Trapezius activity during personal computer work with progressive addition lenses for general purpose and for computer work in neophytes

This study analysed the impact of general purpose progressive addition lenses (GP‐PALs) and computer progressive addition lenses (PC‐PALs) on the activity of the trapezius muscle during computer operation.

widely accepted model for pathogenesis of myofascial pain. 29,30Motor units within the skeletal muscle typically contract in a predetermined order from smallest to largest.Small motor units are the first to be recruited and the last to relax during prolonged muscle contractions.It is believed that these bottom motor units are affected during a prolonged contraction, which leads to an energy crisis from both increasing adenosine triphosphate consumption and decreasing oxygen and glucose supply, even when only low loads are acting on the respective muscle.Sustained low-level muscle activity (SULMA), 17,18 less gaps 16,24,31,32 and little rest time 23,24 are all considered to be risk factors for developing musculoskeletal complaints.Three muscle activation patterns were considered to be risk factors for developing musculoskeletal complaints, namely: • the existence of SULMA periods, which are time periods of continuous activity above a defined threshold with a minimum duration time 17,18 ; • fewer gaps, that is, spontaneous, unconscious and short pauses of muscle activity 16,24,31,32 ; • little rest time, which describes the total time during which the muscle activity is below a defined threshold for a minimum duration. 23,24der users are especially affected, as presbyopia is a risk factor for the development of associated discomfort. 335][36][37] Higher trapezius strain was found while using general purpose progressive addition lenses (GP-PALs), 38,39 but not with PALs designed specifically for computer work (PC-PALs) 40 -also known as occupational progressive lenses. 46In comparison with single vision lenses, office workers using GP-PALs were found, on average, to have 6.8° greater head recline, 33 greater extension in the cervical vertebral angle 41 and greater subjectively perceived or obvious forced posture. 34,42In a pilot study investigating the impact of multifocal contact lenses and GP-PALs on the activity of the trapezius muscle, Kolbe et al. 43 did not observe a relationship between an evoked and a steady head inclination on SULMA periods, gaps and rest time in the upper trapezius muscle in a small sample size.This follow-up study again analysed the impact of two different optical corrections for presbyopia on the muscle activity of the upper trapezius muscle but with an improved methodology and a larger sample size.

METHODS
This randomised, single-blinded, exploratory crossover study was conducted between January and June 2020 at the Division of motor research, Jena University Hospital, Germany.This research was reviewed on 19 December 2019 by the independent ethical review board of Jena University Hospital (5404-01118) and conformed to the principles and applicable guidelines for the protection of human subjects in biomedical research.The study followed the tenets of the Declaration of Helsinki.

Sample size and study population
Missing means and standard deviations (SDs) in previous studies limited the determination of the appropriate sample size for causal research.We aimed to enrol a sample size of 30 subjects to achieve a higher level of significance and to ascertain whether a normal distribution existed (based on the central limit theorem). 45With an assumed dropout rate of 25%, at least 40 subjects should be recruited.
Therefore, 41 right-handed non-presbyopic spectacle wearers (neophytes to PALs) between 18 and 40 years of age with a mean of 24 (±3) years were enrolled in the study.Non-PAL wearers were used to reduce the influence of habituation effects with any kind of presbyopic correction, as this was noted as a significant issue in the pilot study. 43resbyopia was simulated with the use of cycloplegia.
The visual acuity of each subject was 0.10 logMAR (6/7.5) or better, and all subjects had normal binocular vision.Exclusion criteria were: • anisometropia > 1.00 D; hyperopia > 6.00 D; myopia > 10.00 D; • infection in the anterior segment; a history of herpetic keratitis or ectasia (e.g., keratoconus); • use of cortisone; • family history of glaucoma case in first-degree relatives; narrow or closed anterior chamber angle (Van Herrick grade 0, 1 or 2); • chronic back pain (including whiplash in the last 12 months, cervical disk or lumbar disk herniation);

Key points
• The study found no significant difference in the muscular activity of the trapezius muscle when using different types of progressive addition lenses (PALs).• The subjective evaluation data showed that the computer PALs were statistically and clinically superior to the general purpose PAL in every category due to its design, leading to a subjectively perceived larger field of view and a more natural posture.• The study had a high dropout rate, limiting its sample size, and did not provide insights into long-term effects.Moreover, the study suggests that surface electromyography may not be sufficiently accurate to differentiate between the lens types.
• manipulative, osteopathic or physical treatment for musculoskeletal disorders within the last 12 months; • temporomandibular joint dysfunction; • repetitive strain injury; • dizziness and/or tinnitus; • diabetes mellitus; • mental illness (e.g., depression, anxiety or insomnia); • competitive sports or sport activity >4.5 h per week.
Nine subjects had to be excluded from the study because of • problems with centring or tolerance of the PALs (6×); • severely reduced reading speed due to hidden dyslexia (2×); • strong subjective glare and epiphora with mydriasis (1×); • lost to follow-up (1×).
Data from 24 female and 8 male subjects were analysed.The mean (±SD) spherical refractive and astigmatic error of the 32 subjects before cycloplegia was −2.03 ± 2.52 D and −0.84 ± 0.66 D, respectively, for the right eye and −2.01 ± 2.50 D and −0.77 ± 0.63 D, respectively, for the left eye, with a binocular visual acuity of −0.12 ± 0.04 logMAR.Under cycloplegia, the respective sphere and cylinder values were −1.83 ± 2.63 D and −0.84 ± 0.70 D for the right eye and −1.78 ± 2.62 D and −0.80 ± 0.61 D for the left eye, with a binocular visual acuity of −0.08 ± 0.04 logMAR.

Lenses
The GP-PALs and PC-PALs used here were Jubile, VISALL (visal l-brill englas.de/),and i Work, VISALL, respectively.The addition power of progressive spectacles is often calculated as the difference between the common near-viewing distance of 40 cm (i.e., 2.50 D) and half of the remaining accommodative amplitude. 47Since the remaining accommodative amplitude is close to zero following the administration of cyclopentolate, a +2.50 D addition was used for GP-PALs in this study.The power change in PC-PALs is referenced to the near power and is specified as a degression power that is a negative power progression that occurs with increasing height. 46In the present setup, the distance between the eye and the visual display was fixed to 0.73 m for all subjects-see section entitled Workplace and work tasks.Therefore, all PC-PALS used here had a power degression of 1.25 D, resulting in clear vision up to 0.8 m at the intermediate reference point.
Both lenses were based on a non-individual fixed design and were produced with the same technological and production process; thus, no quality differences were expected.The main difference in the designs was a reduced power change with a larger progressive corridor for the computer lens.This led to some remaining plus power at the major reference point, which enabled clear vision at the computer distance with a natural head and body posture.The astigmatic contour plots of the two lenses are illustrated in Figure 1.

Workplace and work tasks
The study was conducted in the motor laboratory of Jena University Hospital on a test workplace consisting of a height adjustable computer desk, a 24″ widescreen matte monitor mounted on an ergonomic monitor arm, an ergonomic office chair (without forearm support) and a standard mouse and keyboard.Viewing distance, relative monitor height (in reference to an imaginary horizontal line at eye level) and character size were aligned with field characteristics, based on the results of the ReVision study 48 (Figure 2).
Remaining ergonomic aspects (e.g., desk and chair height) were aligned with the International Organization for Standardization (ISO) 9241-5:1998. 49The character size of 14.3 angular minutes equals a font size of 11 point Arial at 100% zoom and 73 cm viewing distance.Temperature  and humidity were in the indifference range for surface electromyography (SEMG) measurements, that is, between 20 and 22°C and 40%-50%, respectively.Due to the lack of a standardised, validated protocol for the reproducible generation of a constant workload for visual display unit (VDU) work, a set of tasks was combined that closely simulated a range of screen work without the use of a keyboard.The use of the keyboard was deliberately omitted, since previous studies have found large differences between the keyboard skills of VDU users.When using the keyboard, posture differs greatly between an inexperienced user searching letter by letter on the keyboard and a skilled typist using all their fingers.The work tasks were all based on the fairy tales 'Hansel and Gretel' and 'The Brave Little Tailor' by the Brothers Grimm.The first task (read for 15 min) was reading the fairy tales aloud, while sitting in a neutral position with the hands placed next to the keyboard.For the second task, all sections of the fairy tale were presented in a mixed order and the subjects had to correct the order with the computer mouse by using drag and drop (sort for 8 min).In the third task, different words had to be marked with the computer mouse and formatted to red colour by clicking on the appropriate icon in the 'home' ribbon within Microsoft Word (Micro soft.com)(mark for 7 min).There was no break between the tasks.
All tasks were performed by the subjects one after the other (not randomised) with both lenses (randomised).The subjects received the lenses in an order based on the randomisation plan produced using the software Research Randomizer (rando mizer.org) 50and without knowledge of the type of product (single-blinded).Since both lenses were incorporated into the same spectacle frame, participants could not differentiate the products by their appearance.

Questionnaire for subjective evaluation
A seven-item questionnaire (non-standardised, visual analogue scale) was developed to evaluate subjectively perceived differences in vision and postural load between the lenses (Table 1).Subjects were able to answer the questions simultaneously for each lens with a handheld visual analogue scale containing a slider for each test item.For the person being tested, the scales contained only the extreme expression, for example, 1 (bad)-100 (good) without discrete scales.The examinee could read the exact value on the reverse side with an accuracy of 1 scale point.The visual analogue scale was used because of its superior metrical characteristics compared with discrete scales. 51

Study procedures
The baseline examination (visit 1) consisted of a case history, visual acuity test, binocular subjective refraction

Item
Question (translated to English) Please rate the image quality with test products A and B on a scale from 1 to 100, where 1 is bad and 100 is good.
2 On a scale of 1 (bad)-100 (good), how spontaneously did you get on with the respective test product?
3 How large did you perceive the field of view with test products A and B on a scale from 1 (small) to 100 (large)?
4 On a scale of 1 (little)-100 (a lot), how strongly did you perceive the need to perform a vertical head movement with test products A and B?
5 On a scale of 1 (little)-100 (a lot), how strongly did you perceive the need to perform a horizontal head movement with test products A and B?
6 On a scale of 1 (never)-100 (always), how much did you perceive the need to take a forced head posture (to be able to see sharply) with test products A and B? 7 On a scale of 1 (low)-100 (high), how willing would you be to purchase test product A or B if the presbyopia (caused by artificial ageing) were to remain?
and anterior segment slit lamp biomicroscopy examination including classification of the iridocorneal angle following standard operating procedures 52 and was performed by a trained optometrist.The binocular subjective refraction followed standard operating procedures for estimating myopia, hyperopia and astigmatism. 53inocular disorders were screened using the Stereo Fly Test (Stereo Optical Company, stere oopti cal.com/).Visual acuity threshold testing at far and near followed ISO 8596:2017 54 and was performed using a Landolt-C optotype and psychometric function (three errors in a row of five answers).In the absence of a closed or narrow iridocorneal angle, one drop of cyclopentolate 1% (Alcon, alcon.com)was instilled, a spectacle frame was selected and lens centring parameters were determined using a 3D video centring system (Video Infral; Zeiss, zeiss.de).
After visual acuity became >0.40 logMAR at 0.4 m (due to the induced loss of accommodation), binocular subjective refraction and visual acuity testing were repeated.Both PALs were ordered based on the cycloplegic subjective refraction.
At the beginning of visit 2, the iridocorneal angle was again checked and in the absence of a closed or narrow iridocorneal angle, one drop of cyclopentolate 1% (Alcon, alcon.com)was instilled.During the time until full cycloplegia was achieved, the skin was prepared, electrodes were put in place and the normalisation tasks were performed, see section entitled SEMG recording and signal processing, normalisation, parameters and analysis.Lens centring and visual acuity for intermediate and near tasks were controlled with full cycloplegia.Workplace characteristics were adjusted to the subject, see section entitled Workplace and work tasks.After visual acuity became >0.40 logMAR at 0.4 m (due to the induced loss of accommodation), SEMG signals were recorded bilaterally from the upper trapezius muscles during 30 min of continuous computer work with each lens in a randomised order.Body posture and ergonomic settings (such as display distance) were monitored by an ergonomist and optometrist during the tasks in real time.At the end of visit 2, the seven-item questionnaire was completed by all subjects.

SEMG recording and signal processing, normalisation, parameters and analysis
As noted earlier, Table A1 in Appendix 1 provides a summary of the measurement methodology used and the procedures for analysis.The SEMG data were measured from the left and right descending portions of the trapezius muscle simultaneously from both sides using the analogto-digital converter 'Tower of Measurement' (DeMeTec GmbH, demet ec.de/) at a rate of 2048 samples per second.An antialiasing filter of 1024 Hz was applied.To eliminate artefacts from the QRS complex in the SEMG data, an electrocardiogram was recorded simultaneously to calculate channel-specific templates of the respective ventricular activations which were subtracted from the contaminated signals. 55The position and orientation of the 57 × 37 mm Ag/AgCl electrodes (Kendall™ H93SG; Cardinal Health, cardi nalhe alth.com) are listed in Table 2 and illustrated in Figure 3.The signals were amplified with DC-coupled preamplifiers, placed close to the recording site and stored directly on an electronic notebook (G530-4446-25G; Lenovo, lenovo.com).The SEMG raw signals ran through a passive high-pass filter with a cut-off frequency of 20 Hz (based on De Luca 56 ) and a low-pass filter with a cut-off frequency of 300 Hz.The SEMG signals were digitally band-pass (30-400 Hz) and notch (main frequency and all harmonics) filtered.
To obtain reliable SEMG data, an initial rest period of 20 s was conducted to determine the noise level.Since there is a significant interindividual variability in SEMG data, muscle-specific submaximal reference and maximal contractions were performed.The reference voluntary contraction (RVC) was measured for the trapezius muscle with the arms in 90° abduction in the frontal plane.To evaluate the maximum voluntary contraction (MVC), a device was built consisting of a base plate, a belt with an adjustable iron chain and a force-measuring device connected to the analog-to-digital converter (Figure 4).The MVC was measured three times in a row for 5 s each, with a rest period of at least 2 min between contractions. 57,58The final MVC value was derived from the greatest force value of these consecutive attempts. 57Verbal encouragement was given during every MVC measurement.The root mean square (RMS) value was calculated for epochs of 0.25 s.These RMS values and the raw SEMG were used for identifying and excluding data with movement artefacts and electromagnetic interference.The RMS values were normalised to both the maximal voluntary electrical activity (MVE) obtained during the MVC and the reference voluntary electrical activity (RVE) obtained during the RVC.
Characteristics of muscle activity were evaluated by calculating the amplitude probability distribution function (APDF), gap frequency, relative rest time (RRT) and SULMA periods during the defined working tasks.Gaps were defined in two ways: as muscle activity below 5% of RVE for at least 0.25 s, 28 and as muscle activity below 1% of MVE. 40oth gap definitions were evaluated separately in the results section.RRT was defined as cumulative time with maximum voluntary electrical activation <1% and duration ≥100 ms. 23The 1% threshold level has previously been found to be sensitive to individual differences in terms of gap frequency. 28SULMA was defined as muscle activity >0.5% MVE and duration >1.6 s, 17 respectively.Therefore, a value >1.75 s was used, as this was the first multiple of the 0.25 s measurement interval exceeding 1.6 s, or seven epochs of 0.25 s with continuous muscle activity greater than this threshold.
The SEMG was recorded from 32 subjects.Ten data sets could not be analysed due to permanent atypical head and body movements (1×; scratching the head) and high baseline activity from muscles of unknown cause (9×).During MVC normalisation, there were large inter-individual differences in maximum force in the left and right trapezius.In eight cases, 30 min of error-free measurement could be generated and evaluated for both test times.In the remaining 14 cases, there were irregular, random, short interruptions for one or both test items in the raw data.Since we cannot ensure whether there were interruptions of a SULMA period in these missing data, these 14 measurements were excluded from the analysis of SULMA periods.For the calculation of the APDF, gap frequencies and the RRT, 30-s error-free samples were extracted and evaluated for each minute of continuous measurement in these cases.Thus, in these cases, 30 samples were evaluated for each 30-s measurement period.Except for the SULMA periods, it does not matter whether a continuous measurement or samples of that SEMG measurement were evaluated.

Statistical analysis
Data were analysed using IBM SPSS statistics version 23.0 (ibm.com).A dependent t-test for paired samples (paired t-test) was used to compare the means and their 95% confidence intervals for sample size >30.In those cases, normality was ascertained based on the central limit theorem. 45hen the sample size was between 10 and 30, a pooled nonparametric bootstrap t-test (10,000 replicates, biascorrected and accelerated method) was used for statistical analyses due to its superiority to the Wilcoxon rank sum test. 59For small sample sizes (N < 10), the Wilcoxon rank sum test was performed.A p value of ≤0.05 was applied to determine significant differences.The effect size d z and power were calculated from the mean and SD of the differences using a post-hoc analysis with the software package G*Power (psych ologie.hhu.de/arbeitsgru ppen/allge meine -psych ologi e-und-arbei tspsy cholo gie/gpower). 60

Amplitude probability distribution function
The static (10th percentile) and median (50th percentile) loads of the upper right (Table 3) and upper left trapezius (Table 4) showed no statistical differences between either lens for all work tasks individually (read, sort and mark) or in total (total measurement time over all three work tasks).The APDF for right and left trapezius is illustrated in Figures 5 and 6.

Gap frequency, RRT and SULMA
A high numerical value of the mean gap frequency in the right and left upper trapezius was found for both lenses and both gap criteria (1% MVE; 5% RVE) in the total work task.There were no statistical differences in the gap frequencies between the two lenses (Figure 7 and Table A2 in Appendix 1).
The high numerical value of the mean gap frequency also resulted in increased resting time for the upper right and upper left trapezius muscles during the total work task.However, there were no significant differences observed T A B L E 3 Mean (standard deviation [SD]) and statistical analysis for 10th percentile of amplitude probability distribution function based on reference voluntary electrical activity (RVE) of the upper left and upper right trapezius with each lens analysed for all work tasks (read, sort, mark and in total) (N = 22).between the GP-PAL and the PC-PAL, as shown in Figure 8 and Table A3 in Appendix 1.

Muscle
In eight subjects, SULMA periods could be evaluated for 30 min of continuous work with each lens.There were no statistically significant differences in the SULMA period medians for each time span between the GP-PAL and the PC-PAL (Tables 5 and 6).Since some p values were close to 0.05, significant differences in a larger sample could be conceivable.

Subjective evaluation
Subjective evaluation data for all 32 subjects are presented in Figure 9 and Table A4 in Appendix 1.The mean visual analogue scale scores for visual quality, spontaneous tolerance, field of view and purchase readiness were significantly increased with the PC-PAL compared with the GP-PAL.Means for subjectively perceived vertical head movement, horizontal head movement and forced posture were significantly increased while wearing the GP-PAL compared with the PC-PAL.

Surface electromyography
Contrary to the authors' expectation, the averaged APDF and its percentiles were almost identical for the GP-PAL and the PC-PAL.Only a low muscle activation was found indicating a small risk of developing a musculoskeletal disorder. 61,62The data from this study are consistent with the results of Horgen et al., 40 who investigated the effects of three different PC-PALs on postural load, and a pilot study (SERiOUS PC study 43 ).Despite many differences in the methodology compared with the pilot study, especially using neophyte PAL wearers, increasing the work task duration, using real ergonomic field characteristics and using same day measurements, a large degree of uncertainty remains in the data.For example, the provoked reclination of the head by 5°4 3 -6.8°3 7with the GP-PAL did not lead to an increased static or median load of the trapezius muscle.In contrast with the pilot study, 43 this investigation did not use a task that required keyboard work.Therefore, subjects no longer had to focus between the keyboard and

GP-PAL PC-PAL
the monitor during the tasks.This could explain the difference in the static muscle activity of the trapezius with GP-PALs (5.5% RVE in the present study vs. 18.2%RVE in the SERiOUS PC study 43 ).The examination of gap frequency, RRT and SULMA periods also showed no significant differences between the two lenses.These results are also comparable with the SERiOUS PC study 43 and Horgen et al. 40 The RRT is comparable with the stress and precision task of the control group in Thorn et al. 23 For the evaluation of the SULMA periods, it is unfortunate that complete and error-free measurements were available for both lenses in only eight cases.
Considering the totality of the SEMG data, the findings showed no significant difference in the activity of the trapezius muscle with the two forms of presbyopic examined here.This may indicate that the influence of the correction on the subjects' posture was smaller than previously assumed, or that the SEMG method was not ciently accurate to answer this research question.
During the study, subjects were closely observed by the investigators to obtain further information regarding the high variability of the SEMG data.Although subjects assumed a larger reclination in front of the screen when using GP-PALs, the head did not remain static in this position, since subjects continued to make small, task independent head movements.In a preliminary randomised masked crossover study, Kolbe and Degle showed that head movements did not differ with the use of either GP-PALs or PC-PALs during standardised work tasks in 190 presbyopic VDU workers. 63It is therefore possible that the forced head posture produced by GP-PALs exerted little strain on the trapezius relative to the general, habitual movement at the workstation.In that regard, it is interesting that the Cinderella hypothesis is not without controversy.De Luca et al. were able to show significant differences in the activation patterns of tonic and phasic postural muscles. 64,65Tonic postural muscles do not seem to be activated according to their size, but rather on a rotational principle, and might protect motor units in postural muscles from excessive fatigue when there is a demand for SULMA. 65Therefore, Minerbi and Vulfsons 66 doubted whether the Cinderella hypothesis was a good model for myofascial pain in postural muscles and hypothesised a new model, where increased muscle load and decreased muscle force could lead to shortened motor unit relaxation periods, and subsequently, to an energy crisis.

Subjective evaluation
In contrast to the SEMG data, the subjective evaluation findings were clear.The PC-PAL was statistically and clinically significantly superior to the GP-PAL in every subjective category.The design of PC-PALs with the abandoning of the F I G U R E 8 Means and 95% confidence intervals for relative rest time in % of upper left and right trapezius with both lenses analysed for total task (N = 22).GP-PAL, general purpose progressive addition lens; PC-PAL, progressive addition lens for computer work; RVE, reference voluntary electrical activity.
far distance power and lowering the power range between the near and 'far' reference points effectively decrease the magnitude of unwanted astigmatism, thereby leading to a subjectively perceived larger field of view with specific computer lenses and a more natural posture.This was also reflected in a clear theoretical purchase decision (forced choice) for the specific correction for computer work.[36][37]

Limitations of this study
In this study, a dropout rate of 25% was anticipated.Therefore, the number of cases in this pilot study was set at 41 to include at least 31 subjects in the statistical analysis and to apply more reliable statistical procedures, see section entitled Statistical analysis.Unfortunately, the final dropout rate, including SEMG measurements, turned out to be around 50%, which was much larger than expected.
Hence, the study sample size was smaller than anticipated.However, because of its crossover design, it was comparable with previously conducted studies that analysed SEMG during computer work. 25,39,40,43,67,68,69The subjective evaluation captured a snapshot of a realistic workplace in the laboratory.It is conceivable that the evaluations would be different when used in everyday life at a real workplace.Furthermore, this study did not provide any insights into long-term effects.The subjective evaluation of the lenses and their impact on computer vision syndrome may vary with increasing use. 35,37However, participants could not be allowed a longer familiarisation period (e.g., at their F I G U R E 9 Means and 95% confidence intervals for scores (visual analogue scale, 1-100) of visual quality, spontaneous tolerance, field of view, vertical head movement, horizontal head movement, forced posture and purchase readiness for general purpose progressive addition lens (GP-PAL) and progressive addition lens for computer work (PC-PAL) (n = 32).habitual workplace) because cycloplegic drugs were required for each test phase to achieve the artificially induced presbyopia.Since the use of cyclopentolate not only inhibits accommodation but also dilates the pupil and causes the described optical problems, the results of the study are not fully applicable to people with real presbyopia.For instance, it must be considered that presbyopes are usually at least 20 years if not 30-40 years older than the subjects in the present investigation, and their eyes are likely affected by other gerontological changes.These may include an increase in vitreous and lens opacities, decreased retinal cell density and an increase in dry eye disease.
Although the use of SEMG provides interesting insights into muscle activation and strain, its use and the various analysis algorithms are not without For example, the APDF ignores important aspects of postural variation and pause distribution, 16,70 and its varying percentiles have a high risk of lacking statistical significance. 44Comparison with previously published data is difficult because different research groups choose different normalisation procedures for recording MVC or RVC. 17,57,71,72In particular, it is difficult to ensure that the maximum force is provided only by the muscles being analysed.The MVC measurement setup used here also could not guarantee this.Slight tilting of the body or torso creates a lever on the arm that greatly increases the maximum force.In addition, it is possible to generate part of the force from the legs or the trunk by stretching the entire body lengthwise.Both the experienced instructor and the study team tried to watch for indications of this bias, discard appropriate measurements, reinstruct the participant and repeat the measurement.

CONCLUSION
The SEMG results were inconclusive and there was large variability in the data sets.Considering these results in conjunction with the previous SERiOUS PC Study, 43 the present authors are unsure whether SEMG measurements should be the method of choice for applied research questions such as this.Although it is desirable to assess objectively the large subjectively perceived differences between the different forms of presbyopic corrections, the large variance does not justify the extensive data collection and complex analysis of millions of data points.
Even though the electromyographic approach did not show a clear differentiation between the two lenses examined here, the subjective evaluation was clearly in favour of PC-PALs.Therefore, eye care practitioners should always take an occupational history of presbyopes, ask about the workplace situation and consider prescribing PC-PALs.

AC K N O W L E D G M E N T S
Open Access funding enabled and organized by Projekt DEAL.

FU N D I N G I N FO R M AT I O N
The lenses and subject compensation (in form of a pair of single vision corrective lenses) were provided by VISALL (visal l-brill englas.de/).Otherwise, this research did not receive any specific grant from funding agencies in the public, commercial or not-for-profit sectors.

CO N F L I C T O F I N T E R E S T S TAT E M E N T
The authors have no conflicts of interests to declare.

R E F E R E N C E S APPE NDIX 1 T A B L E A 2
Mean, standard deviation (SD) and statistical data for gap frequencies of upper left and right trapezius with both lenses analysed for total task (N = 22).

Muscle Definition
Gap min −1 (SD) GP-PAL T A B L E A1 Brief description of measurement and analysis of surface electromyography (SEMG).

SEMG
The SEMG measures the electrical activity of the motor unit action potentials (in mV) of one or more muscles Root mean square (RMS) The surface electromyography (SEMG) signal is stochastic in nature, that is, the measurement signal cannot be reproduced exactly when the same movement is measured several times, because the sum interference pattern of the activity of the involved motor units can be different each time.The RMS is a smoothing algorithm that describes the actual occurring average voltage/power values of the SEMG signal and has a direct relation to the strength of the SEMG signal and thus a clear physical meaning Maximum voluntary electrical activation (MVE) and maximum voluntary contraction (MVC) The MVE value corresponds to the maximum electrical activity that a muscle can generate and is often used as a reference value.The MVC value corresponds to the maximum force measured parallel to the SEMG Reference voluntary electrical activity (RVE) and reference voluntary contraction (RVC) RVE and RVC are equivalent to MVE and MVC but for a submaximal reference contraction, for example, holding a 1-kg dumbbell with the arm extended to the side.RVE and RVC are also commonly used as reference values Amplitude probability distribution function (APDF) The APDF describes the cumulative distribution of the different contraction levels of the analysed muscle over the entire measurement time.The APDF indicates the probability with which the threshold of a specific myoelectric activity is exceeded or undershot (e.g., with a probability of 10%, a myoelectric activity of up to 20% relative maximum force occurs) Gap frequency Spontaneous, unconscious, short periods of low muscle activity are referred to as gaps.The investigation of these short pauses is intended to provide information about the health influence of an observed SEMG signal and the associated muscle activity.The gap frequency indicates how many of these gaps occur in a fixed time interval of, for example, 1 s

Relative rest time (RRT)
The RRT describes the total time during which the RMS value is below a defined threshold (e.g., 1% RVC or 5% RVC) for a minimum duration of time (e.g., 100 ms).Depending on the definition of the gaps or the RRT, the RRT can also be understood as the sum of all gaps in a discrete time interval Sustained low level muscle activity (SULMA) SULMA periods are defined as time periods with a duration of at least 1.6 s of continuous activity above 0.5% MVE T A B L E A 4 Mean, standard deviation (SD), statistical data, effect size and power for scores (visual analogue scale, 1-100) of visual quality, spontaneous tolerance, field of view, vertical head movement, horizontal head movement, forced posture and purchase readiness with general purpose progressive addition lenses (GP-PAL) and progressive addition lens for computer work (PC-PAL; N = 32).

F I G U R E 1
Astigmatic contour plot (a) for general purpose progressive addition lenses and (b) progressive addition lenses for computer work are shown for a plano distance power and an addition of +2.50 D (a) and degression of 1.25 D (b).

F I G U R E 2
Field characteristics based on ReVision Study for viewing distance, relative monitor height (in reference to imaginary horizontal line at eye level) and character size.

1
Translated (German to English) questions from the non-standardised seven-item questionnaire to evaluate subjectively perceived differences between the lenses (test products A and B).

T A B L E 2
Position and orientation of the electrodes.Muscle Position and orientation of the electrodes Trapezius (descending part) 1 cm medial to half the distance between C7 and Acromion, on line Myocardium Above the heart axis, with ground on the sternum F I G U R E 3 Position and orientation of the electrodes for electromyography data and electrocardiogram.

F I G U R E 4
Measuring maximum voluntary contraction in frontal plane for the trapezius muscle with a self-made device containing a base plate, a non-elastic belt adjusted to an iron chain customisable for individual body size and a force measuring device connected to the analog-to-digital converter.

F I G U R E 5
Amplitude probability distribution function (solid lines: median and interquartile range, dotted lines: 10th and 50th percentiles) of upper right trapezius during total work task (N = 22).GP-PAL, general purpose progressive addition lens; IQR, interquartile range; PC-PAL, progressive addition lens for computer work.

F I G U R E 6
Amplitude probability distribution function (solid lines: median and interquartile range, dotted lines: 10th and 50th percentiles) of upper left trapezius during the total work task (N = 22).GP-PAL, general purpose progressive addition lens; IQR, interquartile range; PC-PAL, progressive addition lens for computer work.

7
Means and 95% confidence intervals for gap frequencies of the upper left and right trapezius with both lenses analysed for total task (N = 22).GP-PAL, general purpose progressive addition lens; MVE, maximal voluntary electrical activity; PC-PAL, progressive addition lens for computer work; RVE, reference voluntary electrical activity.

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Median, interquartile range (IQR) and statistical analysis for sustained low-level muscle activity (SULMA) periods for continuous 30-min measurements of the upper right trapezius with each lens (N = 8).
Mean (standard deviation[SD]) and statistical analysis for 50th percentile of amplitude probability distribution function based on reference voluntary electrical activity (RVE) of upper left and upper right trapezius with each lens analysed for all work tasks (read, sort, mark and in total) (N = 22).
Abbreviations: BCa, Bias-corrected and accelerated method; CI, confidence interval; GP-PAL, general purpose progressive addition lenses; PC-PAL, progressive addition lens for computer work.
Median, interquartile range (IQR) and statistical analysis for sustained low-level muscle activity (SULMA) periods for continuous 30-min measurements of the upper left trapezius with each lens (N = 8).
T A B L E 6