Purpose Among computer workers, visual complaints, and neck pain are highly prevalent. This study explores how occupa- tional simulated stressors during computer work, like glare and psychosocial stress, affect physiological responses in young females with normal vision. Methods The study was a within-subject laboratory experiment with a counterbalanced, repeated design. Forty-three females performed four 10-min computer-work sessions with different stress exposures: (1) minimal stress; (2) visual stress (direct glare); (3) psychological stress; and (4) combined visual and psychological stress. Muscle activity and muscle blood flow in trapezius, muscle blood flow in orbicularis oculi, heart rate, blood pressure, blink rate and postural angles were continuously recorded. Immediately after each computer-work session, fixation disparity was measured and a questionnaire regarding perceived workstation lighting and stress was completed. Results Exposure to direct glare resulted in increased trapezius muscle blood flow, increased blink rate, and forward bend- ing of the head. Psychological stress induced a transient increase in trapezius muscle activity and a more forward-bent posture. Bending forward towards the computer screen was correlated with higher productivity (reading speed), indicating a concentration or stress response. Forward bent posture was also associated with changes in fixation disparity. Furthermore, during computer work per se, trapezius muscle activity and blood flow, orbicularis oculi muscle blood flow, and heart rate were increased compared to rest. Conclusions Exposure to glare and psychological stress during computer work were shown to influence the trapezius mus- cle, posture, and blink rate in young, healthy females with normal binocular vision, but in different ways. Accordingly, both visual and psychological factors must be taken into account when optimizing computer workstations to reduce physiological responses that may cause excessive eyestrain and musculoskeletal load. Keywords Glare · Psychological stress · Trapezius · Computer work · Vision · Posture Introduction complaints related to the visual stress associated with inten- sive near-visual work (Ranasinghe et al. 2016; Rosenfield Computers and other electronic devices are now widely 2011; Woods 2005). Pain in the neck and shoulder area is used during both occupational and leisure activities. Among also prevalent among individuals who work with comput- computer workers, there is a high prevalence of visual ers (Gerr et al. 2002; Mohanty et al. 2017; Woods 2005), but the mechanisms inducing neck and shoulder pain dur- ing computer work are not fully understood (Andersen et al. * Randi Mork 2011; Jun et al. 2017; Ortego et al. 2016; Wærsted et al. Randi.Mork@usn.no 2010). Research has shown that during computer work, Department of Public Health Science, Norwegian University visual discomfort and pain in the neck and shoulder area of Life Sciences, Ås, Norway are associated, indicating a relation between the visual and Department of Optometry, Radiography and Lighting the musculoskeletal system (Fostervold et al. 2006; Hel- Design, University of South-Eastern Norway, National land et al. 2008; Richter et al. 2011; Wiholm et al. 2007). Centre for Optics, Vision and Eye Care, P.O. Box 235, This yet highlights the importance of visual ergonomics and 3603 Kongsberg, Norway Department of Psychology, University of Oslo, Oslo, Norway Vol.:(0123456789) 1 3 812 International Archives of Occupational and Environmental Health (2018) 91:811–830 optimization of visual conditions to prevent both visual and (Gowrisankaran et al. 2007; Nahar et al. 2007), and pupillary musculoskeletal discomfort among computer workers. contraction (Ellis 1981). Behavioral countermeasures like Based on an evolutionary stress model, Fostervold et al. changing posture, looking away, or shielding the eyes from (2014), described the link between vision, oculomotor fac- the bright light source is also often used during glare condi- tors, and the musculoskeletal system as adaptive and func- tions (Boyce 2014). These adaptations and countermeasures tional. In this model, a key element is the notion of evolu- often eliminates or reduce the effect of glare in ordinary tionarily-novel environments: environments departing from environmental settings. In contrast, modern computer work those for which the human species has developed specific environments represents a problem with a variety of artifi- phenotypic adaptations. Although such adaptations may cial light sources, surfaces with high specular reflection, and seem functional, ongoing efforts to cope in evolutionary- static intensive near-work with high gaze angle and reduced novel environments may give rise to new problems. Sub- possibilities to change position to avoid the glare sources. jective complaints and ailments associated with computer Thus, glare during computer work may accordingly be con- work are in this context seen as a mismatch between species- sidered as evolutionarily novel as traditional adaptations specific adaptations to vision at close distance and demands and countermeasures may not suffice in such environments. imposed by the computer work environment (Fostervold According to the evolutionary stress model, continuing com- 2003; Fostervold et al. 2006; Lie et al. 2000). puter work with glare exposure may thus initiate increased Near-visual tasks, such as computer work, are static and load on the worker leading to negative consequences and often of long duration. This places a high demand on both discomfort. smooth and cross-striated muscles in and around the eyes Increased psychological load has been shown to induce (Lie and Watten 1994; Rosenfield 2011). The eyes focus different physiological responses in humans; such as on near objects by accommodating (contracting the ciliary increased activity in the sympathetic nervous system, muscle to make the lens more spherical), by converging (two increased heart rate, decreased blink rate, and increased of the extraocular muscles, m. rectus medialis, rotate the muscle activity and blood flow in trapezius and facial eyes inward to keep single vision), and by miosis (contract- muscles (Hidaka et al. 2004b; Larsson et al. 1995; Lund- ing the iris sphincter muscle to reduce pupil size to increase berg et al. 2002; Nilsen et al. 2007; Rodriguez et al. 2018; visual acuity) (Levin et al. 2011). Activation of muscles in Skoluda et al. 2015). Some similar responses have been the neck and shoulder area during near-visual work has also found to be induced by excessive light/glare, a visual stressor been shown, and it is likely due to the gaze stabilization (Belkić 1986; Mork et al. 2016; Saito et al. 1996), suggesting necessary for maintaining a clear picture on the retina (Bizzi that effects seen during exposure to visual stress also may et al. 1971; Lie et al. 2000; Lie and Watten 1987; Richter and involve a central mediated stress response. Forsman 2011). Increased visual stress during near-visual However, few studies have investigated both adaptations work, such as inadequate lighting (e.g., glare), uncorrected to glare and reactions to stress in computer work environ- refractive errors, and accommodative and binocular disor- ments. The aim of the present study was thus to elucidate ders, puts extra stress on the visual system and head-stabi- this relationship by means of the two common occupational lizing musculature, and may aggravate symptoms from the stressors: visual stress (direct glare) and psychological eyes and the neck and shoulder area (Gowrisankaran and stress. Based on previous research and deliberations from Sheedy 2015; Rosenfield 2011). In addition, glare has been the evolutionary stress model the following hypotheses were shown to affect subjects with normal binocular vision during tested: (1) the trapezius and orbicularis oculi muscles are computer work, resulting in decreased reading speed/pro- affected by glare and psychological stress. (2) Glare and psy - ductivity (Glimne et al. 2015), increased fixation disparity chological stress affect cardiovascular responses. (3) Glare variation (Glimne et al. 2013), and increased muscle blood and psychological stress induce adjustments in sitting pos- flow in the trapezius (Mork et al. 2016). ture and visual parameters. Glare has always been a potential risk factor in the human environment and should therefore not be considered evolu- tionarily novel. In this regard, glare is comparable to other Methods environmental stressors, where a general stress response would be expected. The effect of glare varies from hardly Design and procedures noticeable to detrimental for functional vision and the visual system contains several adaptations aimed at reducing the The study was a laboratory experiment applying a coun- negative impact of glare. Examples could be: the anatomi- terbalanced, fully factorial, repeated 2 × 2 × 4 design. The cal design of the face with the eyes placed within the orbital within-subject conditions were visual stress (direct glare) cavity to shield for overhead light, increased eyelid squinting (on/off), psychological stress (on/off), and four time points (Mork et al. 2016; Sheedy et al. 2003), increased blink rate [rest, 5, 10 min and recovery (see “Data analysis”)]. To test 1 3 International Archives of Occupational and Environmental Health (2018) 91:811–830 813 the impact of the stress conditions, four 10-min computer- they had read (social-evaluative threat). (3) A visible video work conditions were performed in random order, each camera was turned on to monitor the participants throughout of which contained the same task but with different stress the computer-work session (social-evaluative threat). requirements, as follows. Visual and psychological stress (VPS) Low stress (LS) In the VPS condition, the participants worked with both In the LS condition, the participants worked with appropri- exposure to excessive lighting (glare source turned on) and ate lighting (glare source turned off) and minimal psycho- psychological stress as described in VS and PS. logical stress. The participants were encouraged to perform For each participant, the order of the four computer-work the computer task as well as possible, but it was emphasized conditions were drawn at random from predetermined enve- that their performance would not have a major influence on lopes to control for possible order effects (counterbalanced the test outcome. design). In all four conditions, the participants accomplished the same computer task: they read a text on a computer Visual stress (VS) screen, identified spelling errors in the text, and marked these errors in bold using a regular wireless laser mouse as In the VS condition, the participants worked with exposure a pointing device. The text was in Norwegian and there was to excessive lighting (glare source turned on) and minimal on average one spelling error on every third line (unevenly psychological stress as in LS. See separate section: glare distributed). The examiner was the same for all participants. source and lighting. All four computer-work conditions consisted of (1) a 1-min rest recording before computer work (rest), (2) Psychological stress (PS) 10 min of computer work (LS, VS, PS or VPS), (3) a break [13.9 ± 2.1 min (mean ± SD, n = 43)], and (4) a 1-min rest In the PS condition, the participants worked with appropriate recording after the break to measure the recovery (recovery). lighting (glare source turned off), and exposure to psycho- The recovery period at the end of one condition constituted logical stress. A combination of three stress-inducing proce- the rest period of the following condition (Fig. 1). dures was used: (1) the participants were told to work as fast During all rest recordings (rest and recovery), the par- and accurately as possible and that their performance would ticipants were instructed to sit in a comfortable position have a major influence on the test outcome (time and preci- with their hands in the lap, relaxing the shoulder and neck sion pressure). (2) The participants were told that, after the muscles, and with a comfortable gaze towards an eye-height condition, they would have to answer questions from the text distance target (approximately 6 m away), relaxing their FD FD recovery break rest computer work 1 min preparation 13.9 ±2.1 min 1 min 10 min (rest for next (mean ±SD) condition) Measurement of blood pressure Questionnaire (perceived lighting and stress during computer work) FD Measurement of fixation disparity (n = 20) Continuous measurements Fig. 1 Flowchart of the first test condition. Muscle activity, muscle work session. Directly after the fixation disparity measurement, the blood flow, heart rate, blink rate (in PS and VPS) and postural angles participants completed the questionnaire regarding perceived lighting were recorded continuously during rest, computer work, and recov- and stress. All four conditions included the same parts as shown in ery. Blood pressure was registered twice during the computer task the figure (except preparation), and PPG rest recording before the first (at 4 and 9 min). Fixation disparity was measured during the prepa- condition was used as the baseline for PPG results in all conditions ration period (baseline) and immediately after the 10-min computer- 1 3 814 International Archives of Occupational and Environmental Health (2018) 91:811–830 eyes. To avoid use of the orbicularis oculi muscle, partici- Table 1 Descriptive characteristics of the participants pants were not allowed to close their eyes during the rest Mean ± SD N recordings. Subjects During breaks between conditions (no measurements), Age 21.4 ± 2.4 years 43 all participants were instructed to rest in a sitting or stand- Experience with computers 10.3 ± 2.5 years 43 ing position. Because of the attached equipment, partici- Use of electronic devises (per day) 5.2 ± 2.5 h 43 pants could stand up, but their walking range was limited. Visual status To rest their eyes, they could choose to close their eyes or Distance visual acuity, LogMAR − 0.2 ± 0.1 43 look around freely, but they were not allowed to perform any Spherical equivalent, right eye − 0.8 ± 1.6 43 visually demanding activities, such as looking at their cell Binocular accommodation amplitude, 11 ± 2D 42 phones. To make breaks as similar as possible for all partici- R.A.F-rule pants, a radio played pop music and talk with the examiner Near point of convergence, R.A.F-rule 5 ± 1 cm 43 was minimized. The glare source was turned off during all Horizontal phorias at distance, Cover test − 0.6 ± 1.4Δ 43 rest recordings and during breaks. Horizontal phorias at near, Cover test − 1.8 ± 2.3Δ 42 All participants got verbal and written information about Stereo acuity, TNO 50 ± 16ʺ 43 the study before giving their informed consent. However, all Fixation disparity, the Sheedy Fixation − 1.6 ± 4.4 arcmin 20 were naiive to the specific aim or expected outcomes of the Disparometer (60 cm on test day) study to avoid respondent bias. Personality All information given about the study was standardized Positive trait personality, PANAS (sum 16 ± 2 42 and written down. The participants did not know the condi- score) tion order beforehand and were informed of it verbally just Negative trait personality, PANAS (sum 8 ± 3 43 score) before the beginning of each computer-work session. In a pre-meeting or when attending the lab, all participants per- Measurement was performed by the R.A.F. near-point rule (Neely formed a short computer-work session to familiarize them- 1956) selves with the proofreading task. After finishing the testing procedure, all participants were debriefed and, to ensure that new participants were naive observers, instructed not to tell prevalence of both musculoskeletal pain (Larsson et al. 2007; Paksaichol et al. 2012) and visual symptoms during other participants details about the experiment. All participants attended the lab once, between 8:00 a.m. computer work (Ranasinghe et al. 2016) compared to males. In addition, females and males respond differently to stress and 12:00 p.m., and the visit lasted for approximately 3 h. The first hour was used for information, completing ques - (Collins and Frankenhaeuser 1978; Luine et al. 2017). The participants underwent an optometric examination tionnaires, initial measurement of fixation disparity (base- line), and connection and calibration of the measurement at National Centre for Optics, Vision and Eye care, Kongs- berg, Norway, prior to testing to ensure normal, or corrected equipment. The testing procedure lasted for about 2 h. to normal, vision (Table 1). Twenty participants used no vision correction during testing, 16 wore glasses, and seven Participants used contact lenses. All participants had normal vision func- tions and good eye health. None of the subjects had vertical The sample consisted of 43 healthy female undergraduate students recruited from the University College of Southeast phorias or uncompensated fixation disparities. Exclusion criteria were chronic pain in the neck and shoulder area the Norway (Table 1). Sample size was calculated a priori with a test power of 80%, a significance level of 5% (two-tailed) last 6 months, history of eye trauma and surgery, dyslexia, mental illness, and systemic disease/regular use of medica- and standard errors based on trapezius photopletysmography measurements from Mork et al. (2016) (Owen 1962). Tra- tions affecting circulation, pain sensation, vision, or visual comfort. pezius muscle blood flow was a main variable to investigate in the current study, and the power analysis showed that 36 The participants completed two questionnaires on the testing day (Table 1). The first questionnaire recorded infor - participants should suffice to identify a 15% difference in trapezius blood flow between two repeated means, without mation about visually demanding work prior to attending, coffee or alcohol drinking or exercise during the previous and with glare exposure. Data were collected in two different winter periods 12 h, hours slept the previous night, medications taken the same day, and smoke/snuff use (i.e., smokeless tobacco (December–February): 24 women were tested in 2015 and 20 in 2016. One participant was excluded from the first made from ground or pulverized tobacco leaves). The second questionnaire recorded information about personality traits period because of the exclusion criteria. Only women were included in the study, because females have shown a higher by means of the 10-item mood scale, Positive and Negative 1 3 International Archives of Occupational and Environmental Health (2018) 91:811–830 815 Affect Schedule PANAS (Watson et al. 1988). This form monitor was 60% of maximum. The font size was 12 point rated how the participants usually felt according to five Times New Roman, with a letter size of 3 mm (Capital E). negative and five positive personality traits, scoring each This corresponds to a vertical viewing angle of 0.26° at a specific trait from 1 (nothing) to 5 (much). Dispositional viewing distance of 65 cm. To minimize individual differ - tendencies to experience negative emotions have repeat- ences in light exposure and neck angle due to viewing angle, edly been identified as an important factor in the process of the readable window was reduced to the upper half of the appraising environmental conditions at work (Girardi et al. screen. The background behind the readable window was 2011). Individuals dominated by negative affect have shown light gray. The ambient air temperature and relative humidity higher autonomic activation compared to individuals domi- was 22 ± 1 °C and 38 ± 9% (mean ± SD, n = 42). nated by positive affect (Kreibig 2010). To control for this confounder, negative and positive affect (sum score of five negative/positive items) were included as covariates in the Glare source and lighting statistical analyses. Seven participants performed short periods of visually The glare source consisted of two large luminaries centered demanding work (e.g., reading, computer work) for more 60 cm behind the computer screen. When turned on, the than 2 h before the start of testing. Eight of the participants luminaires simulated an office window exposing the par - had drunk 1–2 cups of coffee 3–4 h prior to testing, two had ticipants to excessive light (visual stress) during computer drunk 1–2 units of alcohol the evening before attending, nine work. The luminaires consisted of translucent acrylic diffus- were habitual smokers or snuff users, and eight had smoked ing fronts (1.25 m × 0.57 m), equipped with six fluorescent or used snuff earlier on the testing day. None of the partici- tubes (T5, F28W/830), and the luminous intensity of the pants had exercised during the previous 12 h, and they had luminar ies was 4634 ± 749 cd/m (mean ± SD, measured slept 7.2 ± 1.8 h (mean ± SD, n = 43) before testing. across the screens), range 3230–5870 cd/m . The luminance levels with the glare source turned off Workstation were 155 cd/m in the working field (computer screen turned on), 90 cd/m in the immediately surrounding area (desktop), The participants were seated in a stationary office chair and 61 cd/m in the background area. This is within the rec- without wheels. Distance to the screen, gaze angle, under- ommended luminance ratio 5:3:1 for a workstation (Anshel arm support, screen tilting, and height of the desktop and 2007). With the glare source turned on, the luminance levels chair were individually optimized according to national were 155, 520, and 4634 cd/m , respectively, and the lumi- and international recommendations (Arbeidsplassforskrif- nance ratio was therefore 1:3:30. The illuminance on the ten 2011, ISO 9241-5 1998; Lillelien et al. 2012). Distance desktop between the participant and the computer screen to the screen was 65 ± 6 cm (mean ± SD, n = 42). The gaze was approximately 1500 and 659 lx with the glare source angle, measured as the angle between the midpoint of the turned on and off, respectively. A Hagner Universal Pho- readable window on the screen and an imaginary horizontal tometer (Modell S4, Sweden) was used for the luminance line at eye level, was 21 ± 2° (mean ± SD, n = 42) downward. measurements, whereas a Hagner Digital Luxmeter (Model Downward gaze angle has been shown to be beneficial dur - EC1, Sweden) was used to measure illuminance. ing computer work (Fostervold et al. 2006). The participants were allowed to move freely within normal ranges during Measurements the conditions. The sitting position was controlled for by measuring postural angles. Before the experiment, the par- Muscle activity in dominant m. trapezius, muscle blood flow ticipants were informed that they could be told to return to in dominant m. trapezius and in m. orbicularis oculi (domi- the initial optimal position if they moved into very unfavora- nant eye), heart rate, blink rate, and postural angles were reg- ble ergonomically postures (e.g., leaning extremely forward istered continuously during computer-work and rest record- or resting the head in one of their hands). This was done to ings (Fig. 1). Blood pressure was registered twice during minimize the possibility of ergonomically loads and sig- each condition (after 4 and 9 min). Fixation disparity was nificantly alternations in viewing distance to the computer measured immediately after each 10-min computer-work screen. To minimize mental/visual disturbances during the period, and directly after that, the participants completed conditions, the examiner sat 3 m away and outside of the the questionnaire regarding workstation lighting and stress participants’ visual field. during computer work. All computer work was performed on an external 24ʺ The orbicularis oculi muscle is a thin elliptical muscle anti-reflection LCD-screen with 1920 × 1200 pixels resolu- surrounding the eye, extending from the lids to the brow, tion and a mean refresh rate of 69.5 Hz (HP LA2405x), temple, and cheek. This muscle consists of two main parts: connected to a closed laptop. The brightness of the external the palpebral (inner) part, responsible for involuntary and 1 3 816 International Archives of Occupational and Environmental Health (2018) 91:811–830 voluntary blinking, and the orbital part, which closes the Norway) (Aaras and Stranden 1988). The inclinometers were lids firmly (Bron et al. 1997; Thorud et al. 2012). During attached to the upper back and to the back of the head. The eye squinting, the orbital part contracts as the palpebral part inclinometers were calibrated (zero value) as the participants relaxes. sat in a reference body position, a balanced position without The trapezius muscle is a large, superficial muscle that a backrest and with the arms hanging down by the sides, extends from the occipital bone, the cervical and the thoracic looking at a point at eye height at a distance of approxi- region, and laterally to the spine of the scapula. Trapezius mately 6 m. Back and head angles were measured in terms of consists of three functional parts: an upper (descending) deviation from this reference position. Flexion (leaning for- part, a middle region (transverse), and a lower (ascending) ward) was given as positive values, extension (leaning back- part. The actions of the upper descending part are elevating ward) as negative values. For the angles of lateral flexion the scapula (supporting the weight of the arm) and assisting (side bending), negative values reflected movements to the with the stabilization and movement of the cervical spine left of the reference position, and positive values reflected (Johnson et al. 1994). movements to the right. If the participants changed their viewing distance during reading, this was reflected in the Muscle activity in m. trapezius measured back angles. Muscle activity was measured unilaterally in the dominant Heart rate trapezius (upper, descending part) during computer work using surface electrode electromyography (EMG). The mus- Heart rate during each rest session and during computer cle activity (and postural angles) measurements were car- work was measured continuously by the PPG method. The ried out using a field-portable apparatus (Physiometer PHY- AC component of the PPG signal is synchronous with the 400, Premed A/S, Oslo, Norway) connected to a computer. heart rate (Lindberg and Oberg 1991), and heart rate was The EMG signal was normalized by calibrating the EMG estimated by counting the number of heartbeats (AC-peaks) response to force [given as % maximum voluntary contrac- per minute. tion (MVC)], using a calibration platform with a force trans- ducer (Aaras and Ro 1997). Further description of the EMG Blood pressure method and establishing of the EMGrms/force relationship for the actual range of work load below 30% MVC can be Systolic and diastolic blood pressure were assessed from the found in Aaras et al. (1996) and Mork et al. (2016). nondominant upper arm after 4 and 9 min of computer work using an automatic oscillometric blood o fl w monitor (A & D Muscle blood flow in m. trapezius and m. orbicularis oculi Medical, Model UA-767Plus 30). The artery position mark was placed 1–2 cm above the elbow, in a medial position on Muscle blood flow was measured unilaterally on the orbital the upper arm (in line with the ring finger with a supinated part of the orbicularis oculi (dominant eye) and on the upper forearm). Before the start of testing, the participants were descending part of the trapezius (dominant side) using asked not to think of the blood pressure measurements, but photoplethysmography (PPG). PPG is a noninvasive opti- rather to stay focused on the computer task. To familiarize cal technique for continuous monitoring of muscle blood participants with the equipment, one measurement was per- flow and can be used to detect blood volume changes in the formed during the preparation phase. microvascular bed of muscle tissue (Sandberg et al. 2005; Zhang et al. 2001). In this study, two custom-designed Blink rate optical probes (Department of Biomedical Engineering, Linköping University, Sweden) were used. The probes were One of the stress-inducing procedures in the conditions with attached on the orbital part of the orbicularis oculi below psychological stress exposure (PS and VPS) was a visible the participant’s dominant eye and medial to the EMG elec- video camera to monitor the participants during computer trodes on the upper descending belly of the trapezius. The work. The videos were used to manually count blink rate PPG method is described in Thorud et al. (2012), and Mork (blinks/min). The overall analysis of blink rate during com- et al. (2016). puter work included data for mean blinks in the first half (0–4 min) and the second half (5–9 min) of the conditions, Postural angles and ‘computer work’ reported in Fig. 7a is therefore the average of these measuring points. In addition, blink rate Postural angles were measured continuously during both the in the first minute of the computer-work period (0–1 min, rest and computer-work periods using two dual axis incli- ‘start of the work session’) was analyzed with respect to ‘the nometers connected to a physiometer (Premed A/S, Oslo, rest of the work session’ (average of blinks/min in 2–9 min, 1 3 International Archives of Occupational and Environmental Health (2018) 91:811–830 817 with the 4th min excluded) (Fig. 7b). The video camera was in each condition, and recorded on 100 mm Visual Analogue not used in the LS and VS conditions to avoid adding psy- Scales (VAS) (Price et al. 1994). The left endpoint (0 mm) chological stress. Hence, blink rate data were only available represented no stress/very comfortable lighting, whereas the from PS and VPS. right endpoint (100 mm) represented highly stressed/very uncomfortable lighting. Fixation disparity Work performance Fixation disparity are small misalignments of the visual axis during binocular vision, without causing double vision. The Productivity was defined as reading speed during 10 min misalignments may be vertical, horizontal, or both, and are of computer work in each condition (number of words read normally compensated by the visual system. In the case of in 10 min). Accuracy was defined as the percentage (%) of horizontal fixation disparity, as measured in the current correctly marked spelling errors (correctly marked errors study, the fixation disparity is a vergence error defined as divided by the actual number of errors in the text read). either an exo-disparity or eso-disparity, where the visual axis fixates behind or crosses in front of the fixation point. Small Data analysis fixation disparities (minutes of arc) are common in subjects with normal binocular vision (Jaschinski 2017). Yet, alter- The sampled signals of muscle activity and postural angles nations in fixation disparity is considered an indicator of were ranked to produce an amplitude distribution function, oculomotor stress (Glimne et al. 2013; Pickwell et al. 1987). ADF (Jonsson 1982). Static and median load were defined as Horizontal fixation disparity was only measured in the ADF levels 0.1 and 0.5, respectively. Muscle blood flow and second test period (n = 20). Immediately after completing heart rate was recorded at a sampling frequency of 240 Hz, the 10-min computer-work period, the participants turned and was analyzed using software developed at Department their chair around, put on a pair of polarized glasses, and of Biomedical Engineering, Linköping University, Sweden placed their head in a chin-and-head support facing a Sheedy (Aqua) and MatLab R2009b (The MathWorks, Inc., US). fixation disparometer (Dwyer 1982). The test distance was The data analysis of muscle blood flow and heart rate is approximately the same as during the computer-work peri- described in Thorud et al. (2012). ods, 60 cm. A Sheedy fixation disparometer consists of suc- For all rest and recovery recordings of the continuous cessive pairs of vernier lines of increasing angular separation measurements, data were analyzed as an average value of the within a structureless field, each line being viewed by one 1-min registrations. To exclude the possibility for the blood eye through the polarizing glasses. The participants were pressure measurements registered after 4 and 9 min during asked to read the text on the disparometer in order focus the the computer-work periods to affect the results, data for the eyes on the correct distance before measuring the fixation continuous measurements captured in the periods 4–5 and disparity. The disparometer was adjusted to zero disparity 9–10 min were left out of the analysis. In the analysis, either at start, and was then adjusted until the participants reported two time points, average of 0–4 min (called 5 min) and aver- that the lines appeared to be aligned. One fixation disparity age of 5–9 min (called 10 min), or one average value for the measurement before the start of testing, called the baseline, entire work period (called computer work) was used. Some was also accomplished (Fig. 1). Variation in disparity after of the tables and graphs included only computer work to each condition relative to baseline is called FD . This make the description of the results easier to follow. change value reflects the change in fixation disparity for each par - Because of technical problems with the photoplethy- ticipant from their own baseline (0), a change in either an mography measurements, especially during the first test eso-direction (+) or exo-direction (−). Therefore, the statis- period, we obtained complete data only for 32 and 23 par- tical analyzes of the fixation disparity measurements were ticipants for trapezius and orbicularis oculi muscle blood conducted with both the measured fixation disparity value flow, respectively. and FD for each condition, both measured in minutes change of arc. Statistics Perceived stress and experience of the lighting Statistical analyses were performed in IBM SPSS Statis- tics (Version 24, US). The overall statistical analysis was To measure how participants perceived the induced stress- performed with ANOVA Repeated Measures (factorial ors in the different conditions, they were asked to report repeated 2 × 2 × 4 design). Planned contrasts were used to how they experienced the lighting and how stressed they compare conditions if the overall analysis indicated either felt during the computer-work periods. The questions were main effects or interaction effects. Inspection of the variables posed immediately after the fixation disparity measurement revealed that several variables departed from the normal 1 3 818 International Archives of Occupational and Environmental Health (2018) 91:811–830 distribution, and base-10 logarithm transformation was exe- direct glare during the computer-work period, they experi- cuted on these variables. For variables with normal distribu- enced the lighting at the workstation as significantly more tion, untransformed data were used in the analysis. For most unpleasant than when they were given appropriate lighting. ANOVA analyses, Mauchly’s test indicated a violation of The analysis also showed a significant main effect the assumption of sphericity. The Greenhouse Geisser cor- of exposure to psychological stress on perceived stress; rection was therefore used if nothing else is reported in the F(1.0, 42.0) = 29.90, p < .001, η = 0.42. This indicates that results. Untransformed data were presented in the figures, the participants felt more stressed during psychological except for the blood flow data, which was shown as percent- stress exposure compared to the conditions with no psy- age muscle blood flow increase relative to baseline. Correla- chological stress (Fig. 2b). tion analysis (Pearson) was performed on transformed data. Furthermore, there was a significant and positive correla- An overall analysis of variance (ANOVA) was performed to tion between perceived stress and lighting in VPS (r = .392, investigate potential overall time effects (test order effects) p = .009), indicating that perceiving more unpleasant ambi- throughout the experiment independent of condition. ent lighting was associated with feeling more stressed. This association was not present during the other conditions. Results Muscle blood flow Analysis of covariates Trapezius Personality measures of negative and positive affect were Visual stress induced increased trapezius muscle blood entered as covariates in the analysis. The results did not flow during computer work (Fig. 3 a). The results showed show any significant interaction effects. Thus, personal- a significant glare by time interaction on trapezius blood ity seems not to have affected the measured variables in f low; F ( 2 . 50 , 77 . 6 0 ) = 3 . 9 3, p = . 0 1 6, η = 0 . 1 1 . D ur- the current study. The co-variates were consequently dis- ing glare exposure, blood flow at 5 min; F (1, 31) = 4.98, carded from further analyses. p = .033, η = 0.14, and 10 min; F(1, 31) = 4.39, p = .045, η = 0.12, was significantly higher than during the rest period. Blood flow during recovery was not significantly Experience of visual and psychological stress different from that during rest. In addition, the results showed a significant main effect The participants were exposed to visual stress (direct glare) of time on trapezius blood flow; F (1.89, 58.68) = 18.84, in two conditions (VS and VPS) and psychological stress p < .001, η = 0.38. Overall, blood flow at 5 min; in two conditions (PS and VPS). The results showed a sig- F ( 1 , 3 1 ) = 3 5 . 9 4, p < . 0 0 1 , η = 0 . 5 4 , a t 1 0 m i n ; F(1, nificant main effect of glare on perceived lighting during 31) = 27.45, p < .001, η = 0.47, and during recovery; F(1, 2 2 computer work; F(1, 42) = 84.51, p < .001, η = 0.67. Fig- 31) = 6.95, p = .013, η = 0.18, was higher compared to that ure 2a shows that when the participants were exposed to during rest in all conditions. Fig. 2 Scores (mm VAS) for a how participants perceived the work- Higher scores indicate a a higher degree of perceived unpleasantness station lighting, and b the degree of stress perceived by participants of the lighting, and b a higher degree of perceived stress. Results are during the four computer-work conditions. LS Low stress, VS visual given as mean ± SEM, n = 43 stress, PS psychological stress, VPS psychological and visual stress. 1 3 International Archives of Occupational and Environmental Health (2018) 91:811–830 819 a b 35 25 30 20 25 15 20 10 15 5 10 0 5 -5 0 -10 Rest 5 min10 min Recovery Rest5 min10 minRecovery LS VS PS VPS LS VS PS VPS Fig. 3 Muscle blood flow in a trapezius (n = 32) and b orbicularis blood flow relative to the initial resting value (baseline). Rest = rest oculi (n = 23) during the four computer-work conditions. LS Low recording before computer work; Recovery = rest recording after a stress, VS visual stress, PS psychological stress, VPS psychological 14-min break (see Fig. 1). All results are given as mean ± SEM and visual stress. Results are given as percentage increase in muscle different from that during rest, indicating an initial and tran- Orbicularis oculi sient increase in trapezius activity due to psychological stress. In addition, Fig. 4 shows that trapezius muscle activ- The statistical analyses showed no significant effect of either visual or psychological stress on orbicularis oculi blood ity was higher during computer work compared to rest. The results showed a main effect of time on both static; flow. However, there was a main effect of time on muscle blood flow in the orbicularis oculi; F (1.74, 38.26) = 4.20, F(1.17, 49.33) = 29.67, p < .001, η = 0.41, and median load; F(1.38, 57.80) = 48.86, p < .001, η = 54. This results p = .027, η = 0.16, where overall muscle blood flow at 10 min was significantly higher than that during recovery; revealed that overall muscle activity (median load) at 5 min; F(1, 42) = 32.10, p < .001, η = 56, and at 10 min; F(1, F(1, 22) = 7.93, p = .010, η = 27. (Fig. 3b). 42) = 30.12, p < .001, η = 57, was significantly higher than that during rest. Muscle activity during recovery was overall Muscle activity in the trapezius not significantly different from that during rest. There was a borderline significant psychological stress by time interaction on trapezius muscle activity, median load; Heart rate F(1.86, 77.95) = 3.13, p = .053, η = 07. During exposure to psychological stress, the results indicated that muscle activ- The results revealed no statistical significant effect of either visual or psychological stress on heart rate. However, Fig. 5 ity at 5 min was significantly higher compared to that during rest; F(1, 42) = 4.85, p = .033, η = 0.10 (Fig. 4b). Muscle shows that there was a main effect of time on heart rate; F(1.27, 43.04) = 18.86, p < .001, η = 0.15. Overall, heart rate activity at 10 min and during recovery was not significantly Fig. 4 Muscle activity in trape- a b 2,5 2,5 zius: a static and b median load, during the four computer-work conditions. LS Low stress, VS visual stress, PS psychologi- cal stress, VPS psychological 1,5 1,5 and visual stress. Rest = rest recording before computer work; Recovery = rest record- ing after a 14-min break (see Fig. 1). Results are given as 0,5 0,5 mean ± SEM, n = 43 Rest5 min10 min Recovery Rest 5 min10 minRecovery LS VS PS VPS LS VS PS 1 3 Trapezius blood flow, % increase Trapezius activity, % MVC (static) Orbicularis blood flow, % increase Trapezius activity, % MVC (median) 820 International Archives of Occupational and Environmental Health (2018) 91:811–830 participants moved closer to the computer screen by flex- ing their back during the induced psychological stress (Fig. 6a). When participants were exposed to psychologi- cal stress, their back was more forward bent at 5 min; F(1, 34) = 6.28, p = .017, η = 0.16, and at 10 min; F(1, 34) = 9.67, p = .004, η = 0.22, than during rest. There was no significant difference in back position between rest and recovery. Figure 6c shows that there was no significant effect on back lateral flexion of exposure to either visual or psychological stress during computer work. Head angles Rest 5 min10 minRecovery LS VS PS VPS Figure 6b shows that both visual and psychological stress induced a more forward-bent head during computer Fig. 5 Heart rate during the four computer-work conditions. LS Low work. The results showed both a statistically significant stress, VS visual stress, PS psychological stress, VPS psychological glare by time interaction; F(2.09, 77.38) = 6.19, p = .003, and visual stress. Rest = rest recording before computer work; Recov- η = 0.14, and a psychological stress by time interac- ery = rest recording after a 14-min break (see Fig. 1). Results are given as mean ± SEM, n = 35 tion; F(1.76, 65.12) = 3.47, p = .043, η = 0.09, on head flexion/extension. When participants were exposed to visual stress, their head was more forward bent at 5 min; at 5 min; F(1, 34) = 6.08, p = .019, η = 0.36, and at 10 min; F(1, 37) = 11.95, p = .001, η = 0.24, and at 10 min; F(1, F(1, 34) = 10.62, p = .003, η = 0.24, was higher compared 37) = 28.44, p < .001, η = 0.44, than during rest. When to that during rest, whereas heart rate during recovery; F(1, exposed to psychological stress, their head position at 34) = 40.57, p < .001, η = 0.54, was significantly lower than 5 min; F(1, 37) = 18.50, p < .001, η = 0.33, and at 10 min; that during rest. F(1, 37) = 11.44, p = .002, η = 0.23, was also more for- ward bent than during rest. There was no significant differ - Blood pressure ence in head flexion between rest and recovery. Figure 6d shows that there was no effect of either visual or psycho- There were no statistical significant effect of exposure to logical stress exposure on head lateral flexion during com- visual or psychological stress on blood pressure (systolic puter work. and diastolic) during computer work. In average, the par- ticipants blood pressure during the computer-work periods Time effects on back and head angles was 106 ± 1 mm Hg (systolic) and 73 ± 1 mm Hg (dias- tolic) (mean ± SEM, n = 43). The results showed significant main effects of time on both back flexion/extension; F (1.56, 39.26) = 30.71, Work performance p < .001, η = 0.46, and head flexion/extension; F(1.61, 59.52) = 235.37, p < .001, η = 0.86. Compared to rest, the Exposure to visual and psychological stress did not affect back and head were overall more forward bent at 5 min work performance, and there was no difference between and at 10 min (p < .001), whereas they were more extended conditions. Mean reading speed (productivity) in all con- during recovery (p < .001). The results also showed a sig- ditions was 1691 ± 64 words per 10 min (mean ± SEM, nificant main effect of time on head lateral flexion; F (1.99, n = 43), whereas mean accuracy was 84 ± 8% correctly 73.69) = 7.49, p = .001, η = 0.17. The participants bent their marked errors in the text (mean ± SEM, n = 42). head more leftward during computer work at 5 min; F(1, 37) = 9.81, p = .003, η = 0.21, and at 10 min; F(1, 37) = 8.40, Postural angles p = .006, η = 0.19, compared to rest, whereas there was no difference in head lateral flexion between rest and recovery. Back angles Blink rate The analysis revealed a psychological stress by time inter- action on back posture during computer work; F(2.17, Figure 7a shows that the participants blinked more fre- 73.79) = 4.61, p = .011, η = 0.12, indicating that the quently during the condition in which glare exposure was 1 3 Heart rate (beats/min) International Archives of Occupational and Environmental Health (2018) 91:811–830 821 Fig. 6 Postural angles for a back flexion/extension (leaning forward/backward, n = 35), b head flexion/extension (n = 38), c back lateral flexion (side bending, n = 31), and d head lateral flexion (n = 38) during the four computer-work condi- tions. LS Low stress, VS visual stress, PS psychological stress, VPS psychological and visual stress. Postural angles are given as degrees relative to a refer- ence sitting position marked in the graphs with a dotted line, representing zero degrees (see “Methods”). Positive/negative values refer to forward/back- ward movements for flexion/ extension and right/left move- ments for lateral flexion. Results are given as mean ± SEM Fig. 7 Blink rate in the two conditions with psychological stress exposure: a mean blink rate during the two computer- work sessions, and b mean blink rate at start of the work session (blinks per minute during 0–1 min) and at the rest of the work session (blinks per minute during the computer- 11 work session, excluding the first minute). PS Psychological stress, VPS psychological and visual stress. Results are given 7 7 as mean ± SEM, n = 18 PS VPS PS VPS Start of the work session (first min) Computer work The rest of the work session (excl. first min) added to psychological stress, than they did during the their blink rate during later phases of the computer-work condition with psychological stress only, indicating a blink conditions. response due to glare. The analysis revealed that there was a statistically significant main effect of condition on blink Fixation disparity rate; F(1.00, 17.00) = 4.99, p = .039, η = 0.23. In addition, Fig. 7b shows the blink rate at the begin- The mean fixation disparity was an exo-disparity at each ning (blinks/min during first minute) and further during measurement point (the eyes focus slightly behind the the computer-work sessions (average blinks/min during plane of focus). The mean fixation disparity was: at base- 2–9 min). The analysis revealed that there was a statistically line: − 1.60 ± 0.99 (mean arc min ± SD, n = 20), af ter signic fi ant ee ff ct of time on blink rate; F(1.00, 17.00) = 9.13, L S: − 1.80 ± 0.88, af ter VS: − 1.60 ± 1.05, af ter PS: p = .008, η = 0.35, indicating that the participants increased − 1.00 ± 0.88, and after VPS: − 1.70 ± 1.10. Figure 8 shows 1 3 Blink rate (blinks/min) Blink rate (blinks/min) 822 International Archives of Occupational and Environmental Health (2018) 91:811–830 results for FD , the change in fixation disparity rela- change tive to baseline for each condition. There was no significant effect of visual or psychological stress on the fixation dispar - ity measurements. Back angle associations Figure 9 shows significant correlations between the par - ticipants’ back angle (flexion/extension) during computer work and FD , reading speed, and orbicularis oculi change blood flow. There were positive correlations between the participants’ back posture (leaning forward/backward) and both change in fixation disparity towards eso/exo-dispar - ity relative to baseline (Fig. 9a) and higher/lower reading speed (Fig. 9b). In addition, there were positive correlations between back angle and orbicularis oculi blood flow during VS and VPS. Fig. 8 Mean FDchange for each of the computer-work conditions. LS Low stress, VS visual stress, PS psychological stress, VPS visual and Heart rate associations psychological stress. Negative/positive values on y-axis represent a change in fixation disparity from baseline towards an exo-/eso-dispar - ity. Dotted line represents the mean baseline value (zero). The results Table 2 shows statistically significant correlations between are given as mean ± SEM, n = 20 heart rate and other measured parameters. In all conditions, angle and a FDchange in VS, PS and VPS, and b reading speed in Fig. 9 Significant correlations between back angle (leaning forward/ LS, VS and PS. LS Low stress, VS visual stress, PS psychological backward = positive/negative values) and FDchange (change towards eso/exo-disparity = positive/negative values), productivity (read- stress, VPS visual and psychological stress. */**Statistically signifi- ing speed, words/10 min), and orbicularis oculi muscle blood flow cant correlation at p < .05 and p < .01, respectively. (*) Close to, but (in table). The correlations are given as Pearson’s correlation coef- not statistically significant correlation (p ≤ .060) ficients. The correlations plots show the associations between back 1 3 International Archives of Occupational and Environmental Health (2018) 91:811–830 823 Table 2 Summary of correlations between heart rate (beats/min) and Eec ff ts of computer work per se trapezius muscle blood flow, systolic blood pressure (mmHg), per - ceived stress (mm VAS), and blink rate (blinks/min) The participants’ trapezius muscle was affected by computer Heart rate vs. N LS VS PS VPS work as such, independent of exposure. This was shown by increased muscle activity and muscle blood flow while Trapezius blood 34 − 0.517** − 0.404* − 0.347* − 0.338(*) working compared to the rest period, and the most obvi- flow ous explanation for this is active use of the trapezius while Systolic blood 38 0.407* 0.204 0.293 0.352* pressure using the mouse to perform the computer task. Similar time Stress 38 0.220 0.302 0.471** 0.350* effects in the trapezius were shown by Mork et al. (2016) Blink rate, 1st min 18 – – − 0.307 − 0.530* during 30 min of reading on a computer. The recordings in Blink rate, mean 18 – – − 0.295 − 0.492* Mork et al. (2016) also showed significant time effects on the trapezius muscle activity and muscle blood during glare Results are given as Pearson’s correlation coefficients exposure, but not during optimal lighting (similar to VS and LS Low stress, VS visual stress, PS psychological stress, VPS visual LS in the current study, respectively). These measurements and psychological stress were performed on the non-dominant and passive trapezius */**Statistically significant correlation at p < 0.05 and p < 0.01, as opposed to the dominant trapezius (active shoulder/arm) respectively in the present study. This is an indication that factors other (*) close to, but not statistically significant correlation (p ≤ .060) than active use of the trapezius also contribute to muscular changes associated with computer work in this neck and increased heart rate was associated with reduced trapezius shoulder muscle. blood flow (or vice versa). Heart rate was also positively Alterations in posture may also have contributed to the correlated with systolic blood pressure in LS and VPS and changes in the trapezius muscle. The participants’ sitting with perceived stress in PS and VPS. Further, there were position was different during computer work compared to negative associations between the participants’ heart rate rest, and this could have affected the neck and shoulder mus- and blink rate in VPS. This indicates that a rise in heart rate culature as a result of differences in postural load. However, was accompanied with both reduced blink rate and trapezius no significant associations between postural angles and tra- blood flow, as well as with increased blood pressure and pezius muscle activity or blood flow were present, which is perceived stress during computer work. consistent with Mork et al. (2016). This indicates that altera- tions in sitting position cannot solely explain the changes Overall effect of time during testing observed in trapezius during computer work. Westgaard and Bjørklund (1987) suggested that increased trapezius muscle For heart rate and head flexion/extension, there was a sta- activity during both mentally and visually demanding com- tistically significantly overall effect of time during test- puter work could be induced by psychologically mediated ing. Heart rate decreased throughout testing from the first muscle tension due to a variety of stressors associated with to the last condition, indicating that the participants were office work, unrelated to postural maintenance. In the current more stressed at the start of the test procedure than they study, the task in all conditions was to read text on the com- were towards the end, independent of condition order. In puter screen and mark spelling errors. Reading is visually addition, the head was also significantly more backward and cognitively demanding (Orchard and Stern 1991), and leaning in the third condition than in the first. Such overall the verbal instructions before all conditions were to perform effects of time might have washed out potentially signifi- the task as well as possible. The task itself may therefore cant effects. have placed an additional mental demand on the participants. During all the four computer-work periods, a general arous- ing effect due to the task may therefore have contributed to Discussion the increased trapezius muscle activity observed. Moreover, trapezius muscle blood flow is also previously reported to In this study, young, healthy females with normal binocu- be affected by mental stress (Larsson et al. 1995). It is there- lar vision were exposed to visual stress (direct glare) and fore likely that the increased trapezius muscle activity and psychological stress during computer work to elucidate the muscle blood flow during computer work were affected by effects of these occupational simulated stressors. A common a similar mechanism. Further, the observed time effect of general stress response was expected both for visual and psy- increased heart rate during computer work also supports that chological stress. In addition, specific effects were expected a general arousal was present during all conditions. In addi- during visual stress as an adaptation to glare. tion, heart rate was lowest during recovery, indicating that 1 3 824 International Archives of Occupational and Environmental Health (2018) 91:811–830 participants were more tense before start of the conditions This was also expected, as stressors similar to those used in compared to after completion. the current study previously have been shown to increase In all conditions, there were negative associations perceived stress levels in humans, as well as other psy- between heart rate and trapezius muscle blood flow. In con- chological responses such as cortisol level and autonomic trast, Larsson et al. (1995) found an increase in both heart responses (Dickerson and Kemeny 2004; Skoluda et al. rate and trapezius muscle blood flow due to mental stress. 2015; Wahlström et al. 2002). In the debriefing after the However, this mental stress was added to a series of static experiment, all participants confirmed that one or more of trapezius contractions (performed with straight arms ele- the induced psychological stressors had affected them, but vated in different angles with a 1-kg load), inducing 30% there were intersubjective differences in what they reported muscle blood flow increase in the exercising trapezius. Dur - to be the most stressful factor. The association between per- ing low-force muscle activity, such as during the computer ceived stress and lighting in the condition with exposure to work in the present study, a different mechanism may be at both glare and psychological stress, further indicates that work. Accordingly, neural control of muscle blood flow is poor ambient lighting may negatively influence perceived shown to be complex (Shoemaker et al. 2016). stress levels, or vice versa, during computer work. Further, the results showed that computer work itself also affected muscle blood flow in the orbicularis oculi, indicat- Orbicularis oculi ing that the orbicularis is activated during near-visual work and attention tasks. This has also been shown in previous There was no significant effect of either glare exposure or research; computer work per se induce eyelid squinting and psychological stress on orbicularis oculi muscle blood flow increased muscle blood flow in the orbicularis oculi (Mork in the present study. The low number of participants with et al. 2016). Squinting improves visual acuity and decreases complete orbicularis oculi blood flow data and the short the amount of light from the surroundings that enters the exposure duration (10 min) may have influenced this result. retina (Sheedy et al. 2003). In the present study, all partici- Mork et al. (2016) used the same glare source as in the pants had normal vision, and their orbicularis oculi response present study, and reported glare to induce significantly during computer work may therefore not have been caused increased orbicularis oculi muscle tension (eyelid squint- by the need for improved vision/visual acuity; rather, it may ing) during 30 min of computer reading (n = 15). In addi- have been an attention/concentration reflex. tion, a borderline significantly increased orbicularis oculi The results corroborates previous research and elaborates blood flow was observed. The eyelid squinting response further the magnitude and the pattern of effects connected may reflect an adaptation of the orbicularis muscle in the to computer work. In the evolutionary stress model (Foster- presence of glare, to reduce the amount of light entering vold et al. 2014) the observed effects are interpreted as func- the eyes (Sheedy et al. 2003). Accordingly, it is likely to tional adaptations to demands imposed by near-visual work. anticipate increased muscle activity in orbicularis oculi also Although work at close distances itself is not evolutionarily in the present study. Further, the results revealed that dur- novel, prolonged static visual work at a short fixed distance, ing glare exposure, orbicularis oculi muscle blood flow was as in computer work, can be seen as an departure from the affected by sitting posture. Bending forward towards the conditions from which the human visual system is adapted. screen/glare source was associated with increased muscle Thus, it follows that continued efforts to cope with the situ- blood flow. This may be related to a stronger eyelid squinting ation, may ultimately give rise to secondary problems. response due to higher retinal illumination as the distance to the glare source became shorter. In relation to this, Thorud Exposure to glare and psychological stress et al. (2012) have previously shown that both muscle blood flow and muscle activity in orbicularis oculi is related to As expected, the participants reported that they perceived increased eye-related symptoms. worse workstation lighting in the glare conditions. The lumi- The hypothesis for a psychological stress response in nance of the visual object (computer screen) was 155 cd/m , orbicularis oculi was outlined based on that psychological whereas the luminance of the glare source placed behind the stress has previously been shown to affect the hemodynam- computer screen was above 4500 cd/m . Previous studies ics and/or muscle activity of other facial muscles (Hidaka have reported that subjects prefer environmental luminance et al. 2004a, b; Nilsen et al. 2007). The current study did not slightly below the luminance of the visual object (Sheedy find any effect of psychological stress on orbicularis oculi et al. 2005), and ambient luminance above 600 cd/m has muscle blood flow, indicating that blood flow in this mus- been rated as disturbing (Berman et al. 1994). cle may not be affected by psychological stress. However, Participants rated perceived stress to be significantly orbicularis oculi muscle activity was not recorded in this higher in conditions with exposure to psychological stress. study. 1 3 International Archives of Occupational and Environmental Health (2018) 91:811–830 825 Cardiovascular responses frequent blinks (Gowrisankaran et al. 2007; Nahar et al. 2011, 2007). Increased blink rate is understood as an indi- In the present study, either glare or psychological stress cation of ocular fatigue or discomfort (Rodriguez et al. 2018; did significantly affect the cardiovascular responses in the Stern et al. 1994), which suggests that the increased blink participants. The literature is inconclusive regarding heart rate observed during glare exposure, may be due to fatigue/ rate and mental loads, showing both increased heart rate discomfort caused by additional stress placed on the visual and no effect due to psychological stress exposure (Hidaka system. et al. 2004b; Iwanaga et al. 2000; Larsson et al. 1995; Nahar Further, the participants’ blink rate was less frequent dur- et al. 2011; Skoluda et al. 2015). Skoluda et al. (2015) found ing the first minute compared to later phases of the com- that the laboratory stress protocol Trier Social Stress Test puter-work conditions, and this occurred simultaneously (TSST), which includes a social-evaluation threat and a with an initial and transient increase in trapezius muscle mental task, induced a perceived stress level of 30 mm VAS activity. This indicated that the participants were most in young men. TSST also induced increased heart rate and concentrated or stressed in the first part of the computer- alpha-amylase concentration (indicating increased sym- work periods, which is consistent with studies showing that pathetic activity) and increased cortisol concentration in reduced blink rate is an aspect of a stress/concentration saliva (indicating activation of a stress response in the hypo- response (Gowrisankaran et al. 2012; Iwanaga et al. 2000; thalamus–pituitary–adrenal axis). The psychological stress Rosenfield et al. 2015). The significant negative relation- conditions in the present study had some similarities to the ship shown between heart rate and blink rate also supports study of Skoluda et al. (2015): it included social-evaluation this view. In addition, muscle activation in the trapezius is threat exposure, the conditions had similar duration, and the also considered an indicator of mental stress (Lundberg et al. participants reported perceived stress at approximately the 2002; Westgaard 1999). same level (30–35 mm VAS). Together with the discrepancy The observed pattern of findings seem to concur both according the effect of mental stress on heart rate, this may with previous findings showing reduced blink frequency in indicate that even though the effect on heart rate was not computer work (Nielsen et al. 2008; Wolkoff 2008) and pre- statistically significant, the psychological exposure might dictions made by the evolutionary stress model (Fostervold have evoked other responses in the participants. In line with et al. 2014). Near-work are generally concentration intensive this, we did find a positive correlation between heart rate and from an evolutionary viewpoint, it seems adaptive to and perceived stress in the conditions with psychological reduce the blink frequency to maintain focus. However, the stress exposure. intensity and duration of near-work found in computer work Another explanation for the lack of cardiovascular vastly exceeds the conditions for which the human visual responses in the current study is that the psychological system is adapted. Continued efforts to cope will gradually stressors may have been too weak. Hidaka et al. (2004b) lead to desiccation of the precorneal tear film and dry eyes observed an increase in sympathetic nervous activity when (Nielsen et al. 2008; Wolkoff 2008). Dry eyes and unstable exposing females to a weak and long-lasting mental stressor, tear film seems to increase scattering and dispersion of light but there was no accompanying heart rate increase even in the eye and contributes to reduced retinal image contrast though the stress was reported as “extremely stressful” (4 and increased risk of disability glare (Huang et al. 2002; on a 0–4 point scale). This may further indicate a possibility Puell et al. 2006). As the main objective of the visual sys- for the psychological stress in the present study to provoke tem is to maintain sharp and single images at the retina, a sympathetic response in the participants, but without an increased blink frequency seems thus to be an adaptive accompanying increase in heart rate. response to the demands imposed by the environment. The literature on the effect of persistent glare exposure on heart rate in human is limited (except from effects of The trapezius muscle bright light/glare in depressed patients, night shift-workers and drivers) (Mork et al. 2016; Saito et al. 1996). However, The trapezius muscle was affected by both glare and psycho- the absence of a glare effect on heart rate indicates that the logical stress in the current study. During glare, the trapezius set up in the current study was not able to provoke a heart muscle blood flow increased significantly, whereas the trape- rate response in young, healthy females. zius muscle activity increased initially due to psychological stress, suggesting that the visual and psychological stress Blink rate responses were dissimilar. Different stimuli that act as stress- ors evoke a wide range of physiological responses, including Blink rate was significantly affected by glare exposure in a variety of autonomic, hormonal, and behavioural responses the current study. This is in accordance with earlier stud- (Dampney 2015; Dayas et al. 2001). In line with this, visual ies showing that glare (and refractive error) induces more stressors and psychosocial stressors have different complex 1 3 826 International Archives of Occupational and Environmental Health (2018) 91:811–830 pathways into the brain (e.g., into the dorsolateral periaq- muscles and from somatic neck muscles has been shown to ueductal grey which has an important role in the coordi- be mutually influential (Biguer et al. 1982; Bruenech et al. nation of responses to a variety of stressors) (Bergmanson 2012; Han and Lennerstrand 1995, 1998). The eyes and 2017; Chellappa et al. 2017; Dampney 2015; Dedovic et al. the head are activated synchronously with eye movements 2009). The dissimilar responses observed regarding trape- when looking at a visual target (Biguer et al. 1982), and zius muscle blood flow and muscle activity is therefore not proprioceptive signals (vibration) of different neck muscles surprising. induce eye movements (Han and Lennerstrand 1995). This The significant increase in trapezius muscle blood flow implies that proprioceptive messages originating in the eye during glare exposure was not present during computer work muscles and/or the neck muscles are processed together with appropriate lighting. This implies the presence of a spe- and lead to coordinated activation. In line with this, rela- cific visual stress response due to glare, as previously shown tionships between eyestrain and eyelid squinting, and neck (Mork et al. 2016). Glare is potentially detrimental for func- and shoulder muscles and musculoskeletal symptoms have tional vision and specific responses to glare was therefore been shown (Helland et al. 2008; Mork et al. 2016; Rich- expected. Whether or not the observed response in trapezius ter et al. 2011; Wiholm et al. 2007; Zetterlund et al. 2009). represents a functional adaptation in evolutionary terms is To the best knowledge of the authors, no study has inves- difficult to say for sure, as its functional and neurological tigated associations between trapezius muscle blood flow basis is still unclear. However, some possible explanations and accommodation or oculomotor muscles. Therefore, a can be outlined. relationship between ocular muscles and neck muscles can- Glare may have produced a response in the autonomic not be dismissed as a factor involved in the increased muscle sympathetic nervous system (Belkić 1986; Saito et al. 1996). blood flow in trapezius during glare exposure. Saito et al. (1996) found increased activity in the sympa- The observed trapezius blood flow increase could also thetic nervous system during exposure to excessive light be influenced by changes in posture affecting muscles in the (5000 lx) for 20 min, and activation of autonomic responses neck and shoulder region. However, there were no differ - is known to affect muscle blood flow in skeletal muscles ences in postural angles in the conditions with glare com- (Shoemaker et al. 2016). It has also been shown that vasodil- pared to the no-glare conditions. In fact, the back angle was atation in the muscles of cats occurs in the early, alerting opposite in the two conditions with glare: extended during stage of the defense reaction, produced both by direct electri- the condition with glare only and flexed during the condition cal stimulation of the hypothalamus and by environmental with both glare and psychological stress. Further, there were stimuli, such as flash of light (Abrahams et al. 1964). The no significant associations between trapezius blood flow and optic nerve in humans leads visual input directly into impor- the postural angles measured. Relationships have been found tant cortical regions regulating autonomic responses, such between rotation of the head away from the midline and as the hypothalamus (Royet et al. 2000), and an activation neck and shoulder complaints and pain (Faucett and Rem- of the autonomic nervous system due to visual stress may pel 1994). Head rotation was not registered in the present therefore be a possible explanation for the increased blood study, and therefore the possibility that head rotation affects flow observed in the trapezius muscle during glare in the the trapezius muscle cannot be excluded. However, Fig. 6 current study. shows that the participants exhibited similar movement pat- Further, glare/excessive stray light on the retina leads to terns for both head flexion and head lateral flexion in all reduced contrast, altered visibility, and blur (Fry and Alp- four conditions and that the differences in angle between the ern 1953; Lie 1981; Van Den Berg 1991). Both blur and conditions was small (within 2°–3°). It is therefore unlikely excessive light exposure has been shown to affect eye-lens that head rotation is the explanation for the observed blood accommodation (Kruger and Pola 1986; Shahnavaz and flow increase during glare exposure. Hedman 1984; Wolska and Switula 1999). In line with this, Richter and coworkers have in several studies shown asso- Posture; associations to concentration and vision ciations between accommodation/ciliary muscle contraction and a bilateral increase in trapezius activity (Richter et al. The participants’ posture was affected by the different 2010; Richter and Forsman 2011; Zetterberg et al. 2013). induced stress exposures during computer work in the pre- They stated that this interaction between the eyes and the sent study. Glare made the participants bend their head for- neck/shoulder muscle may be a neural command between ward, probably to keep excessive light from entering the sustained eye-lens accommodation when fixating on a near eyes. Psychological stress made the participants bend both target and the postural muscles in the neck and shoulder their head and back significantly more forward, and a plau- area, which is activated to stabilize gaze during visually sible explanation for this is that mental work/demanding demanding conditions, such as near-visual work. In line concentration tasks induce a more forward bent position with this, proprioceptive information from the oculomotor to increase proximity to the visual object of interest. The 1 3 International Archives of Occupational and Environmental Health (2018) 91:811–830 827 finding that the participants were significantly more produc- a more forward-bent posture. Bending forward towards the tive when bent forward also supports the suggestion that computer screen was associated with higher reading speed, leaning forward is a concentration response. indicating a concentration or stress response. Forward bent Fixation disparity measured as exo-disparities was posture was also associated with changes in fixation dispar - expected, because all measurements were captured at near ity. Furthermore, during computer work per se, trapezius distances (Jaschinski 2017). However, we did not find any muscle activity and muscle blood flow, orbicularis oculi significant effect on fixation disparity attributable to the muscle blood flow, and heart rate were increased compared visual stress, and this contrasts with other findings. Glimne to rest. et al. (2013) showed significantly more variation in fixation This study emphasizes that problems and ailments associ- disparity after computer work with glare exposure compared ated with computer work should be understood in a broad to non-glare conditions. However, they used a fixed chin environmental context. The traditional medical and biome- and forehead rest to control for the viewing distance dur- chanical model is often restricted to discussing proximate ing the experiment. In the present study, participants were relationships (i.e., questions concerning how the body works allowed to alter their posture and move freely during com- and why some people develop symptoms and diseases). The puter work. There were significantly positive associations evolutionary approach extend this analytical limitation fur- between back angle and change in fixation disparity relative ther by encompassing questions about why some body parts to baseline in the conditions with visual and/or psychologi- are developed to tolerate high amounts of strain, while others cal stress. This indicate that participants compensated for develop malfunctions in their analysis of health problems alterations in x fi ation disparity by moving closer to or farther (Scott-Phillips et al. 2011). away from the computer screen when exposed to additional When adjusting computer workplaces, it is important stress. This seems reasonable, because a longer or shorter to not only minimize potential environmental stress, but viewing distance releases the outward (exo) or inward (eso) also to understand why some situations are tolerated quite demand on the binocular system, respectively. This is also well, while others seem to put undue strain upon the visual supported by Jaschinski (2002), who reported that when able system. This study shows that the optimizing of computer to choose a comfortable viewing distance during computer workstations is a complex field that must take into account work, subjects preferring longer viewing distances had more several different factors, including both physical and psycho- exo-disparity during near-visual work (with forced distance) logical factors. This requires a multidisciplinary approach than did subjects preferring shorter distances. These results and visual ergonomics should be included, because visual indicate that visual factors may affect working posture, or conditions affect the worker during computer work. vice versa, during computer work with exposure to addi- tional environmental stressors. Either way, this suggests that Compliance with ethical standards persistent exposure to visual and/or psychological stress can Conflict of interest The authors declare that they have no conflict of lead to visual and musculoskeletal complaints and reduced interest. work capacity due to altered demand on the binocular system or altered postural load. Further research is needed to under- Ethical approval The study protocol was approved by the Regional Committee for Medical and Health Research Ethics, Norway stand these connections, but the results highlights the neces- (2013/610), and followed the tenets of the 1964 Helsinki declaration sity for optimized visual ergonomics during computer work. and its later amendments or comparable ethical standards. The funding bodies from the Norwegian Extra Foundation for Health and Reha- bilitation/Spine Association, Norway, had no impact on the study: on either design, data collection, analysis or presentation of the results. Conclusion Informed consent All participants received verbal and written informa- Exposure to visual and psychological stress during computer tion about the study, and informed consent was obtained from all indi- work affects young, healthy females with normal binocular vidual participants included. Additional informed consent was obtained from all individual participants for whom identifying information is vision, but the stress mechanisms are obviously dissimilar. included in this article. During visual stress (direct glare), the trapezius muscle blood flow increased and the participants bent their head forward, probably to reduce the amount of light entering Open Access This article is distributed under the terms of the Crea- tive Commons Attribution 4.0 International License (http://creat iveco the retina. 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