i.e., are at the peak of their productivity and experience 15 , and female 16 . Main reasons include repeated movements, physical effort, vibration, accelerated pace of work, environmental aspects, awkward posture and movements over long periods of time without the necessary rest breaks 17 . Such type injury occurs when the tissue recovery is not fast enough 17 , has insidious nature and multifactorial etiology 18 .
Thermography has been tested among workers with work-related MSD, especially those whose jobs involves typing [19][20][21][22] . However, there is wide variability in the reported results, work environments and data analysis. In most such studies assessments lasted 10 minutes and exclusively focused on the back of the hand 19,21 . Reported results included no changes or reduced temperature during and after typing tasks [19][20][21][22] .
Considering the relationship between the skin temperature and MSD among typists and the variation in analysis and findings among previous studies, the aim of the present one was to investigate and compare the thermal behavior of the skin on the wrist and finger extensor muscle area during a typing task among individuals with or without forearm injury.

STUDY DESIGN
The present is a cross-sectional, comparative, observational and quantitative study and was approved by the research ethics committee of Centro Universitário da Serra Gaúcha, ruling no. 3,096,019.

SAMPLING
Through random sampling we selected 24 office workers in Caxias do Sul, Rio Grande do Sul, Brazil, aged 20 to 40 and whose job involved typing. The participants were allocated to two groups: • Without injury (n=12); • With injury (n=12).
Eligibility criteria for the group of participants with injury (GI) were: typing at work at least 5 hours/day, with complaints of tendinitis involving the wrist/finger extensor muscles, lateral epicondylitis or carpal tunnel syndrome confirmed on diagnostic imaging or the Phalen or Cozen and Mills' physical therapy tests 23 . Two participants had abnormal findings on ultrasound, four on magnetic resonance imaging, three on electroneuromyography, one on the Phalen test and two on Cozen and Mills' test. Inclusion criteria for the control group (CG) were typing at work at least 5 hours/day, without any pain or other complaints. No participant could have history of upper extremity fracture, Raynaud's syndrome, diabetes, high blood pressure, abnormal temperature or take blood pressure medications.
The sample size was calculated with software G*Power 3.1.9.2 from data obtained from the literature. On these grounds, the following criteria were: effect size 0.25, error probability of 5%, statistical power 0.8, correlation between measurements 0.5 and sphericity of 1 for F tests (analysis of variance-ANOVA). Thus, the sample had to comprise 24 participants, 12 per group.

PROCEDURES
Data collection was performed by two investigators on sessions held at Integrated Health Center, Centro Universitário da Serra Gaúcha, scheduled according to the participants' availability. The participants were requested to avoid vigorous-intensity exercise, using keyboards, electrical tools, caffeine, alcohol and smoking 60 minutes before assessments. The temperature of the test room (3 × 6 m 2 ) was set to 23ºC; typing equipment (computer, mouse and keyboard) was provided.
After the participants signed an informed consent form they were subjected to clinical interview including data on their work routine. Next, a 2-cm piece of adhesive tape was placed below the lateral epicondyle to be used later for data analysis. The tests were performed with the participants sitting at the desk, with arms and hands in a relaxed position for 20 minutes ( Figure 1A) at a distance from keyboard and screen as recommended in ergonomics standards (height adjustable chair, desk at the level or below the elbow) 21 .
The images were captured at a 1-meter distance from above to include the elbow, forearm, wrist and hand. For this purpose, the participants were requested stopped typing shortly to immediately resume the task. The test ended after 10 minutes.

DATA ANALYSIS
The captured imagens at each time-point were analyzed with software FLIR Tools ® ; areas for analysis were the skin on ventral side of the wrist and finger extensor area ( Figure 2A) and the first to fifth fingers ( Figure 2B) bilaterally ( Figure 2C). The 2-cm piece of adhesive tape placed below the lateral epicondyle served as reference to define the region of interest (ROI) i.e. an equal sized square. To establish the finger ROI, we draw a line from each up to the styloid process of the radius and ulna. Square and lines were used to calculate resultant, minimum, maximum and mean values.

STATISTICAL ANALYSIS
Statistical analysis was performed with software Statistical Package for the Social Sciences (SPSS) version 20.0. Normality and homogeneity were investigated with the Shapiro-Wilk and Levene's test. Temperature data were compared by means of factorial ANOVA according to the following factors:  The results were subjected to descriptive statistics (mean, standard deviation, mean difference [d]), F ratio (ANOVA), p-value (significance) and effect size (eta 2 ). Whenever ANOVA detected significant difference analysis continued with the Bonferroni correction. The significance level was set to p-value<0.05.
The participants' characteristics are described in Table 1. Factor group was associated with significant difference in temperature (F=4.897; p=<0.001; eta 2 =0.352). The participants with injury ( Figure 3)  In turn, we did not find significant differences between time-points (F=1.306; p=0.79; eta 2 =0,127) in the maximum, minimum or mean temperature of neither elbow (Figure 4) nor fingers (p=1.0, Figure 5).While there was no difference between both hands, we did detect difference between fingers (p<0.05).
We neither found significant difference in any of the analyzed variables considering the interaction between the three factors (F=0.772; p=0.930; eta 2 =0.075) also on the post hoc test for specific comparisons (p>0.05).

DISCUSSION
Our aim in the present study was to investigate the thermal behavior of the skin on the wrist and finger extensor muscle area during a short typing task among individuals with or without forearm injury. The former exhibited lower (<1ºC) minimum, mean and maximum temperature on the elbow. Temperature was lower on the right compared to the left side. In turn, there was no difference in the elbow or finger temperature before and after the requested task.
Chronic conditions lead to disuse of the involved musculoskeletal structures, with consequent decrease of the blood flow (vasoconstriction) and local temperature 4 . In turn, acute injury increases the skin temperature 7 . Since in the present study the skin temperature was lower on the elbow of the participants in GI, we may infer theirs was a chronic condition, i.e., without active inflammation. One further reason for the difference in temperature between the analyzed groups might derive from the action of the sympathetic nervous system 19 resulting in a shift of the blood flow from the skin to the muscles, with consequent cooling of the former among the participants in GI. Among chronic patients who perform low-intensity tasks, such as typing, a vicious circle develops involving the sympathetic neural arc which decreases the skin temperature and leads the blood flow deeper into the muscles. This phenomenon accounts for lower temperature in GI. Gold et al. 19 did not find difference among individuals with MSD and cold or not hands at baseline. Nevertheless, the same group 21 reported lower temperature among individuals with injury compared to controls, but not when tested at rooms at 18 (p=0.09) or 21ºC (p=0.26). Also, Ring and Ammer 24 found that the skin temperature was lower among individuals with chronic injury.
The skin temperature was lower on the right compared to the left side in both groups, which probably reflects their hand dominance. Reste     task could have caused skin cooling, since the energy metabolism is lower in the active by comparison to the contralateral side and thus results in less heat. All the participants were right-handed and possibly used the mouse in a way that reduced the blood flow to the analyzed area, which they compensated for through the involvement of other parts of the upper extremities.
In the present study we did not find difference in the temperature of either forearm or fingers as a function of time, thus differently from the results obtained by Gold et al. 19 also with a 10-minute typing task and three groups (injury and cold hands, injury without cold hands, controls). After the end of the task, the skin temperature was lower among the individuals with cold hands, while it exhibited oscillation among controls, with both increase and reduction. In turn, the temperature did not change among the participants without cold hands, probably due to heat dissipation by convection and the action of the sympathetic nervous system shifting the blood flow to the muscles 4,19 . Yet in other studies the same group 20,21 did not find any difference in the hand temperature.
We believe that the typing speed might influence the blood flow volume at the end of the task and that this accounts for the fact we did not find any change in the temperature of the analyzed sites. According to Gold et al. 21 thermoregulation is impaired among individuals with MSD and cold hands. In their study neither the participants with nor those without injury felt symptoms of repetitive strain injury/work-related MSD during the allocated task. The reason is that their usual working hours were much longer than the test duration (10 minutes) which consequently did not suffice to trigger symptoms. These authors 20 further observe that the forearm and arm temperature increases when the typing speed is above 50 words per minute. Reste et al. 22 analyzed temperature dynamics among four individuals along a 3-hour task involving three mouse setups (horizontal computer mouse without mouse pad, horizontal computer mouse with mouse pad and padded wrist support, and vertical computer mouse with mouse pad). The skin temperature decreased in all the cases, but difference was lowest with the vertical mouse.
The main limitations of the present study derive from the task duration, typing speed and the fact we did not impose any demands as is usual in the daily work routine. Then, although we selected a nontechnical text to type, its topic was unfamiliar to the participants and thus the task was harder compared to their routine. There is also the fact the participants had to use unfamiliar equipment, while productivity increases when workers are well acquainted with their tools. Finally, measurements were performed on the skin only, therefore the results cannot be extended to the muscles proper, which temperature is higher compared to the skin [8][9][10] . Nevertheless, we succeeded in gathering evidence for difference in the skin temperature between workers with or without wrist and finger extensor muscle injury. On these grounds, thermography may be useful to assess individuals with disorders involving the skin on the wrist and finger extensor muscle area.
For future studies we suggest increasing the duration of typing tasks or setting definite goals to analyze the typing speed too and with the same keyboards to which subjects are used.

CONCLUSION
The minimum, mean and maximum skin temperature was lower on elbow among individuals with forearm/hand injury compared to injury-free controls. Temperature was lower on the right compared to the left elbow in both groups and did not differ during and after the end of the typing task.