Prolonged computer use, especially if fatigue ensues, is associated with visual and musculoskeletal symptoms. The aim was to determine the time-course of perceived fatigue in the wrist, forearm, shoulder and eyes during a 60-min mouse task (painting rectangles), and whether object size and/or mouse use demands were of influence. Also, we investigated performance (number of rectangles painted), and whether perceived fatigue was paralleled by local muscle fatigue or tissue oxygenation. Ten women performed the task for three conditions (crossover design). At condition 1, rectangles were 45 × 25 mm, square paint cursor size 1.3 × 1.3 mm, and mousepointer movement ratio 1:26. At condition 2, the same cursor size and mousepointer movement ratio was used, but rectangles were smaller. At condition 3, the smaller rectangles were used, but the cursor size was also smaller and mousepointer movement ratio was 1:8. The results showed increased self-reported fatigue over time, with the observed increase greater for the eyes, but no change in physiological responses. Condition 2 resulted in higher performance and increased eye fatigue. Perceived fatigue in the muscles or physiological responses did not differ between conditions. In conclusion, computer work tasks imposing high visual and motor demands, and with high performance, seemed to have an influence on eye fatigue. 

Patients palliated with Total Cavopulmonary Connection have a lower muscle mass and a lower exercise capacity. We assessed calf muscle oxidative metabolism during and after heel raise exercise to exhaustion in young patients with TCPC compared to healthy peers. Near-infrared spectroscopy was used for measuring oxygen metabolism in the medial portion of the gastrocnemius muscle. Forty-three patients with TCPC, aged 6-18 years, were compared with 43 age and sex-matched healthy control subjects. Subgroups were formed to include children (6-12 years) and adolescents (13-18 years) to determine if these age groups influenced the results. During exercise, for the patients compared to controls there was a lower increase in deoxygenated hemoglobin (oxygen extraction) (5.13 ± 2.99au vs. 7.75 ± 4.15au, p = 0.001) and a slower rate of change in total hemoglobin (blood volume) (0.004 ± 0.015au vs 0.016 ± 0.01au, p = 0.001). Following exercise, patients exhibited a slower initial increase in tissue oxygenation saturation index (0.144 ± 0.11au vs 0.249 ± 0.226au, p = 0.007) and a longer half-time to maximum hyperemia (23.7 ± 11.4 s vs 16.8 ± 7.5 s, p = 0.001). On the subgroup level, the adolescents differed compared to healthy peers, whereas the children did not. Young patients with TCPC had impaired oxidative metabolism during exercise and required a longer time to recover. In that the differences were seen in the adolescent group and not in the children group may indicate a declining function with age.

Repetitive work to fatigue is soundly associated with work-related musculoskeletal disorders (WMSD), although the underlying mechanisms are poorly understood. In the present study, we tested the hypothesis that fatiguing work leads to proprioceptive deficits, which can be an initiating factor for the occurrence of WMSD. Thus, the position sense of the shoulder was determined for 13 males and 13 females before and after performing repetitive low-intensity arm work to fatigue in a simulated occupational setting. From a starting position of 45grader to the sagittal plane, position sense tests consisted of subjects attempting to actively reproduce target positions of horizontal move- ments to 15grader and 30grader (shoulder adduction) and to 60grader and 75grader (shoulder abduction). An analysis of variance revealed that the absolute error was significantly increased following fatigue for the subjects as a group (P < 0.001). Furthermore, females had an overall higher error than males (P < 0.01). No difference in error was detected for the shorter movements versus the longer movements. However, the overall absolute error for adduction was significantly higher than for abduc- tion (P < 0.001). The results of the present study support the hypothesis of diminished proprioceptive acuity following low-intensity work to fatigue. A reduction in position sense acuity could lead to impairment in motor control, which would further impact on position sense. Thus, a vicious cycle may be activated that might result in WMSD. The poorer position sense acuity observed for females may contribute to the explanation of why females may contribute to explanation of why females demonstrate a higher incidence of WMSD than males. Key Words: Fatigue, Glenohumeral joint, human, Occupational musculoskeletal problems,Proprioception.

Position sense awareness of the shoulder was determined for 9 males and 9 females before and after performing repetitive low-level arm activity to fatigue. The activity consisted of continuous horizontal internal and external rotations. Position sense awareness tests were performed with subjects blindfolded and seated with the arm in a motorized rig. The subjects were asked to actively reproduce test positions of 10°, 25°, 55° and 70° randomly assigned, while starting at 40° (relative to the sagittal plane). The tests were conducted immediately before and after the fatiguing activity. An ANOVA revealed that the absolute error, measured as test position minus reproduced position, was significantly increased following fatigue for the subjects as a group (p<0.001 ). Furthermore, females had overall higher error than males (p<0.05). The results indicate that repetitive low-level activity to fatigue leads to proprioceptive deficits. These deficits can create a non-optimized motor control, causing an increase in the co-activation of agonist and antagonist muscles and thus increasing the work-load. This could create an unfavorable cycle of events, which would further impact proprioception and thereby enhance the risks for muscle pain and injury. The higher incidence of injuries for female athletes compared to males may be attributed to gender differences in proprioception.

Context: Stretching is common within sports as a potential maneuver for injury prevention. Stretching induced changes in muscle spindIe properties is a suggested mechanism. This may imply a reduction in proprioception following stretching, however, little is known of this association. Our finding showing no effect of acute stretching on shoulder position sense provides insight into this issue.

Objectives: To evaluate if acute stretching of the shoulder muscles affects position sense.

Design: A crossover design with subjects randomized to 3 groups, as regarded by the I sequence of 3 interventions.

Setting: A university human research laboratory.

Patients or Other participants: Nine female (age, 21 +2) and 9 male (24 + 3) healthy volunteers.

lntervention(s): The interventions consisted of stretching of shoulder 1) agonists, 2) antagonists, and 3) non-stretching control.

Main Outcome Measure(s): Position sense acuity of the right shoulder was determined before and arter the interventions by subjects at tempting to reproduce arm positions of 15° and 30° (shoulder adduction) while starting at 45° to the sagittal plane. The outcome variables were the response variability (variable error, VE) and overall accuracy (absolute error, AE).

Results: A multivariate repeated measures analysis ofvariance revealed that the relative change in VE (i.e., VE after/VE before) was not significantly different between the interventions (p = 0.38). Similarly no change in AE was found (p = 0.76). Furthermore, there were no differences regarding test sequence or in the interaction 'intervention x sequence' for either VE (p = 0.73 and 0.53, respectively) or for AE (p = 0.71 and 0.67, respectively)

Conclusions: The present study showed no effect on shoulder position sense after an acute bout of stretching either agonist or antagonist shoulder muscles.

OBJECTIVE: To study the effects of a muscle stretching regimen for the rectus femoris muscle on subjective stretch sensation and range of motion (ROM). DESIGN: A 2 x 2 crossover design comprising 2 treatments and 2 intervention periods.

SETTING: A military base in Sweden.

PARTICIPANTS: A volunteer sample of 29 male military conscripts divided into 2 groups, with each group subjected to both experimental and control treatments at different time periods.

INTERVENTION: Two weeks of supervised stretching (4 times/wk) of the rectus femoris muscle (experimental treatment) and the calf muscles (control treatment).

MAIN OUTCOME MEASURES: Subjective rating of the stretch sensation for the anterior aspect of the thigh determined on a category ratio scale. Passive knee flexion ROM determined on each test with the same applied torque, specific for each subject.

RESULTS: An additive analysis of variance revealed that the stretch sensation after the experimental treatment was decreased, compared with the control treatment (p <.01). The knee flexion, however, remained the same regardless of the treatment.

CONCLUSION: Sensory adaptation seems to be an important mechanistic factor in the effect stretching has on ROM changes. The lack of change in knee flexion suggests that the stretching, as performed in this study, did not influence stiffness of the rectus femoris muscle. Sensory adaptation may also be an underlying mechanism in the alleviating effect of stretching when applied to tired, tender, and painful muscles.

Muscle oxygenation responses are reportedly greater in the distal muscle region than in the proximal muscle region. We combined near infrared spectroscopy and electromyography (EMG) to determine whether regional differences in oxygenation are associated with differences in (1) muscle activation and/or (2) fatigue development. Nine males performed 2-min sustained isometric knee extensions at 15% and 30% maximum voluntary contraction during which oxygenation and EMG were recorded simultaneously from proximal and distal locations of the vastus lateralis muscle. Near infrared spectroscopy variables for oxygen saturation (StO2%) were initial slope at contraction onset, peak drop, and recovery slope at contraction end. Electromyography produced the root mean square to indicate muscle activation and mean power frequency changes over time (decreasing slope) to indicate fatigue development. For StO2%, significantly greater peak drop and steeper recovery slope were found for the distal muscle region than for the proximal muscle region. Root mean square, however, was not different between locations. Mean power frequency decreased throughout the contractions but changes were not different between locations. Our results indicate that for modest submaximal contractions, regional differences in oxygenation are not associated with differences in muscle activation or with fatigue development (as interpreted by changes in mean power frequency over time).

We investigated the effects of active versus passive pauses implemented during computer mouse work on muscle oxygenation and EMG of the forearm extensor carpi radialis muscle, and on wrist position sense. Fifteen healthy female subjects (age: 19-24 years) performed a 60-min mouse-operated computer task, divided into three 20 min periods, on two occasions separated by 3-6 days. On one occasion a passive pause (subjects resting) was implemented at the end of each 20-min period, and on another occasion an active pause (subjects performed a number of high intensity extensions of the forearm) was implemented. Also at the end of each 20-min period, test contractions were conducted and subjective ratings of fatigue and stress were obtained. Another parameter of interest was total haemoglobin calculated as the summation of oxy-and deoxy-haemoglobin, since it reflects blood volume changes. The most interesting findings were an overall increasing trend in total haemoglobin throughout the mouse work (P<0.001), and that this trend was greater for the active pause as compared to the passive pause (P<0.01). These data were accompanied by an overall increase in oxygen saturation (P<0.001), with a tendency, albeit not significant, toward a higher increase for the active pause (P=0.13). EMG amplitude and median frequency tended to decrease (P=0.08 and 0.05, respectively) during the mouse work but was not different between pause types. Borg ratings of forearm fatigue showed an overall increase during the activity (P<0.001), but the perceptions of stress did not change. Position sense did not change due to the mouse work for either pause type. While increasing trends were found for both pause types, the present study lends support to the hypothesis of an enhancement in oxygenation and blood volume for computer mouse work implemented with active pauses. However, a presumption of an association between this enhancement and attenuated fatigue during the mouse work was not supported.

This study determined the day-to-day reliability of NIRS-derived oxygenation responses (ΔStO 2%) for isometric contractions and for cuff occlusion. Twenty-four subjects (12 males and 12 females) were tested on two days (4-6 days interval). Variables generated were: (i) ΔStO 2% for isometric contractions (10%, 30%, 50% and 70% MVC) for descending trapezius (TD) and extensor carpi radialis (ECR) muscles; (ii) slope changes in total haemoglobin (HbTslope) and deoxyhaemoglobin (HHbslope) for the ECR using upper arm venous (VO, 50 mmHg) and arterial occlusion (AO, 250 mmHg); (iii) recovery slopes (Rslope) for oxygen saturation (StO2) following isometric contractions and AO. For each variable an intraclass correlation (ICC) was calculated to assess the ability to differentiate between subjects, and limits of agreement (LOA) were computed to assess day-to-day consistency of the measurement. ICCs for ΔStO2% were lowest at 10% MVC for both ECR (0.58) and TD (0.55), and highest at 30% MVC for ECR (0.95) and at 70% MVC for TD (0.79). For both muscles, LOA for ΔStO 2% was lowest at 10% and highest at 50% and 70% MVC. ICC for HbTslope was 0.17. For HHbslope ICC was higher for AO (0.83) than for VO (0.73), and LOA was lower for AO. For the ECR Rslope ICCs ranged 0.88–0.90 for contraction, but was lower for AO (0.33); LOA was lowest at 70% MVC. For trapezius Rslope ICCs ranged 0.63–0.73 and LOA was lowest at 30% MVC. For this study establishing reliability data for the ECR and TD, and including variables commonly reported, are expected to have meaning for future NIRS studies of work-related upper-extremity pain as well as for other NIRS research and clinical applications.

Chronic trapezius myalgia was shown to be associated with a reduction in microcirculation (via direct measurements of bloodflow). In line with this, morphological data have indicated disturbances in oxidative metabolism. Therefore, data obtained on the muscle oxygenation status could help provide insight into the pathomechanisms behind work related muscle pain. Near-infrared spectroscopy (NIRS) allows for the noninvasive monitoring of continuous changes in skeletal muscle oxygenation (representing the dynamic balance between oxygen delivery and consumption), and for subsequent determinations of changes in blood volume.

Monitoring oxidative metabolic states may provide information on the mechanism behind computer use related musculoskeletal disorders. PURPOSE: To compare tissue oxygen saturation (Sat-O2) profiles in the forearm extensor carpi radialis (ECR) during computer mouse use with and without time pressure and precision demands. METHODS: Twenty-four healthy, right-handed subjects (12 females, 12 males; age 19-28 years) participated in the study. Subjects performed a 45-min mouse operated computer task on two occasions, separated by 3-5 days. The task consisted of painting squares that were presented on the screen. On one occasion, time pressure and precision demands were imposed by limiting the time available for painting a square and introducing a scoring system based on precision of painting (STRESS). On the other occasion, no such restraints were added (NON-STRESS). The order of the two task versions was randomized. During the task, Sat-O2 in the right ECR muscle was measured by near-infrared spectroscopy – NIRS (Inspectra, Hutchinson Technology). In addition, subjective ratings of tenseness and strain, and painting performance measures were recorded. RESULTS: A repeated measures ANOVA revealed a significant decline in Sat-O2 during the STRESS condition (p < 0.05), but no change for the NON-STRESS condition. A gender difference was apparent as females exhibited an overall lower Sat-O2 than males (p < 0.05); however, no interaction was found. Subjects’ ratings of tenseness and strain were significantly higher during the STRESS as compared to the NON-STRESS condition (p < 0.001). These data were paralleled by work pace (i.e. squares painted during STRESS = 119; NON-STRESS = 84, p < 0.001). Furthermore, accuracy of painting was greater for the STRESS as compared to the NON-STRESS condition (p < 0.01), i.e., number of times outside the square, STRESS = 2, NON-STRESS = 4. No gender differences in subjective ratings or performance variables were detected. CONCLUSIONS: Our finding of a change in local metabolic states under stressful conditions may shed light on the mechanism behind computer mouse related forearm muscular disorders. Furthermore, that females demonstrate a lower ECR Sat-O2 could give precedence to their higher incidence of disorders than males.

Monitoring oxidative metabolic states may provide information on the mechanism behind computer use related musculoskeletal disorders. PURPOSE: To compare tissue oxygen saturation (Sat-O2) profiles in the forearm extensor carpi radialis (ECR) during computer mouse use with and without time pressure and precision demands. METHODS: Twenty-four healthy, right-handed subjects (12 females, 12 males; age 19-28 years) participated in the study. Subjects performed a 45-min mouse operated computer task on two occasions, separated by 3-5 days. The task consisted of painting squares that were presented on the screen. On one occasion, time pressure and precision demands were imposed by limiting the time available for painting a square and introducing a scoring system based on precision of painting (STRESS). On the other occasion, no such restraints were added (NON-STRESS). The order of the two task versions was randomized. During the task, Sat-O2 in the right ECR muscle was measured by near-infrared spectroscopy – NIRS (Inspectra, Hutchinson Technology). In addition, subjective ratings of tenseness and strain, and painting performance measures were recorded. RESULTS: A repeated measures ANOVA revealed a significant decline in Sat-O2 during the STRESS condition (p < 0.05), but no change for the NON-STRESS condition. A gender difference was apparent as females exhibited an overall lower Sat-O2 than males (p < 0.05); however, no interaction was found. Subjectsﾒ ratings of tenseness and strain were significantly higher during the STRESS as compared to the NON-STRESS condition (p < 0.001). These data were paralleled by work pace (i.e. squares painted during STRESS = 119; NON-STRESS = 84, p < 0.001). Furthermore, accuracy of painting was greater for the STRESS as compared to the NON-STRESS condition (p < 0.01), i.e., number of times outside the square, STRESS = 2, NON-STRESS = 4. No gender differences in subjective ratings or performance variables were detected. CONCLUSIONS: Our finding of a change in local metabolic states under stressful conditions may shed light on the mechanism behind computer mouse related forearm muscular disorders. Furthermore, that females demonstrate a lower ECR Sat-O2 could give precedence to their higher incidence of disorders than males

The advent of near infrared spectroscopy (NIRS) as a tool for monitoring muscle oxygenation has allowed for important physiological data in sports training and rehabilitation. Commercial methods are generally user-friendly and the technique is non- invasive. By projecting a light beam in to the muscle concentrations of haemoglobin/myoglobin (with and without oxygen) in the vascular bed consisting of small arterioles, capillaries and venules can be determined. Despite its appeal methodological improvements to account for varying muscle depths, and to distinguish between arteriolar and venular contributions separatelyare desired.

We examined the effect of a low-level sustained contraction on the muscle contractile properties, hemodynamics and oxygenation of the flexor digitorum superficialis muscle (FDS) of the finger. We tested the hypothesis that hemodynamics and oxygenation, reflecting the muscle metabolic characteristics, would recovery more quickly than the muscle contractile properties. Eleven subjects (26 ± 4 yrs) were equipped with electrodes for electrical stimulation and a near-infrared spectroscopy (NIRS) probe on the forearm over the FDS. The experimental protocol consisted of three baselines measurements (-60 min, -30 min, pre-exercise), immediately after a sustained 15-min contraction of the FDS at 10% maximal voluntary contraction (post-exercise), and after 30 min, 60 min and 120 min of recovery. For each time point, participants were subjected to a battery of test that included upper arm venous occlusion (at rest), a computer-mouse point and click task (standardized voluntary task), and electrical stimulation. For venous occlusion (50 mmHg, 1 min), slopes were calculated for NIRS-derived total hemoglobin (HbTslope) and deoxyhemoglobin (HHbslope) as estimates of blood flow and oxygen consumption, respectively. The computer-mouse task entailed using the mouse to point and click on targets presented on the screen during which NIRS signals were monitored for determination of change in total hemoglobin (ΔHbT) and oxygen saturation (ΔStO2%). Electrical stimulation (2 Hz, 5 trains of 15 twitches) provided twitch force (Tw-force), contraction time (CT) and one-half relaxation time (½RT) data. Statistical analysis revealed significant changes over time for all contractile parameters as well as for HHbslope (P < 0.05 for each). Post-hoc testing demonstrated significant decreases for Tw-force post-exercise and at 60 min; for CT at post-exercise, 30 min and 60 min; and for ½RT at post-exercise and at 30 min. HHbslope was significantly higher post-exercise as compared to pre-exercise. During the computer-mouse point and click task, no significant differences were detected forΔHbT, however,ΔStO2% showed a tendency to decrease, albeit not significant (P = 0.11). Further testing showedΔStO2% was significantly lower post-exercise and at 30 min as compared to pre-exercise. The present study shows that NIRS provides insight into muscle hemodynamics and oxygenation for low-level sustained activity to fatigue. The overall quick recovery of hemodynamic and oxygenation responses, and a more prolonged recovery of contractile responses confirms our hypothesis, and this may fit well with the classical definition of low frequency fatigue.

This paper reviews three computer mouse studies in our laboratory where our emphasis was on mechanisms behind computer related disorders. Our approach was sequentially (i) to determine validity of a laboratory model of computer mouse use (painting rectangles) for studying musculoskeletal disorders; to use this model (ii) to study time pressure and precision demands on position sense and muscular oxygenation; and (iii) to determine the effect of pauses (active vs passive) on these parameters. (i) Kinematic data for the painting model showed constrained movements of the wrist similar to CAD work; a support for its validity for a real life situation. (ii) Changes in forearm oxygenation were associated with time pressure and precision demands; a potential for insight into the underlying pathophysiological mechanisms. (iii) Increasing trends in oxygenation and blood volume were associated with pauses, especially active; possible explanation for the alleviating effect of discomfort experienced in real life situations when a pause is implemented.

Introduction

Along with increasing number of computer users in work organizations, upper extremity complaints become more and more common. Research has shown that increased exposure to repetitive keyboard and mouse use increases the risk of musculoskeletal disorders. However, the physiological mechanisms remain unclear. Pathological processes as the rate of the metabolism (energy crisis) can be conceivable as a cause of the symptoms and disability related to intensive computer work ¹ . To date, there are no studies made upon local tissue oxygen (O₂) saturation during computer work in patients suffering from computer related disorders in the upper extremity. In the present study, we examined local tissue oxygen (O₂) saturation in m. trapezius and m. extensor carpi radialis brevis before and during mouse operated computer work in patients suffering from computer related disorders, and looked at relations between oxygen saturation, subjective ratings of pain, symptoms and disability.

Material and method

Four right dominant female patient subjects (PS) aged 27 to 46 recruited through the company health care participated in the study. Their mean height was 166 cm and weight of three PS 55.8 kg (130 kg for the fourth PS). They had one or several diagnoses each, comprising of neck myalgia, diffuse forearm pain and lateral epicondylitis. All of them worked full-time and used computer with keyboard and mouse or mousetrapper device. Three PS used the computer for 7-8 hours per workday. The PS related their disorder to intensive computer work for long hours and high precision demands.

Skin fold thickness was measured with a caliper at m. trapezius (TRAP) and m. extensor carpi radialis (ECRB) on the right side, and ranged from 2-6 mm. The mean skin temperature was 32.2° during rest and 32.7° following work on the ECRB (there was one missing post-value due to technical problems). The local tissue oxygen saturation was measured non-invasively with near-infrared spectroscopy, NIRS (Inspectra, Hutchinson Technology). One NIRS-electrode was placed at TRAP and the other on ECRB, both on the right side. After five minutes of rest, the PS performed a mouse operated computer task for 45 minutes with their right hand. The task consisted of painting squares presented on the screen, by using the mouse. Measurements of oxygen saturation were made throughout the computer work. Furthermore, subjective ratings of tiredness and strain were recorded on a 10 cm long VAS-scale. Symptoms and disability of the hand, arm and shoulder was rated with the DASH questionnaire, which contains the areas activities of daily living (ADL) and work ³. Descriptive statistics of the oxygen saturation comprised average and standard deviation of StO2 values in %. Comparison of the local tissue oxygen was made with the subjective ratings.

Results and discussion

Table 1. Local tissue oxygen (during 5 minutes rest and 45 minutes of computer work) and subjective ratings in PS.

The mean saturation value for ECRB during rest was 52.6 (SD 20.2), work 46.7 (SD 14.7), and for TRAP during rest 58.8 (SD 21.2) and work 61.2 (SD 21.7). Skin fold thickness and skin temperature did not show any obvious interaction with the tissue oxygen values. Neither did comparisons of PS´s subjective ratings of tiredness and strain, symptoms, disability and the oxygen saturation, or severity of ratings and oxygen saturation. This might be due to the small amount of subjects and/or that the subjective ratings were not sensitive enough or do not fully reflect the state of art of computer related disorders from the upper extremity. One of the PS in our study reported intense delayed onset of pain 24 hours after the computer work. It is important to be aware that this can occur. The group of PS in this study had lower mean tissue oxygen saturation than healthy female subjects in another study that used the same device and performed an identical mouse operated computer task². More research is needed for a better understanding of the mechanisms behind upper extremity disorders and the extent to which local tissue oxygen saturation reflects their symptoms and disability.

References

1. Boushel, R.; H., L.; Olesen, J.; Gonzales-Alonzo, J.; Bulow, J.; and Kjaer, M.: Monitoring tissue oxygen availability with near infrared spectroscopy (NIRS) in health and disease. Scand J Med Sci Sports, 11: 213-22, 2001.

2. Heiden, M.; Dahlgren, G.; Lyskov, E.; Crenshaw, A.; and Johansson, H.: Effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense. In Nordiska arbetsmiljömötet. Edited, Nyslott, Finland, 2003.

3. http://iwh.on.ca/dash.html.

The aim was to investigate the relationship between oxygen saturation (StO(2)%) measured with near-infrared spectroscopy (NIRS) and myoelectric activity (root mean square, RMS) for the extensor carpi radialis (ECR) and trapezius muscles. In addition, gender differences were examined for submaximal (10-70% MVC) and sustained (10% MVC for 5 min) isometric contractions. Thirteen males and 15 females participated. Changes in StO(2)% (∆StO(2)%) and RMS, expressed as percentages of maximum, were calculated for each submaximal contraction. A good correlation between ∆StO(2)% and RMS was seen for the ECR (r = -0.53) and a moderate correlation seen for the trapezius muscle (r = -0.44). The ANOVA showed a significant decrease in ECR-∆StO(2)% over force with females demonstrating a tendency for larger changes than males. ECR-RMS increased over force with no impact of gender. For the trapezius, ∆StO(2)% decreased over force but was not gender dependent. Trapezius-RMS increased over force with females demonstrating a tendency for greater change than males. For the sustained contraction, ECR-StO(2)% changed over time but was not gender dependent. ECR-RMS increased over time with females showing a greater response than males. Trapezius-StO(2)% changed over time and differed between genders, i.e., males increased while females decreased. RMS increased over time similarly for both genders. In conclusion, our data show that the ECR and trapezius aerobic demands during isometric contractions are negatively correlated to electromyography (EMG) RMS. The present study also suggests some gender specificity for forearm and shoulder myoelectric activity and oxygenation for submaximal and sustained contractions.

Patients suffering from work-related muscle pain (WRMP) fatigue earlier during exercise than healthy controls. Inadequate oxygen consumption and/or inadequate blood supply can influence the ability of the muscles to withstand fatigue. However, it remains unknown if oxygenation and hemodynamics are associated with early fatigue in muscles of WRMP patients. In the present study we applied near-infrared spectroscopy (NIRS) on the extensor carpi radialis (ECR) and trapezius (TD) muscles of patients with WRMP (n = 18) and healthy controls (n = 17). Our objective was to determine if there were group differences in endurance times for a low-level contraction of 15% maximal voluntary contraction (MVC) – sustained for 12-13 min, and to see if these differences were associated with differences in muscle oxygenation and hemodynamics. At baseline, oxygen saturation (StO₂%) was similar between groups for the ECR, but StO₂% was significantly lower for TD for the WRMP patients (76%) compared to controls (85%) (P < 0.01). Also, baseline ECR blood flow was similar in the two groups. For both muscles there were a larger number of patients, compared to controls, that did not maintain the 15% MVC for the allotted time. Consequently, the endurance times were significantly shorter for the WRMP patients than controls (medians, ECR: 347 s vs. 582 s; TD: 430 s vs. 723 s respectively). Responses in StO₂% during the contractions were not significantly different between groups for either muscle, i.e. no apparent difference in oxygen consumption. Overall, we interpret our findings to indicate that the early fatigue for our WRMP patients was not associated with muscle oxygenation and hemodynamics.

Both physical as well as mental demands result in an increased activity in the sympathetic nervous system (SNS) with changes in blood-pressure and heart-rate. Through local release of catecholamines, e.g. noradrenaline (NAd) SNS exerts various actions at the muscle level. The aims of this study were to investigate the effects of low-load repetitive work alone and in combination with mental demands on local muscle interstitial noradrenaline concentration [NAd](i), muscle activity and oxygenation, assessed with microdialysis(,) surface electromyography, and near-infrared spectroscopy, respectively. Healthy females (n = 15) were exposed to (1) 30 min repetitive work (RW) and (2) 30 min repetitive work with superimposed mental load (RWML) on two different occasions. Muscle [NAd](i) and muscle activity increased significantly in response to RW, but did not increase further during RWML. For RW, [NAd](i) was found to be inversely correlated to muscle activity. Oxygenation decreased significantly during work, independently of occasion. Our findings indicate that low-load work causes significantly increased trapezius muscle [NAd](i) in healthy females, and short periods of superimposed mental load do not add to this increase and further, that both muscle activity and oxygenation were unaffected by the superimposed mental load.

Epidemiological research provides strong evidence for a link between repetitive work (RW) and the development of chronic trapezius myalgia (TM). The aims were to further elucidate if an accumulation of sensitising substances or impaired oxygenation is evident in painful muscles during RW. Females with TM (n=14) were studied during rest, 30 min RW and 60 min recovery. Microdialysate samples were obtained to determine changes in [glutamate], [PGE2], [lactate], and [pyruvate] relative to work. Muscle oxygenation (%StO2) was assessed using near-infrared spectroscopy. During work all investigated substances, except PGE2, increased significantly: [glutamate] (54%, P<0.0001), [lactate] (26%, P<0.005), [pyruvate] (19%, P<0.0001), while the %StO2 decreased (P<0.05). During recovery [PGE2] decreased (P<0.005), [lactate] remained increased (P<0.001), [pyruvate] increased progressively (P<0.0001), and %StO2 had returned to baseline. Changes in substance concentrations and oxygenation in response to work indicate normal increase in metabolism but no ongoing inflammation in subjects withTM.

In the present study, we assessed the wrist kinetics (range of motion, mean position, velocity and mean power frequency in radial/ulnar deviation, flexion/extension, and pronation/supination) associated with performing a mouse-operated computerized task involving painting rectangles on a computer screen. Furthermore, we evaluated the effects of the painting task on subjective perception of fatigue and wrist position sense. The results showed that the painting task required constrained wrist movements, and repetitive movements of about the same magnitude as those performed in mouse-operated design tasks. In addition, the painting task induced a perception of muscle fatigue in the upper extremity (Borg CR-scale: 3.5, p<0.001) and caused a reduction in the position sense accuracy of the wrist (error before: 4.6 degrees , error after: 5.6 degrees , p<0.05). This standardized painting task appears suitable for studying relevant risk factors, and therefore it offers a potential for investigating the pathophysiological mechanisms behind musculoskeletal disorders related to computer mouse use.

The aim of this study was to investigate the effects of duration of low-load repetitive work on intramuscular lactate, pyruvate, glutamate and prostaglandin E2 (PGE2), and oxygen saturation in the trapezius muscle. Twenty healthy females were studied during baseline rest, during low-load repetitive work for either 30 (REP 30) or 60 (REP 60) minutes, and 60 minutes recovery. Intramuscular microdialysate (IMMD) samples were obtained, and local muscle tissue oxygenation (StO2 %) assessed with near infrared spectroscopy (NIRS). Subjects rated their perceived exertion (Borg CR-10 scale) and capillary blood was sampled for lactate analysis. The results showed a significant increase in IMMD lactate in response to both REP 30 and REP 60 (P<0.05 and P<0.01 respectively) and glutamate (P<0.0001), but no progressive increase with increasing work duration. Both IMMD pyruvate and lactate tended to be significantly increased during the recovery period. No corresponding increase in blood-lactate was found. Local muscle StO2 % did not change significantly in response to work and was not correlated to the IMMD lactate concentration. The ratings of perceived exertion increased in response to work, and remained increased after recovery for REP 60.

In conclusion, the results of this study show significantly increased IMMD lactate and, glutamate concentrations in the trapezius muscle of healthy females in response to low-load work, but no progressive increase with increased work duration. Further, they do not indicate that the increased IMMD lactate concentration was caused by a locally decreased or insufficient muscle tissue oxygenation.

Much is still unknown concerning the mechanisms underlying the development of chronic muscle pain. The presence and magnitude of inflammatory substances and neurotransmitters in chronic painful conditions is not clear. The aims of the present study were to determine, with the use of microdialysis, the interstitial concentrations and the equilibration times for PGE(2) and glutamate in the trapezius muscles of nine female subjects with chronic muscle pain, and nine pain-free age-matched controls. A microdialysis probe was implanted in the upper part of the trapezius muscle and perfused with Ringer-acetate solution at a flow rate of 0.3 muL/min. Samples were obtained every 30 min, during a 4-h rest period. At equilibration, the mean concentrations (+/-SE) of PGE(2) were 0.71 (+/-0.11) ng/mL for the pain-group and 0.97 (+/-0.35) ng/mL for the controls. For glutamate the mean concentrations for the pain-group were 66.3 (+/-13.3) mumol/L and 60.6 (+/-22.9) mumol/L for the controls. For the pain group and the control group, respectively, equilibration for PGE(2) was reached at 180 and 150 min, and for glutamate at 150 and 120 min. The present study showed no differences between groups in the concentrations of PGE(2) and glutamate in the trapezius muscle. Further, it revealed that when using the slow-flow method, a period of at least 2.0-2.5 h is needed, after probe insertion, to reach steady state for glutamate and PGE(2).

Objective:  This study systematically summarizes biochemical biomarker research in non-traumatic musculoskeletal disorders (MSDs).  Two research questions guided the review:  1) Are there biochemical markers associated with neck and upper extremity MSDs? and, 2) Are there biochemical markers associated with the severity of neck and upper extremity MSDs? 

Methods:  A literature search was conducted in PubMed and SCOPUS.  Eighty-seven studies met primary inclusion criteria.  Following a quality screen, data were extracted from 44 sufficient quality articles.

Results:  Most of the 87 studies were cross-sectional and utilized convenience samples of patients as both cases and controls.  A response rate was explicitly stated in only 11 (13%) studies.  Less than half of the studies controlled for potential confounding through restriction or in the analysis.  Most sufficient quality studies were conducted in older populations (mean age in one or more analysis group > 50 yrs).

In sufficient quality articles, 82% demonstrated at least one statistically significant association between the MSD(s) and biomarker(s) studied.  Evidence suggested that: a) the collagen repair marker TIMP-1 is decreased in fibroproliferative disorders, b) 5-HT (serotonin) is increased in trapezius myalgia, and c) triglycerides are increased in a variety of MSDs.  Only five studies showed an association between a biochemical marker and MSD severity.

Conclusion: While some MSD biomarkers were identified, limitations in the articles examined included possible selection bias, confounding, spectrum effect (potentially heterogeneous biomarker associations in populations according to symptom severity or duration) and insufficient attention to co-morbid conditions. A list of recommendations for future studies is provided.

Background

This study systematically summarizes quantitative imaging biomarker research in non-traumatic neck and shoulder musculoskeletal disorders (MSDs). There were two research questions: 1) Are there quantitative imaging biomarkers associated with the presence of neck and shoulder MSDs?, 2) Are there quantitative imaging biomarkers associated with the severity of neck and shoulder MSDs?

Methods

PubMed and SCOPUS were used for the literature search. One hundred and twenty-five studies met primary inclusion criteria. Data were extracted from 49 sufficient quality studies.

Results

Most of the 125 studies were cross-sectional and utilized convenience samples of patients as both cases and controls. Only half controlled for potential confounders via exclusion or in the analysis. Approximately one-third reported response rates. In sufficient quality articles, 82% demonstrated at least one statistically significant association between the MSD(s) and biomarker(s) studied. The literature synthesis suggested that neck muscle size may be decreased in neck pain, and trapezius myalgia and neck/shoulder pain may be associated with reduced vascularity in the trapezius and reduced trapezius oxygen saturation at rest and in response to upper extremity tasks. Reduced vascularity in the supraspinatus tendon may also be a feature in rotator cuff tears. Five of eight studies showed an association between a quantitative imaging marker and MSD severity.

Conclusions

Although research on quantitative imaging biomarkers is still in a nascent stage, some MSD biomarkers were identified. There are limitations in the articles examined, including possible selection bias and inattention to potentially confounding factors. Recommendations for future studies are provided.

Background. Type 1 diabetic patients and non-diabetic patients were referred for evaluation for chronic exertional compartment syndrome (CECS) based on clinical examination and complaints of activity-related leg pain in the region of the tibialis anterior muscle. Previous studies using near-infrared spectroscopy (NIRS) showed greater deoxygenation during exercise for CECS patients versus healthy controls; however, this comparison has not been done for diabetic CECS patients.

Methods. We used NIRS to test for differences in oxygenation kinetics for Type 1 diabetic patients diagnosed with (CECS-diabetics, n = 9) versus diabetic patients without (CON-diabetics, n = 10) leg anterior chronic exertional compartment syndrome. Comparisons were also made between non-diabetic CECS patients (n = 11) and healthy controls (CON, n = 10). The experimental protocol consisted of thigh arterial cuff occlusion (AO, 1-minute duration), and treadmill running to reproduce symptoms. NIRS variables generated were resting StO2%, and oxygen recovery following AO. Also, during and following treadmill running the magnitude of deoxygenation and oxygen recovery, respectively, were determined.

Results. There was no difference in resting StO2%between CECS-diabetics (78.2±12.6%) vs. CONdiabetics (69.1±20.8%), or between CECS (69.3±16.2) vs. CON (75.9±11.2%). However, oxygen recovery following AO was significantly slower for CECS (1.8±0.8%/sec) vs. CON (3.8±1.7%/sec) (P = 0.002); these data were not different between the diabetic groups. StO2%during exercise was lower (greater deoxygenation) for CECS-diabetics (6.3±8.6%) vs. CON-diabetics (40.4±22.0%), and for CECS (11.3±16.8%) vs. CON (34.1±21.2%) (P<0.05 for both). The rate of oxygen recovery post exercise was faster for CECS-diabetics (3.5±2.6%/sec) vs. CON-diabetics (1.4±0.8%/sec) (P = 0.04), and there was a tendency of difference for CECS (3.1±1.4%/sec) vs. CON (1.9±1.3%/sec) (P = 0.05).

Conclusion. The greater deoxygenation during treadmill running for the CECS-diabetics group (vs. CON-diabetics) is in line with previous studies (and with the present study) that compared non-diabetic CECS patients with healthy controls. Our findings could suggest that NIRS may be useful as a diagnostic tool for assessing Type 1 diabetic patients suspected of CECS.

PURPOSE:

The mechanisms behind trapezius myalgia are unclear. Many hypotheses have been presented suggesting an altered metabolism in the muscle. Here, muscle microdialysate from healthy and myalgic muscle is analysed using metabolomics. Metabolomics analyse a vast number of metabolites, enabling a comprehensive explorative screening of the cellular processes in the muscle.

METHODS:

Microdialysate samples were obtained from the shoulder muscle of healthy and myalgic subjects that performed a work and stress test. Samples from the baseline period and from the recovery period were analysed using gas chromatography-mass spectrometry (GC-MS) together with multivariate analysis to detect differences in extracellular content of metabolites between groups. Systematic differences in metabolites between groups were identified using multivariate analysis and orthogonal partial least square discriminate analysis (OPLS-DA). A complementary Mann-Whitney U test of group difference in individual metabolites was also performed.

RESULTS:

A large number of metabolites were detected and identified in this screening study. At baseline, no systematic differences between groups were observed according to the OPLS-DA. However, two metabolites, L-leucine and pyroglutamic acid, were significantly more abundant in the myalgic muscle compared to the healthy muscle. In the recovery period, systematic difference in metabolites between the groups was observed according to the OPLS-DA. The groups differed in amino acids, fatty acids and carbohydrates. Myristic acid and putrescine were significantly more abundant and beta-D-glucopyranose was significantly less abundant in the myalgic muscle.

CONCLUSION:

This study provides important information regarding the metabolite content, thereby presenting new clues regarding the pathophysiology of the myalgic muscle.

Introduction

As the number of employees involved in computer work increases, neck and upper extremity complaints grow more common. Tight deadlines lead to high time pressure, which, in combination with precision demands, may increase the risk of musculoskeletal disorders. The physiological manifestation of such demands, however, is not entirely clear. In the present study, we examined local tissue oxygen saturation in the upper extremity as well as subjective stress responses during computer mouse work with and without time pressure and precision demands, and investigated whether these working situations have different effects on wrist position sense.

Material and methods

Twenty-four healthy, right-handed subjects (12 females, 12 males; age 19-28 years) participated in the study. Except for one subject who reported using a computer for 480 minutes per day, their average daily computer use was 84 minutes (SD 65 minutes). Subjects performed a 45-min mouse operated computer task on two occasions, separated by 3-5 days. The task consisted of painting squares that were presented on the screen. On one occasion, time pressure and precision demands were imposed on the task by limiting the time available for painting a square and introducing a scoring system based on precision of painting. On the other occasion, no such restraints were added. The order of the two task versions was randomized. During the task, tissue oxygen saturation in trapezius and extensor carpi radialis on the right arm was measured non-invasively by near-infrared spectroscopy, NIRS (Inspectra, Hutchinson Technology). In addition, subjective ratings of tenseness and strain, painting performance measures, and skin temperature were recorded. The position matching ability of the wrist was measured before and after the computer task. In the position matching tests, subjects attempted to actively reproduce target positions of horizontal movements about the right wrist joint. From a starting position of 30° of extension, target positions were randomized between 0° and 30° of flexion. The absolute value of the difference between presented target and reproduced position (AE) was used as outcome measure. Skin fold thickness at the locations of the NIRS electrodes was measured with a caliper at the beginning of the experiment.

Repeated measures analyses of variance were used to test for differences in oxygen saturation throughout the task, and for testing differences in position sense, as estimated by position matching ability, before and after the task. Subjective ratings during the task were analyzed by Wilcoxon’s non-parametric tests. Remaining parameters were tested for differences by t-tests or Wilcoxon’s non-parametric tests, depending on distribution of data. In all tests, p<0.05 was considered significant.

Results

A significant decline in oxygen saturation during mouse work under time pressure and precision demands was seen for extensor carpi radialis (F=4.68, p=0.036). This pattern was not present when mouse work was performed under more relaxed conditions. For trapezius, no difference in oxygen saturation between task versions was seen (F=0.01, p=0.932), although a general increase in saturation during work was found (F=10.35, p=0.002). Gender differences were apparent for extensor carpi radialis as well as for trapezius. Females showed an overall lower oxygen saturation in extensor carpi radialis than men (F=4.81, p=0.034). Furthermore, they showed a significantly different trend in trapezius oxygen saturation during work than men (F=6.27, p=0.016). Somewhat surprisingly, these gender differences could not be explained by differences in skin fold thickness at electrode positions (extensor carpi radialis: t=0.77, p=0.449; trapezius: t= 1.34, p=0.193). Subjects’ mean skin temperature changes during work was +0.41°C (SD 0.83°C).

AE measured before the computer task did not significantly differ between occasions (paired t-test: t=0.08, p=0.940), indicating that the time period between occasions was long enough for effects of work to wash out. A significant increase in AE was seen following the task (F=15.59, p<0.001), irrespective of task version (F<0.01, p=0.968). No gender differences in AE were found (F=0.08, p=0.774). Subjects’ ratings of tenseness and strain were significantly higher during work under time pressure and precision demands (Wilcoxon’s signed-ranks test: Z>3.41, p<0.001), compared to work without such demands. This is in agreement with the fact that subjects increased their work pace (squares painted in work with demands: 119, without demands: 84, paired t-test: t=8.38, p<0.001), and made an effort to paint as accurate and precise as possible (no. of times outside the square in work with demands: 2, without demands: 4, paired t-test: t=3.68, p=0.001) during the more demanding task version. No gender differences in subjective ratings (Mann-Whitney U-test: Z<2.14, ns) or performance variables (t-test: t<1.71, ns) were found.

Discussion

Subjects’ showed diminished oxygen saturation in extensor carpi radialis when working under time pressure and precision demands. This could be attributed to an increased mental load and/or a higher work intensity associated with this working situation. In support of this, subjective ratings of tenseness and strain scored higher during the more demanding task. It was also shown that subjects painted squares at a higher rate during the task. The fact that no differences in trapezius oxygen saturation between task versions was found might suggest that the physical strain associated with the work, being more prominent in extensor carpi radialis, could be a major contributor to oxygen saturation changes. Subjects’ tenseness ratings, however, would argue against it. Position sense, although poorer following work, was not affected by work type. One may speculate that physiological mechanisms involved in our measurement of position sense are not affected by local tissue oxygen saturation in extensor carpi radialis. The present data show considerable gender differences in oxygen saturation during rest as well as computer mouse work, that does not seem to be caused by skin fold thickness or painting performance.

The present study investigated the effects of time pressure and precision demands during computer mouse work on muscle oxygenation and position sense in the upper extremity. Twenty-four healthy subjects (12 males and 12 females) performed a 45-min standardized mouse operated computer task on two occasions. The task consisted of painting rectangles that were presented on the screen. On one occasion, time pressure and precision demands were imposed (more demanding task, MDT) whereas, on the other occasion, no such restraints were added (less demanding task, LDT). The order of the two task versions was randomized. Tissue oxygen saturation in the trapezius and extensor carpi radialis muscles was recorded throughout, and the position matching ability of the wrist was measured before and after the tasks. In addition, measurements of autonomic nervous system reactivity and subjective ratings of tenseness and physical fatigue were obtained. Performance was measured in terms of the number of rectangles that were painted during the task. During MDT, oxygen saturation in extensor carpi radialis decreased (p<0.05) compared to LDT. These data were paralleled by increased electrodermal activity (p<0.05), skin blood flow (p<0.05), ratings of tenseness and fatigue (p<0.01), and increased performance (p<0.01) during MDT. Females exhibited lower oxygen saturation than males, during rest as well as during the computer tasks (p<0.01). Wrist repositioning error increased following LDT as compared to MDT (p<0.05). In conclusion, computer mouse work under time pressure and precision demands caused a decrease in forearm muscle oxygenation, but did not affect wrist position sense accuracy. We attribute our changes in oxygenation more to increased oxygen consumption as a result of enhanced performance, than to vasoconstriction.

The main aim was to examine the effects of measurement depth on tissue oxygenation, and haemoglobin kinetics in the oxidative supraspinatus/trapezius muscle and in the deltoid muscle. Seven female subjects performed dynamic arm abductions (0.5 Hz) with an external load of 1.2 kg at level of the elbows. Tissue oxygenation (StO2) expressed as %StO2 saturation, total haemoglobin (HbT) , oxyhaemoglobin (HbO2) and deoxyhaemoglobin (Hb) were measured in two different measurement depth using near infrared spectroscopy (NIRS). HbT reflects blood volume. Measurements were performed simultaneously in the supraspinatus/trapezius and in the anterior part of the deltoid muscles.

The average StO2 level at rest was in general lower in the supraspinatus/trapezius muscles (67.7(5.3) % StO2) than in the deltoid muscle (85.4(4.4) % StO2). Submaximal dynamic arm abductions resulted in decreased StO2 in both muscles. However, the initial decrease was steeper and larger for the deltoid muscle than for the supraspinatus/trapezius region. Correspondingly, the initial reoxygenation in the recovery phase was faster for the deltoid muscle than for the supraspinatus/trapezius muscle.

At rest no differences in StO2 were found with increasing measurement depth in any of the two muscles. However, during exercise StO2 decreased with increasing measurement depth in the deltoid muscle whereas StO2 was more homogeneous in the supraspinatus/trapezius muscle.

Furthermore, HbT and HbO2 were largely maintained in the supraspinatus/trapezius muscle whereas HbT and HbO2 decreased during exercise and with increasing measurement depth (HbT) in the deltoid muscle. Hb increased during exercise in both muscles.

The hemodynamic differential responses to exercise for the two regions may reflect a combined effect of differences in muscle fibre composition and a dependency of depth related changes in the intramuscular pressure during exercise. Thus, the supraspinatus/trapezius muscles seems to be more

efficiently adapted to the oxygen demand during submaximal dynamic exercise than the deltoid muscle although the intramuscular pressure during the contractions is expected to be higher in the supraspinatus muscle than in the deltoid muscle.

In conclusion, muscle hemodynamic responses to dynamic exercise are highly muscle specific and may be spatially homogeneous or inhomogeneous depending on the muscle

Objective. The present study aimed to elucidate possible physiological mechanisms behind impaired endurance during chewing as previously reported in WAD. We tested the hypothesis of a stronger autonomic reaction in WAD than in healthy subjects in response to dynamic loading of the jaw-neck motor system. Design. Cardiovascular reactivity, muscle fatigue indicies of EMG, and perceptions of fatigue, exhaustion and pain were assessed during standardised chewing. Twenty-one WAD subjects and a gender/age matched control group participated. Baseline recordings were followed by two sessions of alternating unilateral chewing of a bolus of gum with each session followed by a rest period. Results. More than half of the WAD subjects terminated the test prematurely due to exhaustion and pain. In line with our hypothesis the chewing evoked an increased autonomic response in WAD exhibited as a higher increase in heart rate as compared to controls. Furthermore, we saw consistently higher values of arterial blood pressure for WAD than for controls across all stages of the experiment. Masseter EMG did not indicate muscle fatigue nor were there group differences in amplitude and mean power frequency. Pain in the WAD group increased during the first session and remained increased, whereas no pain was reported for the controls. Conclusion. More intense response to chewing in WAD might indicate pronounced vulnerability to dynamic loading of the jaw-neck motor system with increased autonomic reactivity to the test. Premature termination and autonomic involvement without EMG signs of muscle fatigue may indicate central mechanisms behind insufficient endurance during chewing.

Background: Unlike the usual skeletal muscles, ciliary muscles responsible for focusing the crystalline eye lens and extraocular muscles responsible for convergence eye movements appear resistant to fatigue. Purpose: The dual goals of this article are to briefly outline the current evidence that suggests that probing into blood flow and hemodynamic prefrontal brain activity with Near-Infrared Spectroscopy (fNIRS) could advance progress in visual ergonomic research, and to provide pilot data exemplifying the proposed approach. Methods: The vision task consisted of sustained focusing on a contrast-varying black and white Gabor grating. Four participants with a median age of 46 (IQR 44 – 50) fixated the grating from a distance of 65 cm. Three counterbalanced 10-min tasks required central fixation and accommodation/convergence on the grating target through: (i) 0.0 diopter (D) lenses, (ii) −1.5 D lenses, and (iii) −3.5yD lenses while maintaining maximal focus. Non-invasive measurements of local oxyhemoglobin (HbO₂) were quantified with a one-channel Near Infrared Spectrometer, NIRS. The NIRS probe was placed on the prefrontal cortex in the vicinity of the right dorsolateral prefrontal cortex or Brodmann area 46 (DLPFC, BA 46). Accommodation response and contrast threshold was measured in parallel. Results: General estimating equation analyses showed that baseline subtracted DLPFC blood flow (ΔHbO2) increased significantly over time in all three lens conditions. The effect of time may be caused by a continuous increase in mental effort to compensate for progressively more mental fatigue induced by increased visual attention. The increase of DLPFC ΔHbO2 was also larger in magnitude in participants with larger amplitudes accommodation response (i.e., in participants who minimized deterioration in visual performance). Conclusion: The results from this study indicate that oxyhemoglobin changes recorded over DLPFC with NIRS can be used to assay the degree to which the visual system is strained during demanding near work.

Objectives This experimental pilot study assessed the effects of sustained low-level accommodative vergence loads on oculomotor performance, eyestrain, and musculoskeletal functioning.

Methods A high-contrast fixation-point stimulus [light-emitting diode (LED)] was introduced into the optical axis of the viewing eye or into the midline in case of binocular viewing. The participants (N=6) were asked to compensate for the blur incurred by adjusting the strength of their eye lens. The participants performed in the following three standardized sequential viewing tasks: (i) resting with eyes open in darkness, (ii) accommodating alternately on a near versus a far LED illuminated sequentially (near–far response), and (iii) sustained fixation upon a LED at near. After the third task, the first and second tasks were repeated once.

Results The main effects of the third task were to decrease the overall rate of binocular accommodative relaxation time (diopters/s) in the repetition of the second task trial. The baseline shifts in individual response times also correlated with changes in the response amplitudes under the binocular stimulus conditions, which required contraction of the ciliary muscle.

Conclusions The results taken as a whole validate a technique of essential interest to applied vision research.